MODULE in Consumer Electronics

September 8, 2017 | Author: Nyepni Nelmi | Category: Capacitor, Series And Parallel Circuits, Direct Current, Root Mean Square, Resistor
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Consumer Electronics Module...

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TABLE OF CONTENTS             

PREFACE INTRODUCTION SYMBOLS AND FUNCTIONS BLOCK DIAGRAMS TYPES OF COMPONENTS RESISTANCE CAPACITANCE AC/DC CIRCUITS ELECTRICITY AND ELECTRONS OHM’S LAW WATT LAW MULTIMETERS OSCILLOSCOPE

            

TRANSISTORS MOSFETS TRANSDUCERS VOLTAGE DIVIDERS TRANSFORMERS COILS DIODES POWER SUPPLY SOLDERING GUIDE VACUUM TUBE POWER SUPPLY PROJECT EXERCISES DEFINITION OF TERMS

PREFACE This module is for beginning students without any background in electricity and electronics, starting with basic components, ohm’s laws , series and parallel circuits , power dissipation in resistance , and the topics progress to multimeters and low voltage power supplies. For each subject, the basic principle are explained followed by exercises.

Mr. Sancho L. Revilla Trainer Consumer Electronics / Audio Video Servicing Muntinlupa City Technical Institute ,TESDA San Guillermo,Putatan Muntinlupa City

Introduction Electronics and radio communications are practical applications of the general principles of electricity. The same electricity produced by a battery for a flashlight can be modified to do any number of jobs, from running a motor or producing heat and light to more advanced uses such as working a computer or providing wireless broadcasting for radio and television. The word radio is an abbreviated form of radiotelegraph or radiotelephone. In its first form, wireless communication was by radiotelegraph, using short dots and longer dashes as symbols for letters in the morse code. Now radiotelephone is used more,providing wireless voice communications or broadcasting voice and music programs for entertainment. In general, then, radio is the art of wireless communications. The word electronics derives from the electron, which is a tiny, invisible quantity of electricity present in all materials. In terms of its many uses, electronics can be defined to include all applications of electricity flowing in a vacuum, as in vacuum tubes, in gas vapor, and in certain solid materials such as transistors. More generally, electronics includes all effects of electricity where the action of individual electrons determines the application. Radio and electronics are closely related. Sometimes they are even joined in their use. For example, an electronic heating unit generates radio waves that go through the work to produce heat. The heat bonds the solid materials together. Even if applications are not so close, the principles of radio and electronics are essentially the same. Both are based on the fundamentals laws of electricity.

SYMBOLS AND FUNCTIONS

Wires and connections Component

Circuit Symbol

Function of Component To pass current very easily from one part of a circuit to another.

Wire

Wires joined

A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two Tjunctions, as shown on the right.

Wires not joi ned

In complex diagrams it is often necessary to draw wires crossing even though they are not connected. The simple crossing on the left is correct but may be misread as a join where the 'blob' has been forgotten. The bridge symbol on the right leaves no doubt!

Power Supplies Component

Circuit Symbol

Function of Component

Cell

Supplies electrical energy. The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.

Battery

Supplies electrical energy. A battery is more than one cell. The larger terminal (on the left) is positive (+).

Solar Cell

Converts light to electrical energy. The larger terminal (on the left) is positive (+).

DC supply

Supplies electrical energy. DC = Direct Current, always flowing in one direction.

Supplies electrical energy. AC = Alternating Current, continually changing direction.

AC supply

Fuse

A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.

Transformer

Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.

Earth (Ground)

A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.

Output Devices: Lamps, Heater, Motor, etc. Component

Circuit Sym bol

Function of Component

Lamp (lighting)

A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.

Lamp (indicator )

A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.

Heater

A transducer which converts electrical energy to heat.

Motor

A transducer which converts electrical energy to kinetic energy (motion).

Bell

A transducer which converts electrical energy to sound.

Buzzer

A transducer which converts electrical energy to sound.

Inductor (Coil, Solenoid)

A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a

transducer converting electrical energy to mechanical energy by pulling on something. Switches Component

Circuit Sy mbol

Push Switch (push-tomake)

Function of Component A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.

Push-to-Break Switch

This type of push switch is normally closed (on), it is open (off) only when the button is pressed.

On-Off Switch (SPST)

SPST = Single Pole, Single Throw. An on-off switch allows current to flow only when it is in the closed (on) position.

2-way Switch (SPDT)

SPDT = Single Pole, Double Throw. A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.

Dual On-Off Switch (DPST)

DPST = Double Pole, Single Throw. A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections. DPDT = Double Pole, Double Throw. This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.

Reversing Switch (DPDT)

An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit. NO = Normally Open, COM = Common, NC = Normally Closed.

Relay

Resistors Component

Resistor

Circuit Sy mbol

Function of Component A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit.

Some publications use the old resistor symbol: Variable Resis tor (Rheostat)

This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

Variable Resis tor (Potentiometer )

This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.

Variable Resis tor (Preset)

This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.

Capacitors Component

Circuit Symbo l

Function of Component

Capacitor

A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

Capacitor, polarised

A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

Variable Capa citor

A variable capacitor is used in a radio tuner.

Trimmer Capacitor

Diodes

This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.

Component

Circuit Symbo l

Function of Component

Diode

A device which only allows current to flow in one direction.

LED Light Emitting Diode

A transducer which converts electrical energy to light.

Zener Diode

A special diode which is used to maintain a fixed voltage across its terminals.

Photodiode

A light-sensitive diode.

Transistors Component

Circuit Sym bol

Function of Component

Transistor NPN

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

Transistor PNP

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

Phototransistor

A light-sensitive transistor.

Audio and Radio Devices Component Microphone

Circuit Sy mbol

Function of Component A transducer which converts sound to electrical energy.

Earphone

A transducer which converts electrical energy to sound.

Loudspeaker

A transducer which converts electrical energy to sound.

Piezo Transducer

A transducer which converts electrical energy to sound.

Amplifier (general symbol)

An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.

A device which is designed to receive or transmit radio signals. It is also known as an antenna.

Aerial (Antenna) Meters and Oscilloscope Component

Circuit S ymbol

Function of Component

Voltmeter

A voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!

Ammeter

An ammeter is used to measure current. A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.

Galvanometer

An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.

Ohmmeter

An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Oscilloscope Sensors (input devices) Component LDR Thermistor

Circuit Sym bol

Function of Component A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor A transducer which converts temperature (heat) to resistance (an electrical property).

Logic Gates Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the Logic Gates page. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission). Gate Type NOT

Traditional IEC Symbol Symbol

Function of Gate A NOT gate can only have one input. The 'o' on the output means 'not'. The output of a NOT gate is the inverse (opposite) of its input,

so the output is true when the input is false. A NOT gate is also called an inverter. AND

An AND gate can have two or more inputs. The output of an AND gate is true when all its inputs are true.

NAND

A NAND gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not ANDgate. The output of a NAND gate is true unless all its inputs are true.

OR

An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.

NOR

A NOR gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not OR gate. The output of a NOR gate is true when none of its inputs are true.

EX-OR

An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).

EXNOR

An EX-NOR gate can only have two inputs. The 'o' on the output means 'not' showing that it is a Not EX-ORgate. The output of an EX-NOR gate is true when its inputs are the same (both true or both false).

Block Diagrams Block diagrams are used to understand (and design) complete circuits by breaking them down into smaller sections or blocks. Each block performs a particular function and the block diagram shows how they are connected together. No attempt is made to show the components used within a block, only the inputs and outputs are shown. This way of looking at circuits is called the systems approach. Power supply (or battery) connections are usually not shown on block diagrams.

Audio Amplifier System

The power supply (not shown) is connected to the pre-amplifier and power amplifier blocks.   

 

Microphone - a transducer which converts sound to voltage. Pre-Amplifier - amplifies the small audio signal (voltage) from the microphone. Tone and Volume Controls - adjust the nature of the audio signal. The tone control adjusts the balance of high and low frequencies. The volume control adjusts the strength of the signal. Power Amplifier - increases the strength (power) of the audio signal. Loudspeaker - a transducer which converts the audio signal to sound.

Radio Receiver System

The power supply (not shown) is connected to the audio amplifier block.    



Aerial - picks up radio signals from many stations. Tuner - selects the signal from just one radio station. Detector - extracts the audio signal carried by the radio signal. Audio Amplifier - increases the strength (power) of the audio signal. This could be broken down into the blocks like the Audio Amplifier System shown above. Loudspeaker - a transducer which converts the audio signal to sound.

Regulated Power Supply System

   

Transformer - steps down 230V AC mains to low voltage AC. Rectifier - converts AC to DC, but the DC output is varying. Smoothing - smooths the DC from varying greatly to a small ripple. Regulator - eliminates ripple by setting DC output to a fixed voltage.

Feedback Control System

The power supply (not shown) is connected to the control circuit block.   

  

Sensor - a transducer which converts the state of the controlled quantity to an electrical signal. Selector (control input) - selects the desired state of the output. Usually it is a variable resistor. Control Circuit - compares the desired state (control input) with the actual state (sensor) of the controlled quantity and sends an appropriate signal to the output transducer. Output Transducer - converts the electrical signal to the controlled quantity. Controlled Quantity - usually not an electrical quantity, e.g. motor speed. Feedback Path - usually not electrical, the Sensor detects the state of the controlled quantity.

Five Basic components Considering the many different applications of electronics and radio, we can be a little surprised that there are only five basic types of components for all the different kinds of equipment. Of course, each type has many variations for specific uses. Still, the following is a short list:  Electron tubes, including vacuum-tube amplifiers, gas-filled tubes, and the cathode-ray tube (CRT)

 Transitors.this is probably the most important use of solidstate semiconductors, which includes diodes and the intergrated circuits.

 Resistors

 Capacitors, or condensers

 Inductors, or coils

Tubes and transistors are used in electonic circuits with resistors, capacitors and inductors. The transistors and tubes are active components, meaning they can amplify or rectify.

Resistors, capacitors and inductors are passive components.intergrated circuits combined solid-state transistors, and diodes in one IC chip for a complete circuit with the passive components.

Resistor Resistance is inserted into a circuit, either to reduce the current to a desired value or to produce a specific IR voltage drop. The components for this purpose are called RESISTORS. They are the most used components in all kinds of electronic equipment. The two main types of resistors commonly used are the carbon composition and the wire wound. They are the available from a fraction of an ohm to many megohms, with a power rating of several hundred watts down to a value as low as on tenth(1/10) of a watt.the power rating is the maximum amount of watts the resitor can dissipate without excessive heat. Wire wound resistors are used for applications where the power dissipation in the resistor is about 3 watts or more. Carbon-composition type is usually applied for 2 watts or less because its smaller and costs less. Most of the resistor used in the radio, television and electronic equipmant are carbon composition. The carbon composition cann either fixed or variable. Fixed resistors have a specific amount or resistance that cannot be adjusted from one value between zero ohm and its maximum value. Potentiometers and rheostats are variable resistance controls, either carbon composition or wirewound, used for varying the voltage and current in a circuit. A rheostat is a variable resistance with two terminals connected in series to vary the current.on the other hand, the potentiometer has three teminals. The fixed maximum resistance between the two end terminals is connected across a voltage source and the variable is used to vary the voltage division. Sometimes potentiometers are used as rheostat by short-circuiting one end of the terminal to the middle terminal. Because carbon resistors are small physically, they are color-coded to indicate their resistance value in ohms. Resistor tolerance Tolerance is the amount by which actual resistance can be different from the color coded value.tolerance is usually given in percent.

Classes of Resistors Variable resistor which value is easily changed, like the volume adjustment of Radio. The other is semi-fixed resistor that is not meant to be adjusted by anyone but a technician.

Thermistor ( Thermally sensitive resistor ) The resistance value of the thermistor changes according to temperature. This part is used as a temperature sensor.

Fixed resistors-is one in which the value of its resistance cannot change.

2nd band

3rd band (multiplier)

4th band (multiplier)

color

1st band

Black



0

×1

1 brown

1

1

×10

2 red

2

2

×10²

3orange

3

3

×10³

4 yellow

4

4

×10⁴

5 green

5

5

×10⁵

6 blue

6

6

×10⁶

7 violet

7

7

×10⁷

8 gray

8

8

×10⁸

9 white

9

9

×10⁹

Gold





×0.1

+ or - 5%

silver





×0.01

+ or - 10%

+ or - 20%

Resistors connected in Series When resistors are connected in series their combined resistance is equal to the individual resistances added together. For example if resistors R1 and R2 are connected in series their combined resistance, R, is given by: Combined resistance in series: R = R1 + R2 This can be extended for more resistors: R = R1 + R2 + R3 + R4 + ... Note that the combined resistance in series will always be greater than any of the individual resistances. Resistors connected in Parallel When resistors are connected in parallel their combined resistance is less than any of the individual resistances. There is a special equation for the combined resistance of two resistors R1 and R2: Combined resistance of R1 × R2 R = two resistors in parallel: R1 + R2 For more than two resistors connected in parallel a more difficult equation must be used. This adds up thereciprocal ("one over") of each resistance to give the reciprocal of the combined resistance, R: 1 R

=

1 1 1 + + + ... R1 R2 R3

The simpler equation for two resistors in parallel is much easier to use! Note that the combined resistance in parallel will always be less than any of the individual resistances.

Capacitors A capacitor is a small device that can be charged up with electrical energy, store it and then release it. Just like a rechargeable battery. But unlike a battery, it does not use a chemical reaction and it can only hold a very small charge. A very large capacitor can only light up an LED for a few seconds. They come in many shapes and sizes and a few are shown below. The bigger the capacitor, the more charge it will hold.

A capacitor is made from two metal plates or metal foils separated by an insulator

called

a

Dielectric

material.

The Dielectric materials can be made from Ceramic, Mica, Polypropylene, Polyester,

Electrolytic,

Tantalum

and

even

air.

The larger capacitors look like tubes, this is because the metal foil plates are rolled up with an insulating dielectric material sandwiched in between.

The value of capacitance is determined by The size of the plates, The distance between them, The type of dilectric material used. The Unit of Capacitance (C) Capacitance is measured in Farads. (after Michael Faraday 1791 - 1867) The Farad is too big a unit so values are measured in:microfarads (μF), nanofarads (nF) and picofarads (pF). Largest value is 22000μF Lowest value is 1.0pF 1F =1,000,000μF

1μF = 1000nF

1nF = 1000pF

Circuit Identification. In circuit diagrams a fixed capacitor is identified with the letter C. i.e. C1 C2 ... C12 Variable capacitors/Trimmers are identified with the letters VC1 VC2 .....

Polarized Capacitors. Electrolytic and Tantalum capacitors are POLARISED and they must be connected the correct way round. (correct polarity). The casing is marked showing the Negative lead which should be connected to the Negative rail (0 Volt). The circuit symbol shows the + Positive lead. All others capacitors can be connected either way round. Use of Capacitors. Capacitors are used in following ways:1. 2. 3. 4. 5. 6.

Store a voltage for a period of time, Create a time delay circuit. Shorten or extend pulse lengths, Smooth fluctuating voltages, Filter unwanted frequencies, Allows Alternating Current (ac) to pass to another part of a circuit but blocks Direct Current (dc).

Working Voltage. The working voltage of the capacitor must not be exceeded. It is good practice to choose a capacitor with a working voltage 50% higher than the circuits normal working voltage. Care should be taken with polarised Electrolytic and Tantalum capacitors as they have low working voltages. For a

9 Volt circuit choose a 16V or higher capacitor. The higher the voltage, the bigger and more expensive they get. Manufacturer's catalogue will give you all the information you need. Leakage Current. The dielectric is an insulator and the current should not flow through it. However a perfect insulator does not exist and a small leakage current will flow out eventually discharging the capacitor. Capacitance Code:- Most capacitors have a tolerance of 20% and have the following numerical values 10 15 22 33 47 68 82 As many capacitors are small, the values are printed with a three number code. The first two refer to the numerical values and the last gives the numbers of zeros. Some old capacitors are colour coded in a way similar to resistors.

Capacitors in Series and Parallel Combined capacitance (C) of capacitors connected in series: Combined capacitance (C) of capacitors connected in parallel:

1 C

1 C1 =

1 C2 +

1 C3 +

+ ...

C = C1 + C2 + C3 + ...

Two or more capacitors are rarely deliberately connected in series in real circuits, but it can be useful to connect capacitors in parallel to obtain a very large capacitance, for example to smooth a power supply. Note that these equations are the opposite way round for resistors in series and parallel.

Charging a capacitor The capacitor (C) in the circuit diagram is being charged from a supply voltage (Vs) with the current passing through a resistor (R). The voltage across the capacitor (Vc) is initially zero but it increases as the capacitor charges. The capacitor is fully charged when Vc = Vs. The charging current (I) is determined by the voltage across the resistor (Vs Vc): Charging current, I = (Vs - Vc) / R (note that Vc is increasing) At first Vc = 0V so the initial current, Io = Vs / R Vc increases as soon as charge (Q) starts to build up (Vc = Q/C), this reduces the voltage across the resistor and therefore reduces the charging current. This means that the rate of charging becomes progressively slower.

time constant = R × C

where:

time constant is in seconds (s) R = resistance in ohms ( ) C = capacitance in farads (F)

For example: If R = 47k and C = 22µF, then the time constant, RC = 47k × 22µF = 1.0s. If R = 33k and C = 1µF, then the time constant, RC = 33k × 1µF = 33ms. A large time constant means the capacitor charges slowly. Note that the time constant is a property of the circuit containing the capacitance and resistance, it is not a property of a capacitor alone. The time constant is the time taken for the charging (or discharging) current (I) to fall to 1/e of its initial value (Io). 'e' is the base of natural logarithms, an important number in mathematics (like ). e = 2.71828 (to 6 significant figures) so we can roughly say that the time constant is the time taken for the current to fall to 1/3 of its initial value. After each time constant the current falls by 1/e (about 1/3). After 5 time constants (5RC) the current has fallen to less than 1% of its initial value and we can reasonably say that the capacitor is fully charged, but in fact the capacitor takes for ever to charge fully!

The bottom graph shows how the voltage (V) increases as the capacitor charges. At first the voltage changes rapidly because the current is large; but as the current decreases, the charge builds up more slowly and the voltage increases more slowly. After 5 time constants (5RC) the capacitor is almost fully charged with its voltage almost equal to the supply voltage. We can reasonably say that the capacitor is fully charged after 5RC, although really charging continues for ever (or until the circuit is changed).

Time Voltage Charge 0RC

0.0V

0%

1RC

5.7V

63%

2RC

7.8V

86%

3RC

8.6V

95%

4RC

8.8V

98%

5RC

8.9V

99%

Discharging a capacitor The top graph shows how the current (I) decreases as the capacitor discharges. The initial current (Io) is determined by the initial voltage across the capacitor (Vo) and resistance (R): Initial current, Io = Vo / R. The bottom graph shows how the voltage (V) decreases as the capacitor discharges.

Time Voltage Charge

At first the current is large because the voltage is large, so charge is lost quickly and the voltage decreases rapidly. As charge is lost the voltage is reduced making the current smaller so the rate of discharging becomes progressively slower.

0RC

9.0V

100%

1RC

3.3V

37%

2RC

1.2V

14%

After 5 time constants (5RC) the voltage across the capacitor is almost zero and we can reasonably say that the capacitor is fully discharged, although really discharging continues for ever (or until the circuit is changed).

3RC

0.4V

5%

4RC

0.2V

2%

5RC

0.1V

1%

AC, DC and Electrical Signals AC means Alternating Current and DC means Direct Current. AC and DC are also used when referring to voltages and electrical signals which are not currents! For example: a 12V AC power supply has an alternating voltage (which will make an alternating current flow). Anelectrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit.

Alternating Current (AC) Alternating Current (AC) flows one way, then the other way, continually reversing direction. An AC voltage is continually changing between positive (+) and negative (-).

AC from a power supply This shape is called a sine wave.

The rate of changing direction is called the frequency of the AC and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second. This triangular signal is AC because it changes between positive (+) and negative (-).

Mains electricity in the UK has a frequency of 50Hz.See below for more details of signal properties. An AC supply is suitable for powering some devices such as lamps and heaters but almost all electronic circuits require a steady DC supply (see below).

Direct Current (DC) Direct Current (DC) always flows in the same direction, but it may increase and decrease. A DC voltage is always positive (or always negative), but it may increase and decrease. Electronic circuits normally require a steady DC supply which is constant at one value or asmooth DC supply which has a small variation called ripple.

Steady DC from a battery or regulated power supply, this is ideal for electronic circuits.

Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits. Power supplies contain a transformer which converts the mains AC supply to a safe low voltage AC. Then the AC is converted to DC by a bridge rectifier but the output is varying DC which is unsuitable for electronic circuits. Some power supplies include a capacitor to provide smooth DC which is suitable for lesssensitive electronic circuits, including most of the projects on this website.

Smooth DC from a smoothed power supply, this is suitable for some electronics.

Varying DC from a power supply without smoothing, this is not suitable for electronics.

Lamps, heaters and motors will work with any DC supply. Please see the Power Supplies page for further information. Power supplies are also covered by the Electronics in Meccano website.

Properties of electrical signals An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit.

The voltage-time graph on the right shows various properties of an electrical signal. In addition to the properties labelled on the graph, there is frequency which is the number of cycles per second. The diagram shows a sine wave but these properties apply to any signal with a constant shape.    



Amplitude is the maximum voltage reached by the signal. It is measured in volts, V. Peak voltage is another name for amplitude. Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace it is usual to measure peak-peak voltage. Time period is the time taken for the signal to complete one cycle. It is measured in seconds (s), but time periods tend to be short so milliseconds (ms) and microseconds (µs) are often used. 1ms = 0.001s and 1µs = 0.000001s. Frequency is the number of cycles per second. It is measured in hertz (Hz), but frequencies tend to be high so kilohertz (kHz) and megahertz (MHz) are often used. 1kHz = 1000Hz and 1MHz = 1000000Hz. frequency =



1 time period

and

time period =

Mains electricity in the UK has a frequency of 50Hz, so it has a time period of 1/50 = 0.02s = 20ms.

1 frequency

Root Mean Square (RMS) Values The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. Clearly for most of the time it is less than the peak voltage, so this is not a good measure of its real effect.

Instead we use the root mean square voltage (VRMS) which is 0.7 of the peak voltage (Vpeak): VRMS = 0.7 × Vpeak and Vpeak = 1.4 × VRMS These equations also apply to current. They are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 are different values for other shapes. The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.

For example a lamp connected to a 6V RMS AC supply will light with the same brightness when connected to a steady 6V DC supply. However, the lamp will be dimmer if connected to a 6V peak AC supply because the RMS value of this is only 4.2V (it is equivalent to a steady 4.2V DC).

You may find it helps to think of the RMS value as a sort of average, but please remember that it is NOT really the average! In fact the average voltage (or current) of an AC signal is zero because the positive and negative parts exactly cancel out!

What do AC meters show, is it the RMS or peak voltage? AC voltmeters and ammeters show the RMS value of the voltage or current. What does '6V AC' really mean, is it the RMS or peak voltage? If the peak value is meant it should be clearly stated, otherwise assume it is the RMS value. In everyday use AC voltages (and currents) are always given as RMS values because this allows a sensible comparison to be made with steady DC voltages (and currents), such as from a battery. For example a '6V AC supply' means 6V RMS, the peak voltage is 8.6V. The UK mains supply is 230V AC, this means 230V RMS so the peak voltage of the mains is about 320V! So what does root mean square (RMS) really mean? First square all the values, then find the average (mean) of these square values over a complete cycle, and find the square root of this average. That is the RMS value. Confused? Ignore the maths (it looks more complicated than it really is), just accept that RMS values for voltage and current are a much more useful quantity than peak values.

Electricity and the Electron Electricity is an invisible force that can produce heat., light , motion , and many other physical effects. The force is an attraction or repulsion between electric charges. More specifically, electricity can be explained in terms of electric charge , current , voltage , and resistance. The corresponding electrical units are the coulomb for measuring charge , the ampere for current, the volt for potential difference, and the ohm for resistance. These characteristics can then be applied to the electric circuits. Charges of opposite polarity attract

If two small charged bodies of light weight are mounted so that they are free to move easily and are placed close to each other, one can be attracted to the other when the two charges have opposite polarity. In terms of electrons and protons, they tend to be attracted to each other by the force of attraction between opposite charges. Charges of the same polarity repel When the two bodies have an equal amount of charge with the same polarity, they repel each other.

Electricity is the flow of charge around a circuit carrying energy from the battery (or power supply) to components such as lamps and motors. Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again. The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit. With the switch open the circuit is broken - so electricity cannot flow and the lamp is off.

With the switch closed the circuit is complete - allowing electricity to flow and the lamp is on. The electricity is carrying energy from the battery to the lamp. We can see, hear or feel the effects of electricity flowing such as a lamp lighting, a bell ringing, or a motor turning - but we cannot see the electricity itself, so which way is it flowing? Which way does electricity flow? We say that electricity flows from the positive (+) terminal of a battery to the negative (-) terminal of the battery. We can imagine particles with positive electric charge flowing in this direction around the circuit, like the red dots in the diagram. This flow of electric charge is called conventional current. This direction of flow is used throughout electronics and it is the one you should remember and use to understand the operation of circuits.

Imaginary positive particles moving in the direction of the conventional current

However this is not the whole answer because the particles that move in fact have negative charge! And they flow in the opposite direction! Please read on...

The electron When electricity was discovered scientists tried many experiments to find out which way the electricity was flowing around circuits, but in those early days they found it was impossible to find the direction of flow. They knew there were two types of electric charge, positive (+) and negative (-), and they decided to say that electricity was a flow of positive charge from + to -. They knew this was a guess, but a decision had to be made! Everything known at that time could also be explained if electricity was negative charge flowing the other way, from - to +.

The electron was discovered in 1897 and it was found to have a negative charge. The guess made in the early days of electricity was wrong! Electricity in almost all conductors is really the flow of electrons (negative charge) from - to +. By the time the electron was discovered the idea of electricity flowing from + to - (conventional current) was firmly established. Luckily it is not a problem to think of electricity in this way because positive charge flowing forwards is equivalent to negative charge flowing backwards. To prevent confusion you should always use conventional current when trying to understand how circuits work, imagine positively charged particles flowing from + to -.

Electrical characteristics Quantity Current Charge Power Voltage Resistance Reactance Impendance Conductance

symbol I or i Q or q P V or v R X Z G

Admittance Susceptance Capacitance Inductance Frequency Period

Y B C L F or f T

Basic unit Ampere Coulomb Watt Volt Ohm Ohm Ohm Siemens Siemens Siemens Farad Henry Hertz Second

Multiples and submultiples of units value

prefix

1,000,000,000,000=10¹² 1,000,000,000=10⁹ 1,000,000=10⁶ 1,000=10³ 100=10² 10=10 0.1=10 ¹ 0.01=10 ² 0.001=10 ³ 0.000 001=10 ⁶ 0.000 000 001=10 ⁹ 0.000 000 000 001= 10 ¹²

Tera Giga Mega Kilo Hecto deka deci centi mili micro nano pico

symbol T G M K h da d c m u n p

Conversion of units How to convert basic units ti sub-units/ multiple units Rule#1: When converting smaller to a bigger unit, move the decimal point to the left. Example:  1,000 Ω = 1 kΩ  200 Ω = 0.2 kΩ Rule#2: When converting bigger to smaller unit, move the decimal point to the right. Example:  2.2kΩ= 2,200Ω  1mΩ = 1,000,000 

OHM’s Law This unit explains how the amount of current I in the cicuit depends on its resistance R and the applied voltage. Specifically, I=V/R, determined in 1828 by the experiments of George Simon Ohm. Ohm’s law also determine the amount of electrical power in the circuit. Formula:



V-- is the Voltage measured in volts I --is the Current measured in amperes R --is the resistance measured in Ohms



Volts = Amps times Ohms or



Amps = Volts divide by Ohms or



Ohms = Volts divide by Amps

Current= I=V/R Resistance= R=V/I Voltage= V=IR

Series Circuit When the components in a circuit are connected in successive order with an end of each joined to an end of the next,they form a series circuit. The result is only one path of electron flow.current I is the same in all series components. Total resistance RT=R₁ + R₂ + R₃ etc.. Total voltage

VT=V₁ + V₂ + V₃ etc..

Parallel Circuit When two or more components are connected across one voltage source ,they form a parallel circuit. Each parallel path is then a branch with its own individual current I. parallel circuits therefore, have one common voltage across all the branches but individual branch currents that can be different.these characteristics are opposite from series circuits. That have one common current but individual voltage or ops that can be different. Formulas: RT=

for two branches

RT =

RT =

long method

IT=I1 + I2+ I3 etc. PT=P1 + P2 + P3 An open in one branch results in no current through the branch, but the other branches can have their normal current .

Series and Parallel Connections Connecting Components There are two ways of connecting components: In series so that each component has the same current. The battery voltage is divided between the two lamps Each lamp will have half the battery voltage if the lamps are identical. In parallel so that each component has the same voltage. Both lamps have the full battery voltage across them. The battery current is divided between the two lamps.

Most circuits contain a mixture of series and parallel connections The terms series circuit and parallel circuit are sometimes used, but only the simplest of circuits are entirely one type or the other. It is better to refer to specific components and say they are connected in series orconnected in parallel. For example: the circuit on the right shows a resistor and LED connected in series (on the right) and two lamps connected in parallel (in the centre). The switch is connected in series with the two lamps.

Lamps in Series If several lamps are connected in series they will all be switched on and off together by a switch connected anywhere in the circuit. The supply voltage is divided equally between the lamps (assuming they are all identical). If one lamp blows all the lamps will go out because the circuit is broken.

Christmas Tree Lights The lamps on a Christmas tree are connected in series. Normally you would expect all the lamps to go out if one blew, but Christmas tree lamps are special! They are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one 'fuse' lamp which blows normally. If there are 20 lamps and the mains electricity voltage is 240V, each lamp must be suitable for a 12V supply because the 240V is divided equally between the 20 lamps: 240V ÷ 20 = 12V WARNING! The Christmas tree lamps may seem safe because they use only 12V but they are connected to the mains supply which can be lethal. Always unplug from the mains before changing lamps. The voltage across the holder of a missing lamp is the full 240V of the mains supply! (Yes, it really is!) Lamps in Parallel If several lamps are connected in parallel each one has the full supply voltage across it. The lamps may be switched on and off independently by connecting a switch in series with each lamp as shown in the circuit diagram. This arrangement is used to control the lamps in buildings. This type of circuit is often called a parallel circuit but you can see that it is not really so simple - the switches are in series with the lamps, and it is these switch and lamp pairs that are connected in parallel.

Switches in Series If several on-off switches are connected in series they must all be closed (on) to complete the circuit. The diagram shows a simple circuit with two switches connected in series to control a lamp. Switch S1 AND Switch S2 must be closed to light the lamp.

Switches in Parallel If several on-off switches are connected in parallel only one needs to be closed (on) to complete the circuit. The diagram shows a simple circuit with two switches connected in parallel to control a lamp. Switch S1 OR Switch S2 (or both of them) must be closed to light the lamp.

“Power Dissipation in Resistance” When current flows in a resistance, heat is produced because friction between the holing free electrons and the atoms obstructs the path of electron flow. the heat is the evidence that power is used in producing current. This is how a fuse opens, as heat resulting from excessive current melts the metal link in the fuse. There are 3 basic formulas for power but 9 combinations, as follows: P=V×I

I=P/V

V=P/I

P=I²×R

R=P/I²

I=√

P=V²/R

R=V²/P

V=√

Watts and horse power units: a further example of how electrical power corresponds to mechanical power is the fact that, 746w= 1hp = 650 ft. lb/s This relation can be remembered more easily as 1hp equals approximately ¾ kilowatt (kw). Ikw=1,000w

KILOWATTHOURS (KWH) = This is a unit commonly used for large amounts of electrical work or energy.The amount is calculated simply as the product of the power in kilowatts multiplied by the time in hours during which the power is used.As an example,if a light bulb uses 300w or 0.3kw for 4 hours (h),the amount of energy is 0.3 x 4, which equals 1.2 kwh.

Comparison of Series and Parallel Circuit Series Circuit:

Parallel circuit:

Current the same in all components

Voltage the same across all the branches

V across each series R is I×R

I in each branch R is V/R

VT=V₁ + V₂ + V₃ etc..

IT=I₁ + I₂ + I₃ etc..

RT=R₁ + R₂ + R₃ etc..

GT=G₁ + G₂ + G₃ etc..

RT must be more than the largest individual R

RT must be less than the smallest branch R

PT=P₁ + P₂ + P₃ etc..

PT=P₁ + P₂ + P₃

Applied voltage is divided into IR voltage drops

Main-line current is divided into branch currents

The largest IR drop is across the largest series R

The largest branch I is in the smallest parallel R

Open in one component causes Open in one branch does not entire circuit to be open prevent I in other branches

Multimeter A multimeter or tester is an indespensable tool for a technician, the interpretation of meter reading gives technician clues to troubleshooting. Parts of a multimeter Range selector knob Scale Pointer or needle Moving coil assembly Scale plate 0Ω adjuster knob

Test probe or test prod

Negative and positive input Output socket

Zero position adjuster

General Precaution on Using Multitester          

never leave the multitester on the edge of working table when not in use, always set the multitester on the highest range (1,000 vac) always place in the appropriate range position, the range selector before using the multimeter. Never ever leave the multimeter with test probe connected to each other specially when its range selector is on resistance scale. When measuring an ac or dc voltage with undetermined power rating, set it on the highest range either ac or dc depending on voltage source When measuring resistance and voltages, connect the two test probe across the load. When measuring current, connect the instrument inseries with the load. Take note the polarity of being measure Always check the battery of the V.O.M Handle it with care.

Choosing a multimeter The photographs below show modestly priced multimeters which are suitable for general electronics use, you should be able to buy meters like these for less than £15. A digital multimeter is the best choice for your first multimeter, even the cheapest will be suitable for testing simple projects. If you are buying an analogue multimeter make sure it has a high sensitivity of 20k /V or greater on DC voltage ranges, anything less is not suitable for electronics. The sensitivity is normally marked in a corner of the scale, ignore the lower AC value (sensitivity on AC ranges is less important), the higher DC value is the critical one. Beware of cheap analogue multimeters sold for electrical work on cars because their sensitivity is likely to be too low.

Digital multimeters All digital meters contain a battery to power the display so they use virtually no power from the circuit under test. This means that on their DC voltage ranges they have a very high resistance (usually called input impedance) of 1M or more, usually 10M , and they are very unlikely to affect the circuit under test. Typical ranges for digital multimeters like the one illustrated: (the values given are the maximum reading on each range)     

DC Voltage: 200mV, 2000mV, 20V, 200V, 600V. AC Voltage: 200V, 600V. Digital Multimeter DC Current: 200µA, 2000µA, 20mA, 200mA, 10A*. *The 10A range is usually unfused and connected via a special socket. AC Current: None. (You are unlikely to need to measure this). Resistance: 200 , 2000 , 20k , 200k , 2000k , Diode Test.

Digital meters have a special diode test setting because their resistance ranges cannot be used to test diodes and other semiconductors.

Analogue multimeters Analogue meters take a little power from the circuit under test to operate their pointer. They must have a high sensitivity of at least 20k /V or they may upset the circuit under test and give an incorrect reading. See the section below on sensitivity for more details. Batteries inside the meter provide power for the resistance ranges, they will last several years but you should avoid leaving the meter set to a resistance range in case the leads touch accidentally and run the battery flat.

Analogue Multimeter

Typical ranges for analogue multimeters like the one illustrated: (the voltage and current values given are the maximum reading on each range)     

DC Voltage: 0.5V, 2.5V, 10V, 50V, 250V, 1000V. AC Voltage: 10V, 50V, 250V, 1000V. DC Current: 50µA, 2.5mA, 25mA, 250mA. A high current range is often missing from this type of meter. AC Current: None. (You are unlikely to need to measure this). Resistance: 20 , 200 , 2k , 20k , 200k . These resistance values are in the middle of the scale for each range.

It is a good idea to leave an analogue multimeter set to a DC voltage range such as 10V when not in use. It is less likely to be damaged by careless use on this range, and there is a good chance that it will be the range you need to use next anyway!

Sensitivity of an analogue multimeter Multimeters must have a high sensitivity of at least 20k /V otherwise their resistance on DC voltage ranges may be too low to avoid upsetting the circuit under test and giving an incorrect reading. To obtain valid readings the meter resistance should be at least 10 times the circuit resistance (take this to be the highest resistor value near where the meter is connected). You can increase the meter resistance by selecting a higher voltage range, but this may give a reading which is too small to read accurately! On any DC voltage range: Analogue Meter Resistance = Sensitivity × Max. reading of range e.g. a meter with 20k /V sensitivity on its 10V range has a resistance of 20k /V × 10V = 200k . By contrast, digital multimeters have a constant resistance of at least 1M (often 10M ) on all their DC voltage ranges. This is more than enough for almost all circuits.

Measuring voltage and current with a multimeter 1. Select a range with a maximum greater than you expect the reading to be. 2. Connect the meter, making sure the leads are the correct way round. Digital meters can be safely connected in reverse, but an analogue meter may be damaged. 3. If the reading goes off the scale: immediately disconnect and select a higher range. Multimeters are easily damaged by careless use so please take these precautions:   

Always disconnect the multimeter before adjusting the range switch. Always check the setting of the range switch before you connect to a circuit. Never leave a multimeter set to a current range (except when actually taking a reading). The greatest risk of damage is on the current ranges because the meter has a low resistance.



Measuring voltage at a point When testing circuits you often need to find the voltages at various points, for example the voltage at pin 2 of a 555 timer IC. This can seem confusing where should you connect the second multimeter lead?  Connect the black (negative -) lead to 0V, normally the negative terminal of the battery or power supply.  Connect the red (positive +) lead to the point you where you need to measure the voltage. Measuring voltage at a point.  The black lead can be left permanently connected to 0V while you use the redlead as a probe to measure voltages at various points.



You may wish to fit a crocodile clip to the black lead of your multimeter to hold it in place while doing testing like this.

Voltage at a point really means the voltage difference between that point and 0V (zero volts) which is normally the negative terminal of the battery or power supply. Usually 0V will be labelled on the circuit diagram as a reminder. Reading analogue scales Check the setting of the range switch and choose an appropriate scale. For some ranges you may need to multiply or divide by 10 or 100 as shown in the sample readings below. For AC voltage ranges use the red markings because the calibration of the scale is slightly different.

Analogue Multimeter Scales These can appear daunting at first but remember Sample readings on the that you only need to read one scale at a time! scales shown: The top scale is used when measuring resistance. DC 10V range: 4.4V (read 010 scale directly) DC 50V range: 22V (read 0-50 scale directly) DC 25mA range: 11mA (read 0-250 and divide by 10) AC 10V range: 4.45V (use the red scale, reading 0-10) If you are not familiar with reading analogue scales generally you may wish to see theanalogue display section on the general meters page. Measuring resistance with a multimeter To measure the resistance of a component it must not be connected in a circuit. If you try to measure resistance of components in a circuit you will obtain false readings (even if the supply is disconnected) and you may damage the multimeter. The techniques used for each type of meter are very different so they are treated separately:

Measuring resistance with a DIGITAL multimeter 1. Set the meter to a resistance range greater than you expect the resistance to be. Notice that the meter display shows "off the scale" (usually blank except for a 1 on the left). Don't worry, this is not a fault, it is correct - the resistance of air is very high! 2. Touch the meter probes together and check that the meter reads zero. If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try again. 3. Put the probes across the component. Avoid touching more than one contact at a time or your resistance will upset the reading! Measuring resistance with an ANALOGUE multimeter The resistance scale on an analogue meter is normally at the top, it is an unusual scale because it reads backwards and is not linear (evenly spaced). This is unfortunate, but it is due to the way the meter works. 1. Set the meter to a suitable resistance range. Choose a range so that the resistance you expect will be near the middle of the scale. For example: with the scale shown below and an expected resistance of about 50k choose the × 1k range. 2. Hold the meter probes together and adjust the control on the front of the meter which is usually labelled "0 ADJ" until the pointer reads zero (on the RIGHT remember!). If you can't adjust it to read zero, the battery inside the meter needs replacing. 3. Put the probes across the component. Avoid touching more than one contact at a time or your resistance will upset the reading!

Reading analogue resistance scales For resistance use the upper scale, noting that it reads backwards and is not linear (evenly spaced). Check the setting of the range switch so that you know by how much to multiply the reading. Sample readings on the scales shown: × 10 range: 260 × 1k range: 26k

Analogue Multimeter Scales The resistance scale is at the top, note that it reads backwards and is not linear (evenly spaced).

If you are not familiar withreading analogue scales generally you may wish to see theanalogue display section on the general meters page. Testing a diode with a multimeter The techniques used for each type of meter are very different so they are treated separately: Testing a diode with a DIGITAL multimeter  



Digital multimeters have a special setting for testing a diode, usually labelled with the diode symbol. Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V). Reverse the connections. The diode should NOT conduct this way so the meter will display "off the scale" (usually blank except for a 1 on the left).

Diodes a = anode k = cathode

Testing a diode with an ANALOGUE multimeter  





Set the analogue multimeter to a low value resistance range such as × 10. It is essential to note that the polarity of analogue multimeter leads is reversed on the resistance ranges, so the black lead is positive (+) and the red lead is negative (-)! This is unfortunate, but it is due to the way the meter works. Connect the black (+) lead to anode and the red (-) to the cathode. The diode should conduct and the meter will display a low resistance (the exact value is not relevant). Reverse the connections. The diode should NOT conduct this way so the meter will show infinite resistance (on the left of the scale).

For further information please see the diodes page. You may find it easier to test a diode with the simple tester project. Testing a transistor with a multimeter Set a digital multimeter to diode test and an analogue multimeter to a low resistance range such as × 10, as described above for testing a diode. Test each pair of leads both ways (six tests in total): 





The base-emitter (BE) junction should behave like a diode and conduct one way only. The base-collector (BC) junction should Testing an NPN transistor behave like a diode and conduct one way only. The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.

The ohmmeter scale is used to measure resistance, the topmost scale of a multimeter is for ohmmeter reading. Zero is located on the right side of the meter: the infinity∞ is on the left side.

Ohm/division ×1 0-2Ω=0.2Ω 2-10Ω=0.5 10-20Ω=1Ω 20-50Ω=2Ω

50-100Ω=5Ω 100-200Ω=20Ω 200-500Ω=100Ω 1k=1,000Ω= 2k=2,000Ω

The voltmeter is used when it is necessary to determine the presence or absence of an electrical pressure or voltage on a unit undertest. Our tester has two voltmeter functions, one is DC and the other is AC . although this functions are separated from each other both AC and DC are indicated in the same scales of the meter. Volts/division ACV: 10v=0.2v 50v=1v 250v=5v 1,000v=20v

DCV: 10v=0.2v 50v=1v 250v=5v 1,000v=20v 0.1v=0.002v/2mv 2.5v=0.05v/50mv

Connecting meters It is important to connect meters the correct way round:  

The positive terminal of the meter, marked + or coloured red should be connected nearest to + on the battery or power supply. The negative terminal of the meter, marked - or coloured black should be connected nearest to - on the battery or power supply.

Voltmeters

   

Voltmeters measure voltage. Voltage is measured in volts, V. Voltmeters are connected in parallel across components. Voltmeters have a very high resistance.

Connecting a voltmeter in parallel

Measuring voltage at a point When testing circuits you often need to find the voltages at various points, for example the voltage at pin 2 of a 555 timer IC. This can seem confusing - where should you connect the second voltmeter lead? 

  

Connect the black (negative -) voltmeter lead to 0V, normally the negative terminal of the battery or power supply. Connect the red (positive +) voltmeter lead to the point you where you need to measure the voltage. The black lead can be left permanently connected to 0V while you use the redlead as a probe to measure voltages at various points. You may wish to use a crocodile clip on the black lead to hold it in place.

Voltage at a point really means the voltage difference between that point and 0V (zero volts) which is normally the negative terminal of the battery or power supply. Usually 0V will be labelled on the circuit diagram as a reminder. Analogue meters take a little power from the circuit under test to operate their pointer. This may upset the circuit and give an incorrect reading. To avoid this voltmeters should have a resistance of at least 10 times the circuit resistance (take this to be the highest resistor value near where the meter is connected). Most analogue voltmeters used in school science are not suitable for electronics because their resistance is too low, typically a few k . 100k or more is required for most electronics circuits. Ammeters

 





Ammeters measure current. Current is measured in amps (amperes), A. 1A is quite large, so mA (milliamps) and µA (microamps) are often used. 1000mA = 1A, 1000µA = 1mA, 1000000µA = 1A. Ammeters are connected in series. To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram. Connecting an ammeter in series Ammeters have a very low resistance.

The need to break the circuit to connect in series means that ammeters are difficult to use on soldered circuits. Most testing in electronics is done with voltmeters which can be easily connected without disturbing circuits. Galvanometers Galvanometers are very sensitive meters which are used to measure tiny currents, usually 1mA or less. They are used to make all types of analogue meters by adding suitable resistors as shown in the diagrams below. The photograph shows an educational 100µA galvanometer for which various multipliers and shunts are available.

Making an Making a Voltmeter Ammeter A galvanometer A galvanometer with a high with a low resistance multiplie resistance shunt i r in series to make n parallel to make a voltmeter. an ammeter.

Galvanometer with multiplier and shunt Maximum meter current 100µA (or 20µA reverse). This meter is unusual in allowing small reverse readings to be shown.

Ohmmeters An ohmmeter is used to measure resistance in ohms ( ). Ohmmeters are rarely found as separate meters but all standard multimeters have an ohmmeter setting. 1 is quite small so k and M are often used. 1k

= 1000 , 1M

= 1000k

= 1000000 .

Oscilloscopes (CROs)

An oscilloscope is a test instrument which allows you to look at the 'shape' of electrical signals by displaying a graph of voltage against time on its screen. It is like a voltmeter with the valuable extra function of showing how the voltage varies with time. A graticule with a 1cm grid enables you to take measurements of voltage and time from the screen.

Circuit symbol for an oscilloscope

The graph, usually called the trace, is drawn by a beam of electrons striking the phosphor coating of the screen making it emit light, usually green or blue. This is similar to the way a television picture is produced. Cathode Ray Oscilloscope (CRO) Oscilloscopes contain a vacuum tube with a cathode (negative electrode) at one end to emit electrons and an anode (positive electrode) to accelerate them so they move rapidly down the tube to the screen. This arrangement is called an electron gun. The tube also contains electrodes to deflect the electron beam up/down and left/right.

The electrons are called cathode rays because they are emitted by the cathode and this gives the oscilloscope its full name of cathode ray oscilloscope or CRO.

A dual trace oscilloscope can display two traces on the screen, allowing you to easily compare the input and output of an amplifier for example. It is well worth paying the modest extra cost to have this facility.

Precautions  

An oscilloscope should be handled gently to protect its fragile (and expensive) vacuum tube. Oscilloscopes use high voltages to create the electron beam and these remain for some time after switching off - for your own safety do not attempt to examine the inside of an oscilloscope!

Setting up an oscilloscope Oscilloscopes are complex instruments with many controls and they require some care to set up and use successfully. It is quite easy to 'lose' the trace off the screen if controls are set wrongly! There is some variation in the arrangement and labelling of the many controls so the following instuctions may need to be adapted for your instrument. 1. Switch on the oscilloscope to warm up (it takes a minute or two). 2. Do not connect the input lead at this stage. 3. Set the AC/GND/DC switch (by the Y INPUT) to DC. 4. Set the SWP/X-Y switch to SWP (sweep). This is what you should see 5. Set Trigger Level to AUTO. after setting up, when there 6. Set Trigger Source to INT (internal, is no input signal connected the y input). 7. Set the Y AMPLIFIER to 5V/cm (a moderate value). 8. Set the TIMEBASE to 10ms/cm (a moderate speed). 9. Turn the timebase VARIABLE control to 1 or CAL. 10. Adjust Y SHIFT (up/down) and X SHIFT (left/right) to give a trace across the middle of the screen, like the picture. 11. Adjust INTENSITY (brightness) and FOCUS to give a bright, sharp trace. 12. The oscilloscope is now ready to use! Connecting the input lead is described in the next section.

Connecting an oscilloscope The Y INPUT lead to an oscilloscope should be a co-axial lead and the diagram shows its construction. The central wire carries the signal and the screen is connected to earth (0V) to shield the signal from electrical interference (usually called noise).

Construction of a co-axial lead

Most oscilloscopes have a BNC socket for the y input and the lead is connected with a push and twist action, to disconnect you need to twist and pull. Oscilloscopes used in schools may have red and black 4mm sockets so that ordinary, unscreened, 4mm plug leads can be used if necessary. Professionals use a specially designed lead and probes kit for best results with high frequency signals and when testing high resistance circuits, but this is not essential for Oscilloscope lead and probes kit simpler work at audio frequencies (up to 20kHz). An oscilloscope is connected like a voltmeter but you must be aware that the screen (black) connection of the input lead is connected to mains earth at the oscilloscope! This means it must be connected to earth or 0V on the circuit being tested. Obtaining a clear and stable trace Once you have connected the oscilloscope to the circuit you wish to test you will need to adjust the controls to obtain a clear and stable trace on the screen: 



The Y AMPLIFIER (VOLTS/CM) control determines the height of the trace. Choose a setting so the trace occupies at least half the screen height, but does not disappear off the screen. The trace of an AC signal The TIMEBASE (TIME/CM) control determines with the oscilloscope the rate at which the dot sweeps across the controls correctly set screen. Choose a setting so the trace shows at least one cycle of the signal across the screen.



Note that a steady DC input signal gives a horizontal line trace for which the timebase setting is not critical. The TRIGGER control is usually best left set to AUTO.

If you are using an oscilloscope for the first time it is best to start with an easy signal such as the output from an AC power pack set to about 4V. Measuring voltage and time period The trace on an oscilloscope screen is a graph of voltage against time. The shape of this graph is determined by the nature of the input signal. In addition to the properties labelled on the graph, there is frequency which is the number of cycles per second. The diagram shows a sine wave but these properties apply to any signal with a constant shape.    



Amplitude is the maximum voltage reached by the signal. It is measured in volts, V. Peak voltage is another name for amplitude. Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace it is usual to measure peak-peak voltage. Time period is the time taken for the signal to complete one cycle. It is measured in seconds (s), but time periods tend to be short so milliseconds (ms) and microseconds (µs) are often used. 1ms = 0.001s and 1µs = 0.000001s. Frequency is the number of cycles per second. It is measured in hertz (Hz), but frequencies tend to be high so kilohertz (kHz) and megahertz (MHz) are often used. 1kHz = 1000Hz and 1MHz = 1000000Hz. frequency =

1 time period

and

time period =

1 frequency

Voltage Voltage is shown on the vertical y-axis and the scale is determined by the Y AMPLIFIER (VOLTS/CM) control. Usually peak-peak voltage is measured because it can be read correctly even if the position of 0V is not known. The amplitude is half the peakpeak voltage. The trace of an AC signal If you wish to read the amplitude voltage directly you must check the position of 0V (normally halfway up Y AMPLIFIER: 2V/cm the screen): move the AC/GND/DC switch to GND TIMEBASE: 5ms/cm (0V) and use Y-SHIFT (up/down) to adjust the position of the trace if necessary, switch back to DC Example measurements: afterwards so you can see the signal again. peak-peak voltage = 8.4V Voltage = distance in cm × volts/cm amplitude voltage = 4.2V Example: peak-peak voltage = 4.2cm × 2V/cm = 8.4V amplitude (peak voltage) = ½ × peak-peak voltage = time period = 20ms 4.2V frequency = 50Hz Time period Time is shown on the horizontal x-axis and the scale is determined by the TIMEBASE (TIME/CM) control. Thetime period (often just called period) is the time for one cycle of the signal. The frequency is the number of cyles per second, frequency = 1/time period Ensure that the variable timebase control is set to 1 or CAL (calibrated) before attempting to take a time reading. Time = distance in cm × time/cm Example: time period = 4.0cm × 5ms/cm = 20ms and frequency = 1/time period = 1/20ms = 50Hz

Slow timebase, no input You can see the dot moving

Timebase (time/cm) and trigger controls The oscilloscope sweeps the electron beam across the screen from left to right at a steady speed set by the TIMEBASE control. Each setting is labelled with the time the dot takes to move 1cm, effectively it is setting the scale on the x-axis. The timebase control may be labelled TIME/CM. At slow timebase settings (such as 50ms/cm) you can see a dot moving across the screen but at faster settings (such as 1ms/cm) the dot is moving so fast that it appears to be a line.

Fast timebase, no input The dot is too fast to see so it appears to be a line

The VARIABLE timebase control can be turned to make a fine adjustment to the speed, but it must be left at the position labelled 1 or CAL (calibrated) if you wish to take time readings from the trace drawn on the screen. The TRIGGER controls are used to maintain a steady trace on the screen. If they are set wrongly you may see a trace drifting sideways, a confusing 'scribble' on the screen, or no trace at all! The trigger maintains a steady trace by starting the dot sweeping across the screen when the input signal reaches the same point in its cycle each time. For straightforward use it is best to leave the trigger level set to AUTO, but if you have difficulty obtaining a steady trace try adjusting this control to set the level manually. Y amplifier (volts/cm) control The oscilloscope moves the trace up and down in proportion to the voltage at the Y INPUT and the setting of the Y AMPLIFIER control. This control sets the voltage represented by each centimetre (cm) on the the screen, effectively it is setting the scale on the y-axis. Positive voltages make the trace move up, negative voltages make it move down. The y amplifier control may be labelled Y-GAIN or VOLTS/CM.

Varying DC (always positive)

The input voltage moving the dot up and down at the same time as the dot is swept across the screen means that the trace on the screen is a graph of voltage (y-axis) against time (x-axis) for the input signal.

The AC/GND/DC switch The normal setting for this switch is DC for all signals, including AC! Switching to GND (ground) connects the y input to 0V and allows you to quickly check the position of 0V on the screen (normally halfway up). There is no need to disconnect the input lead while you do this because it is disconnected internally. Switching to GND allows you to quickly check the position Switching to AC inserts a capacitor in series with of 0V (normally halfway up). the input to block out any DC signal present and pass only ACsignals. This is used to examine signals showing a small variation around one constant value, such as the ripple on the output of a smooth DC supply. Reducing the VOLTS/CM to see more detail of the ripple would normally take the trace off the screen! The AC setting removes the constant (DC) part of the signal, allowing you to view just the varying (AC) part which can now be examined more closely by reducing the VOLTS/CM. This is shown in the diagrams below: Displaying a ripple signal using the AC switch

Switch in normal DC Switch moved to AC VOLTS/CM reduced to position. position. enlarge the ripple. The ripple is difficult to see, The constant (DC) part of The ripple can now be but if VOLTS/CM is reduced the examined more closely. to enlarge it the trace will signal is removed, leaving disappear off the screen! just the ripple (AC) part.

Transistor The transistor, invented by three scientists at the Bell Laboratories in 1947, rapidly replaced the vacuum tube as an electronic signal regulator. A transistor regulates current or voltageflow and acts as a switch or gate for electronic signals. A transistor consists of three layers of a semiconductor material, each capable of carrying a current. A semiconductor is a material such as germanium and silicon that conducts electricity in a "semi-enthusiastic" way. It's somewhere between a real conductor such as copper and an insulator (like the plastic wrapped around wires). The semiconductor material is given special properties by a chemical process called doping. The doping results in a material that either adds extra electrons to the material (which is then called N-type for the extra negative charge carriers) or creates "holes" in the material's crystal structure (which is then called P-type because it results in more positive charge carriers). The transistor's three-layer structure contains an N-type semiconductor layer sandwiched between P-type layers (a PNP configuration) or a P-type layer between N-type layers (an NPN configuration). A small change in the current or voltage at the inner semiconductor layer (which acts as the control electrode) produces a large, rapid change in the current passing through the entire component. The component can thus act as a switch, opening and closing an electronic gate many times per second. Today's computers use circuitry made with complementary metal oxide semiconductor (CMOS) technology. CMOS uses two complementary transistors per gate (one with N-type material; the other with P-type material). When one transistor is maintaining a logic state, it requires almost no power. Transistors are the basic elements in integrated circuits (ICs), which consist of very large numbers of transistors interconnected with circuitry and baked into a single siliconmicrochip or "chip."

Transistor Function Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier(always partly on). The amount of current amplification is called the current gain, symbol hFE. For further information please see the Transistor Circuits page. Types of transistor There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.

Transistor circuit symbols

The leads are labelled base (B), collector (C) and emitter (E). These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels! A Darlington pair is two transistors connected together to give a very high current gain. In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Connecting Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on. If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue to identify the leads. The drawings on the right show Transistor leads for some common case styles. the leads for some of the most common case styles. Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above. Please see below for a table showing the case styles of some common transistors.

Soldering Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink.

Crocodile clip.

Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation.

Heat sinks Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.

Heat sink

Testing a transistor Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it: 1. Testing with a multimeter Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range. Test each pair of leads both ways (six tests in total): 

 

The base-emitter (BE) junction should Testing an NPN transistor behave like a diode and conduct one way only. The base-collector (BC) junction should behave like a diode and conduct one way only. The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.

2. Testing in a simple switching circuit Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboardfor example. Take care to include the 10k resistor in the base connection or you will destroy the transistor as you test it! If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released. To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.

A simple switching circuit to test an NPN transistor

Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC current gain hFE.

Transistor codes There are three main series of transistor codes used in the UK: 





Codes beginning with B (or A), for example BC108, BC478 The first letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable. Codes beginning with TIP, for example TIP31A TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings. Codes beginning with 2N, for example 2N3053 The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.

Choosing a transistor Most projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current IC and the current gain hFE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating. To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced. The quantities shown in the table are explained below NPN transistors Code

Structure

Case style

IC VCE hFE max. max. min.

Ptot max.

Category Possible (typical substitutes use) Audio, low power

BC182 BC547

BC107

NPN

TO18 100mA 45V 110 300mW

BC108

NPN

General TO18 100mA 20V 110 300mW purpose, low power

NPN

General TO18 100mA 20V 420 600mW purpose, low power

NPN

Audio (low TO18 200mA 20V 200 300mW noise), low power

BC184 BC549

BC182

NPN

General TO92C 100mA 50V 100 350mW purpose, low power

BC107 BC182L

BC182L

NPN

General TO92A 100mA 50V 100 350mW purpose, low power

BC107 BC182

BC547B

NPN

TO92C 100mA 45V 200 500mW

BC548B

NPN

TO92C 100mA 30V 220 500mW General

BC108C

BC109

Audio, low power

BC108C BC183 BC548

BC107B BC108B

purpose, low power BC549B

2N3053

BFY51

BC639

TIP29A

TIP31A

TIP31C

TIP41A

2N3055

NPN

Audio (low TO92C 100mA 30V 240 625mW noise), low power

BC109

NPN

TO39 700mA 40V

General 50 500mW purpose, low power

BFY51

TO39

30V

General purpose, 40 800mW medium power

BC639

80V

General purpose, 40 800mW medium power

BFY51

60V

30W

General purpose, high power

40W

General purpose, high power

TIP31C TIP41A

40W

General purpose, high power

TIP31A TIP41A

65W

General purpose, high power

117W

General purpose, high power

NPN

NPN

NPN

NPN

NPN

NPN

NPN

TO92A

TO220

TO220

TO220

TO220

TO3

1A

1A

1A

3A

3A

6A

15A

60V

40

10

100V 10

60V

60V

15

20

Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data. PNP transistors

Code

Case Structure style

IC VCE hFE max. max. min.

Ptot max.

Category Possible (typical substitutes use)

BC177

PNP

TO18 100mA 45V 125 300mW

Audio, low power

BC477

BC178

PNP

General TO18 200mA 25V 120 600mW purpose, low power

BC478

BC179

PNP

Audio (low TO18 200mA 20V 180 600mW noise), low power

BC477

PNP

TO18 150mA 80V 125 360mW

Audio, low power

BC177

PNP

General TO18 150mA 40V 125 360mW purpose, low power

BC178

BC478

TIP32A

TIP32C

PNP

PNP

TO220

TO220

3A

3A

60V

25

100V 10

40W

General purpose, high power

TIP32C

40W

General purpose, high power

TIP32A

Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data. Structure

This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type.

Case style

There is a diagram showing the leads for some of the most common case styles in the Connecting section above. This information is also available in suppliers' catalogues.

IC max.

Maximum collector current.

VCE max.

Maximum voltage across the collector-emitter junction. You can ignore this rating in low voltage circuits.

hFE

This is the current gain (strictly the DC current gain).

The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units. The gain is often quoted at a particular collector current IC which is usually in the middle of the transistor's range, for example '[email protected]' means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts. Why hFE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here. Ptot max.

Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly IC × VCE. For transistors operating as switches the maximum collector current (IC max.) is more important.

Category

This shows the typical use for the transistor, it is a good starting point when looking for a substitute. Catalogues may have separate tables for different categories.

Possible substitutes These are transistors with similar electrical properties which will be suitable substitutes in most circuits. However, they may have a different case style so you will need to take care when placing them on the circuit board. Darlington pair This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. You can make up your own Darlington pair from two transistors. For example:

 

For TR1 use BC548B with hFE1 = 220. For TR2 use BC639 with hFE2 = 40.

The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800. The pair's maximum collector current IC(max) is the same as TR2.

MOSFET

Two power MOSFETs in the surface-mount package D2PAK. Operating as switches, each of these components can sustain a blocking voltage of 120 volts in theOFF state, and can conduct a continuous current of 30 amperes in the ON state, dissipating up to about 100 watts and controlling a load of over 2000 watts. Amatchstick is pictured for scale.

A cross section through an nMOSFET when the gate voltage VGS is below the threshold for making a conductive channel; there is little or no conduction between the terminals source and drain; the switch is off. When the gate is more positive, it attracts electrons, inducing an n-type conductive channel in

the substrate below the oxide, which allows electrons to flow between thendoped terminals; the switch is on.

Simulation result for formation of inversion channel (electron density) and attainment of threshold voltage (IV) in a nanowire MOSFET. Note that the threshold voltage for this device lies around 0.45V. The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a transistor used for amplifying or switching electronic signals. The basic principle of this kind oftransistor was first proposed by Julius Edgar Lilienfeld in 1925. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common. The 'metal' in the name is now often a misnomer because the previously metal gate material is now often a layer of polysilicon (polycrystalline silicon). Aluminium had been the gate material until the mid 1970s, when polysilicon became dominant, due to its capability to form self-aligned gates. Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates. IGFET is a related term meaning insulated-gate field-effect transistor, and is used almost synonymously with MOSFET, being more accurate since many "MOSFETs" use a gate that is not metal and a gate insulator that is not oxide. Another synonym is MISFET for metal–insulator–semiconductor FET.

P-channel

N-channel

JFET

MOSFET enh

MOSFET enh (no bulk)

MOSFET de

Transducers A transducer is a device which converts a signal from one form to another.Most electronics circuits use both input and output transducers:

Input Transducers Input Transducers convert a quantity to an electrical signal (voltage) or to resistance (which can be converted to voltage). Input transducers are also called sensors. Examples:    

LDR converts brightness (of light) to resistance. Thermistor converts temperature to resistance. Microphone converts sound to voltage. Variable resistor converts position (angle) to resistance. LDR

Output Transducers Output Transducers convert an electrical signal to another quantity. Examples:     

Lamp converts electricity to light. LED converts electricity to light. Loudspeaker converts electricity to sound. Motor converts electricity to motion. Heater converts electricity to heat.

Loudspeaker

Using input transducers (sensors) Most input transducers (sensors) vary their resistance and this can be used directly in some circuits but it is usually converted to an electrical signal in the form of a voltage. The voltage signal can be fed to other parts of the circuit, such as the input to an IC or a transistor switch. The conversion of varying resistance to varying voltage is performed by a simple circuit called a voltage divider.

Voltage divider circuit

Voltage Dividers They are also called Potential Dividers Voltage divider (potential divider) A voltage divider consists of two resistances R1 and R2 connected in series across a supply voltage Vs. The supply voltage is divided up between the two resistances to give an output voltage Vo which is the voltage across R2. This depends on the size of R2 relative to R1: 





If R2 is much smaller than R1, Vo is small (low, almost 0V) (because most of the voltage is across R1) If R2 is about the same as R1, Vo is about half Vs (because the voltage is shared about equally between R1 and R2) If R2 is much larger than R1, Vo is large (high, almost Vs) (because most of the voltage is across R2)

Vo =

Vs × R2 R1 + R2

If you need a precise value for the output voltage Vo you can use Ohm's law and a little algebra to work out the formula for Vo shown on the right. The formula and the approximate rules given above assume that negligible current flows from the output. This is true if Vo is connected to a device with a high resistance such as voltmeter or an IC input. For further information please see the page on impedance. If the output is connected to a transistor Vo cannot become much greater than 0.7V because the transistor's base-emitter junction behaves like a diode.

Voltage dividers are also called potential dividers, a name which comes from potential difference (the proper name for voltage). One of the main uses of voltage dividers is to connect input transducers into circuits...

Using an input transducer (sensor) in a voltage divider Most input transducers (sensors) vary their resistance and usually a voltage divider is used to convert this to a varying voltage which is more useful. The voltage signal can be fed to other parts of the circuit, such as the input to an IC or a transistor switch. The sensor is one of the resistances in the voltage divider. It can be at the top (R1) or at the bottom (R2), the choice is determined by when you want a large value for the output voltage Vo:  

Put the sensor at the top (R1) if you want a large Vo when the sensor has a small resistance. Put the sensor at the bottom (R2) if you want a large Vo when the sensor has a large resistance.

Then you need to choose a value for the resistor... Choosing a resistor value The value of the resistor R will determine the range of the output voltage Vo. For best results you need a large 'swing' (range) for Vo and this is achieved if the resistor is much larger than the sensor's minimum resistance Rmin, but much smaller than the sensor's maximum resistance Rmax. You can use a multimeter to help you find the minimum and maximum values of the sensor's resistance (Rmin and Rmax). There is no need to be precise, approximate values will do. Then choose resistor value: R = square root of (Rmin × Rmax) Choose a standard value which is close to this calculated value. For example: An LDR: Rmin = 100 , Rmax = 1M , so R = square root of (100 × 1M) = 10k .

OR

swapping over the resistor and sensor The resistor and sensor can be swapped over to invert the action of the voltage divider. For example an LDR has a high resistance when dark and a low resistance when brightly lit, so:  

If the LDR is at the top (near +Vs), Vo will be low in the dark and high in bright light. If the LDR is at the bottom (near 0V), Vo will be high in the dark and low in bright light.

Using a variable resistor A variable resistor may be used in place of the fixed resistor R. It will enable you to adjust the output voltage Vo for a given resistance of the sensor. For example you can use a variable resistor to set the exact brightness level which makes an IC change state. The variable resistor value should be larger than the fixed resistor value. For finer control you can use a fixed resistor in series with the variable resistor. For example if a 10k fixed resistor is suitable you could replace it with a fixed 4.7k resistor in series with a 10k variable resistor, allowing you to adjust the resistance from 4.7k to 14.7k . The sensor and variable If you are planning to use a variable resistor resistor can be swapped connected between the +Vs supply and the base over if necessary of a transistor you must include a resistor in series with the variable resistor. This is to prevent excessive base current destroying the transistor when the variable resistor is reduced to zero. For further information please see the page onTransistor Circuits.

POWER TRANSFORMERS LAMINATED CORE

Laminated Core Transformer This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics 

Widely available in power ratings ranging from mW to MW



Insulated lamination minimizes eddy current losses



Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings



Rectangular core



Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used



Mu-metal shields can be fitted to reduce EMI (electromagnetic interference)



A screen winding is occasionally used between the 2 power windings



Small appliance and electronics transformers may have a thermal cut out built in



Occasionally seen in low profile format for use in restricted spaces



Laminated core made with silicon steel with high permeability

Toroidal

Toroidal Transformer Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring (and thus threaded through the ring during winding), and tape for insulation. Toroidal transformers compared to EI core transformers: 

Lower external magnetic field



Smaller for a given power rating



Higher cost in most cases, as winding requires more complex and slower equipment



Less robust



Central fixing is either 

bolt, large metal washers and rubber pads



bolt and potting resin



Over-tightening the central fixing bolt may short the windings



Greater inrush current at switch-on

Autotransformer An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 V across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 V supply to drive 115 V equipment, or reversed to drive 230 V equipment from 115 V. Since the current in the windings is lower, the transformer is smaller, lighter cheaper and more efficient. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than an isolating (two-winding) transformer of the same rating. Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33 kV and 66 kV sub-transmission networks.

Coils A coil is nothing more than copper wire wound in a spiral. This symbol is used to indicate a coil in a circuit diagram. Inductance value is designated in units called the Henry(H). The more wire the coil contains, the stronger its characteristics become. The inductance value can become quite large. If a coil is wound around an iron rod, or ferrite core (strengthened with iron powder), the inductance of the coil will be greatly increased. Coils used in typical electric circuits varely widely in values, ranging from a few micro-henry (µH) to many henry (H). Coils are sometimes called "inductors." Inductance is the measure of the strength of a coil. Capacitors have capacitance, resistors have resistance, and Inductors (coils) have inductance. When alternating current flows through a coil, the magnetic flux that occurs in the coil changes with the current. When a second coil is put close to the first

coil (with the changing flux), alternating voltage is caused to flow in the second coil by an effect known as "mutual induction." Mutual inductance (inductance) is measured in units of the Henry. The changing magnetic flux in a coil affects itself as well as other coils. This is called self induction, the degree of this self induction is called Self Inductance. Self inductance is a measure of a coil's ability to establish an induced voltage as a result of a change in its current. Self inductance is commonly referred to as simply "inductance," and is symbolized by "L". The unit of inductance is the Henry (H). The definition of "Henry" is "When a current of 1 ampere flows through a given coil in 1 second such that 1 volt is induced to flow in a second coil, the mutual inductance between the coils is said to be 1 Henry." The definition of self inductance is the same, except that the 1 volt is induced in the first coil; there is no second coil. Characteristic of coils When wire is coiled, it takes on various characteristics that are different from straight wire. Below I will explain some of the characteristics coils that I know. Current Stabilization Characteristic When current begins to flow in the coil, the coil resists the flow. When current decreases, the coil makes current continue to flow (briefly) at the previous rate. This is called "Lenz's law". 'The direction of induced current in a coil is such that is opposes the change in the magnetic field that procduced it.'

This characteristic is used for the ripple filter circuit of a power supply where it transforms alternating current(AC) to direct current(DC). When a rectifier is used to make DC from AC, the output of the rectifier without a ripple filter circuit is ripple current. Ripple current is DC that has a large AC component.

A ripple filter circuit often combines coil and capacitors. The coil resists the change of current and capacitors supplement the flow of current by discharging into the circuit if the input voltage drops. Thus, clear, ripple-free DC is obtained from the ripple filter circuit. Resistor is used instead of coil in simple ripple filter circuit.

Mutual induction As I wrote above, electric power can be transfered between two coils by mutual induction. The transformer utilizes this characteristic. The input coil that gives the electric power is called the primary side, while the output coil that takes out the electric power is called the secondary side. The output voltage is determined by the ratio of turns of wire between the primary coil and secondary coil. Some transformers have a tap (or several) on the secondary coil to provide multiple voltage levels. Electromagnet When current flows through a conductor, a magnetic field is created. This field is much stronger in a coil. An electromagnet is just like a regular magnet. It attracts iron, nickel, and some other metals. Relays utilize this characteristic. When the current flows to the coil of relay, the magnetic field attracts a steel plate, and the switch that is attached to the steel plate goes ON. And the doorbell chime also utilizes electromagnets.

Resonance When a coil and a capacitor are combined, the resulting circuit has special characteristics. The impedance (resistance to current flow) of the circuit changes with the frequency of the voltage. Current will flow easily at a given frequency, but has difficulty flowing at another frequency. The tunning circut that select a particular radio station utilizes this characteristic. Explaning resonance in more detail is very difficult. If you want to know more detail, please read further in a book about electronics.

High Frequency Coils The photograph shows an example of a small coil component. The component on the left is wound with thin copper wire to a small barbellshaped ferrite core, and has a value of 100µH.

It is used for high frequency resonance, or for detering of high frequency. As for size, the diameter is about 4 mm, the height about 7 mm. The value of the small coil like this is indicated with a color code, just like a resistor. The strengh of this type of coil varies from 1µH to several hundred µH. 1µH, 2.2µH, 3.3µH, 3.9µH, 4.7µH, 5.6µH, 6.8µH, 8.2µH, 10µH, 15µH, 18µH, 22µH, 27µH, 33µH, 39µH, 46µH, 56µH, 68µH, 82µH, 100µH other. The second coil from the left has thin copper wire wound around a stick-shaped ferrite core. It is used the same as the component above. The value is 470µH. The diameter of the core is 4 mm, height is 10 mm, and the diameter of the coil is 8 mm. The two devices on the right in the photograph are high frequency transformers. They are used for intermediate frequency (455KHz) tuning of transistor radios, or for oscillator circuits. To shield the coils from magnetic flux, and to prevent the coils from interfering with other circuits, the high frequency coils are housed in a metal case called shield case. This case must be connected to ground. As for tuning or oscillation, this type of transformer can change its value of inductance. Adjustment of the Inductance Value The ferrite core of the coil is made like a screw. The core can be made to move in or out of the coil by turning it with a screwdriver. A special plastic screwdriver is better to use for adjustment of the coils. By moving the ferrite core in or out of the coil, the value of the coil's inductance can be changed. The value of inductance can also be changed by changing the number of turns of wire that comprise the coil, but in fact it is not possible in any practical way. You want try it ? The tuner of an FM radio handles very high frequencies (about 70MHz

to 100MHz). The coils used in the tuning circuit are hollow; i.e. they have no ferrite core. A coil with a ferrite core has too much inductance too be used in such a circuit. To adjust the inductance value of a hollow coil, the spacing between the loops of the coil is changed. When you disassemble an FM radio, you may find these coils to appear a bit untidy. Do not try to "fix" the coil by making it a perfect set of loops. The coil has been bent intentionally, in order to be adjusted precisely.

The Toroidal Coil The toroidal coil consists of copper wire wrapped around a cylindrical core. It is possible to make it so that the magnetic flux which occurs within the coil doesn't leak out, the coil efficiency is good, and that the magnetic flux has little influence on other components.

Diodes A diode is a semiconductor device which allows current to flow through it in only one direction. Although a transistor is also a semiconductor device, it does not operate the way a diode does. A diode is specifically made to allow current to flow through it in only one direction. Some ways in which the diode can be used are listed here. A diode can be used as a rectifier that converts AC (Alternating Current) to DC (Direct Current) for a power supply device. Diodes can be used to separate the signal from radio frequencies. Diodes can be used as an on/off switch that controls current. This symbol is used to indicate a diode in a circuit diagram. The meaning of the symbol is (Anode) (Cathode). Current flows from the anode side to the cathode side. Although all diodes operate with the same general principle, there are different types suited to different applications. For example, the following devices are best used for the applications noted.

Voltage regulation diode (Zener Diode) The circuit symbol is . It is used to regulate voltage, by taking advantage of the fact that Zener diodes tend to stabilize at a certain voltage when that voltage is applied in the opposite direction. Light emitting diode The circuit symbol is . This type of diode emits light when current flows through it in the forward direction. (Forward biased.) Variable capacitance diode The circuit symbol is . The current does not flow when applying the voltage of the opposite

direction to the diode. In this condition, the diode has a capacitance like the capacitor. It is a very small capacitance. The capacitance of the diode changes when changing voltage. With the change of this capacitance, the frequency of the oscillator can be changed.

The graph on the right shows the electrical characteristics of a typical diode. When a small voltage is applied to the diode in the forward direction, current flows easily. Because the diode has a certain amount of resistance, the voltage will drop slightly as current flows through the diode. A typical diode causes a voltage drop of about 0.6 - 1V (VF) (In the case of silicon diode, almost 0.6V) This voltage drop needs to be taken into consideration in a circuit which uses many diodes in series. Also, the amount of current passing through the diodes must be considered. When voltage is applied in the reverse direction through a diode, the diode will have a great resistance to current flow. Different diodes have different characteristics when reverse-biased. A given diode should be selected depending on how it will be used in the circuit. The current that will flow through a diode biased in the reverse direction will vary from several mA to just µA, which is very small. The limiting voltages and currents permissible must be considered on a case by case basis. For example, when using diodes for rectification, part of the time they will be required to withstand a reverse voltage. If the diodes are not chosen carefully, they will break down.

Rectification / Switching / Regulation Diode

The stripe stamped on one end of the diode shows indicates the polarity of the diode. The stripe shows the cathode side. The top two devices shown in the picture are diodes used for rectification. They are made to handle relatively high currents. The device on top can handle as high as 6A, and the one below it can safely handle up to 1A. However, it is best used at about 70% of its rating because this current value is a maximum rating. The third device from the top (red color) has a part number of 1S1588. This diode is used for switching, because it can switch on and off at very high speed. However, the maximum current it can handle is 120 mA. This makes it well suited to use within digital circuits. The maximum reverse voltage (reverse bias) this diode can handle is 30V. The device at the bottom of the picture is a voltage regulation diode with a rating of 6V. When this type of diode is reverse biased, it will resist changes in voltage. If the input voltage is increased, the output voltage will not change. (Or any change will be an insignificant amount.) While the output voltage does not increase with an increase in input voltage, the output current will. This requires some thought for a protection circuit so that too much current does not flow. The rated current limit for the device is 30 mA. Generally, a 3-terminal voltage regulator is used for the stabilization of a power supply. Therefore, this diode is typically used to protect the circuit from momentary voltage spikes. 3 terminal regulators use voltage regulation diodes inside

Diode bridge Rectification diodes are used to make DC from AC. It is possible to do only 'half wave rectification' using 1 diode. When 4 diodes are combined, 'full wave rectification' occurrs. Devices that combine 4 diodes in one package are called diode bridges. They are used for full-wave rectification.

The photograph on the left shows two examples of diode bridges. The cylindrical device on the right in the photograph has a current limit of 1A. Physically, it is 7 mm high, and 10 mm in diameter. The flat device on the left has a current limit of 4A. It is has a thickness of 6 mm, is 16 mm in height, and 19 mm in width.

The photograph on the right shows a large, high-power diode bridge. It has a current capacity of 15A. The peak reverse-bias voltage is 400V. Diode bridges with large current capacities like this one, require a heat sink. Typically, they are screwed to a piece of metal, or the chasis of device in which they are used. The heat sink allows the device to radiate excess heat. As for size, this one is 26 mm wide on each side, and the height of the module part is 10 mm.

Light Emitting Diode ( LED ) Light emitting diodes must be choosen according to how they will be used, because there are various kinds. The diodes are available in several colors. The most common colors are red and green, but there are even blue ones. The device on the far right in the photograph combines a red LED and green LED in one package. The component lead in the middle is common to both LEDs. As for the remaing two leads, one side is for the green, the other for the red LED. When both are turned on simultaneously, it becomes orange. When an LED is new out of the package, the polarity of the device can be determined by looking at the leads. The longer lead is the Anode side, and the short one is the Cathode side. The polarity of an LED can also be determined using a resistance meter, or even a 1.5 V battery. When using a test meter to determine polarity, set the meter to a low resistance measurement range. Connect the probes of the meter to the LED. If the polarity is correct, the LED will glow. If the LED does not glow, switch the meter probes to the opposite leads on the LED. In either case, the side of the diode which is connected to the black meter probe when the LED glows, is the Anode side. Positive voltage flows out of the black probe when the meter is set to measure resistance.

It is possible to use an LED to obtain a fixed voltage. The voltage drop (forward voltage, or VF) of an LED is comparatively stable at just about 2V. I explain a circuit in which the voltage was stabilized with an LED in "Thermometer of bending apparatus-2". Shottky barrier diode Diodes are used to rectify alternating current into direct current. However, rectification will not occur when the frequency of the alternating current is too high. This is due to what is known as the "reverse recovery characteristic." The reverse recovery characteristic can be explained as follows: IF the opposite voltage is suddenly applied to a forward-biased diode, current will continue to flow in the forward direction for a brief moment. This time until the current stops flowing is called the Reverse Recovery Time. The current is considered to be stopped when it falls to about 10% of the value of the peak reverse current. The Shottky barrier diode has a short reverse recovery time, which makes it ideally suited to use in high frequency rectification.

The shottky barrier diode has the following characteristics. The voltage drop in the forward direction is low. The reverse recovery time is short. However, it has the following disadvantages. The diode can have relatively high leakage current. The surge resistance is low. Because the reverse recovery time is short, this diode is often used for the switching regulator in a high frequency circuit.

Power Supplies Types of Power Supply There are many types of power supply. Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices. A power supply can by broken down into a series of blocks, each of which performs a particular function.

For example a 5V regulated supply:

Each of the blocks is described in more detail below:    

Transformer - steps down high voltage AC mains to low voltage AC. Rectifier - converts AC to DC, but the DC output is varying. Smoothing - smooths the DC from varying greatly to a small ripple. Regulator - eliminates ripple by setting DC output to a fixed voltage.

Power supplies made from these blocks are described below with a circuit diagram and a graph of their output:    

Transformer Transformer Transformer Transformer

Dual Supplies

only + Rectifier + Rectifier + Smoothing + Rectifier + Smoothing + Regulator

Some electronic circuits require a power supply with positive and negative outputs as well as zero volts (0V). This is called a 'dual supply' because it is like two ordinary supplies connected together as shown in the diagram. Dual supplies have three outputs, for example a ±9V supply has +9V, 0V and 9V outputs. Transformer only

The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor. Transformer + Rectifier

The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for electronic circuits unless they include a smoothing capacitor.

Transformer + Rectifier + Smoothing

The smooth DC output has a small ripple. It is suitable for most electronic circuits. Transformer + Rectifier + Smoothing + Regulator

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

Transformer Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC. Transformer circuit symbol Step-up transformers increase voltage, stepdown transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V in UK) to a safer low voltage.

The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core.

Transformer

Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up. The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage. turns ratio =

Vp Np = Vs Ns

and

power out = power in Vs × Is = Vp × Ip

Vp = primary (input) voltage Np = number of turns on primary coil Ip = primary (input) current

Vs = secondary (output) voltage Ns = number of turns on secondary coil Is = secondary (output) current

Rectifier There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is the most important and it produces fullwave varying DC. A full-wave rectifier can also be made from just two diodes if a centre-tap transformer is used, but this method is rarely used now that diodes are cheaper. A single diode can be used as a rectifier but it only uses the positive (+) parts of the AC wave to produce half-wave varying DC. Bridge rectifier A bridge rectifier can be made using four individual diodes, but it is also available in special packages containing the four diodes required. It is called a full-wave rectifier because it uses all the AC wave (both positive and negative sections). 1.4V is used up in the bridge rectifier because each diode uses 0.7V when conducting and there are always two diodes conducting, as shown in the diagram below. Bridge rectifiers are rated by the maximum current they can pass and the maximum reverse voltage they can withstand (this must be at least three times the supply RMSvoltage so the rectifier can withstand the peak voltages). Please see the Diodes page for more details, including pictures of bridge rectifiers.

Bridge rectifier Alternate pairs of diodes conduct, changing over the connections so the alternating directions of AC are converted to the one direction of DC.

Output: full-wave varying DC (using all the AC wave)

Single diode rectifier A single diode can be used as a rectifier but this produces half-wave varying DC which has gaps when the AC is negative. It is hard to smooth this sufficiently well to supply electronic circuits unless they require a very small current so the smoothing capacitor does not significantly discharge during the gaps. Please see the Diodes page for someexamples of rectifier diodes.

Single diode rectifier

Output: half-wave varying DC (using only half the AC wave)

Smoothing Smoothing is performed by a large value electrolytic capacitor connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output.

Note that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.

Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply voltage is satisfactory and the equation below gives the required value for the smoothing capacitor. A larger capacitor will give less ripple. The capacitor value must be doubled when smoothing half-wave DC. Smoothing capacitor for 10% ripple, C =

5 × Io Vs × f

C = smoothing capacitance in farads (F) Io = output current from the supply in amps (A) Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC f = frequency of the AC supply in hertz (Hz), 50Hz in the UK Regulator Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output voltages. They are also rated by the maximum current they can pass. Negative voltage regulators are available, mainly for use in dual supplies. Most regulators include some automatic protection from excessive current ('overload protection') and overheating ('thermal protection').

Voltage regulator

Many of the fixed voltage regulator ICs have 3 leads and look like power transistors, such as the 7805 +5V 1A regulator shown on the right. They include a hole for attaching aheatsink if necessary.

Zener diode regulator For low current power supplies a simple voltage regulator can be made with a resistor and a zener diode connected in reverse as shown in the diagram. Zener diodes are rated by their breakdown voltage Vz andmaximum power Pz (typically 400mW or 1.3W). The resistor limits the current (like an LED resistor). The current through the resistor is constant, so when there is no output current all the current flows through the zener diode and its power rating Pz must be large enough to withstand this. Please see the Diodes page for more information about zener diodes.

zener diode a = anode, k = cathode

Choosing a zener diode and resistor: 1. The zener voltage Vz is the output voltage required 2. The input voltage Vs must be a few volts greater than Vz (this is to allow for small fluctuations in Vs due to ripple) 3. The maximum current Imax is the output current required plus 10% 4. The zener power Pz is determined by the maximum current: Pz > Vz × Imax 5. The resistor resistance: R = (Vs - Vz) / Imax 6. The resistor power rating: P > (Vs - Vz) × Imax Example: output voltage required is 5V, output current required is 60mA. 1. Vz = 4.7V (nearest value available) 2. Vs = 8V (it must be a few volts greater than Vz) 3. Imax = 66mA (output current plus 10%) 4. Pz> 4.7V × 66mA = 310mW, choose Pz = 400mW 5. R = (8V - 4.7V) / 66mA = 0.05k = 50 , choose R = 47 6. Resistor power rating P > (8V - 4.7V) × 66mA = 218mW, choose P = 0.5W

Soldering Guide How to Solder What is solder? Solder is an alloy (mixture) of tin and lead, typically 60% tin and 40% lead. It melts at a temperature of about 200°C. Coating a surface with solder is called 'tinning' because of the tin content of solder. Lead is poisonous and you should always wash your hands after using solder. Solder for electronics use contains tiny cores of flux, like the wires inside a mains flex. The flux is corrosive, like an acid, and it cleans the metal surfaces as the solder melts. This is why you must Reels of solder melt the solder actually on the joint, not on the iron tip. Without flux most joints would fail because metals quickly oxidise and the solder itself will not flow properly onto a dirty, oxidised, metal surface. The best size of solder for electronics is 22swg (swg = standard wire gauge).

Desoldering At some stage you will probably need to desolder a joint to remove or reposition a wire or component. There are two ways to remove the solder:

1. With a desoldering pump (solder sucker)      

Using a desoldering pump (solder sucker)

Set the pump by pushing the spring-loaded plunger down until it locks. Apply both the pump nozzle and the tip of your soldering iron to the joint. Wait a second or two for the solder to melt. Then press the button on the pump to release the plunger and suck the molten solder into the tool. Repeat if necessary to remove as much solder as possible. The pump will need emptying occasionally by unscrewing the nozzle.

2. With solder remover wick (copper braid)    

Apply both the end of the wick and the tip of your soldering iron to the joint. As the solder melts most of it will flow onto the wick, away from the joint. Remove the wick first, then the soldering iron. Cut off and discard the end of the wick coated with solder.

Solder remover wick

After removing most of the solder from the joint(s) you may be able to remove the wire or component lead straight away (allow a few seconds for it to cool). If the joint will not come apart easily apply your soldering iron to melt the remaining traces of solder at the same time as pulling the joint apart, taking care to avoid burning yourself. First a few safety precautions:  

 

Never touch the element or tip of the soldering iron. They are very hot (about 400°C) and will give you a nasty burn. Take great care to avoid touching the mains flex with the tip of the iron. The iron should have a heatproof flex for extra protection. An ordinary plastic flex will melt immediately if touched by a hot iron and there is a serious risk of burns and electric shock. Always return the soldering iron to its stand when not in use. Never put it down on your workbench, even for a moment! Work in a well-ventilated area. The smoke formed as you melt solder is mostly from the flux and quite irritating. Avoid breathing it by keeping you head to the side of, not above, your work.



Wash your hands after using solder. Solder contains lead which is a poisonous metal.

If you are unlucky (or careless!) enough to burn yourself please read the First Aid section.

Preparing the soldering iron: 



  

Place the soldering iron in its stand and plug in. The iron will take a few minutes to reach its operating temperature of about 400°C. Dampen the sponge in the stand. The best way to do this is to lift it out the stand and hold it under a cold tap for a moment, then squeeze to remove excess water. It should be damp, not dripping wet. Wait a few minutes for the soldering iron to warm up. You can check if it is ready by trying to melt a little solder on the tip. Wipe the tip of the iron on the damp sponge. This will clean the tip. Melt a little solder on the tip of the iron. This is called 'tinning' and it will help the heat to flow from the iron's tip to the joint. It only needs to be done when you plug in the iron, and occasionally while soldering if you need to wipe the tip clean on the sponge.

You are now ready to start soldering: 



Hold the soldering iron like a pen, near the base of the handle. Imagine you are going to write your name! Remember to never touch the hot element or tip. Touch the soldering iron onto the joint to be made. Make sure it touches both the component lead and the track. Hold the tip there for a few seconds and...



 

Feed a little solder onto the joint. It should flow smoothly onto the lead and track to form a volcano shape as shown in the diagram. Apply the solder to the joint, not the iron. Remove the solder, then the iron, while keeping the joint still. Allow the joint a few seconds to cool before you move the circuit board. Inspect the joint closely. It should look shiny and have a 'volcano' shape. If not, you will need to reheat it and feed in a little more solder. This time ensure that boththe lead and track are heated fully before applying solder.

If you are unlucky (or careless!) enough to burn yourself please read the First Aid section.

Crocodile clip

Using a heat sink Some components, such as transistors, can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the component body. You can buy a special tool, but a standard crocodile clip works just as well and is cheaper.

Soldering Advice for Components It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. You are much less likely to make a mistake if you do this! 1. Stick all the components onto a sheet of paper using sticky tape. 2. Identify each component and write its name or value beside it. 3. Add the code (R1, R2, C1 etc.) if necessary. Many projects from books and magazines label the components with codes (R1, R2, C1, D1 etc.) and you should use the project's parts list to find these codes if they are given.

4. Resistor values can be found using the resistor colour code which is explained on our Resistors page. You can print out and make your own Resistor Colour Code Calculator to help you. 5. Capacitor values can be difficult to find because there are many types with different labelling systems! The various systems are explained on our Capacitors page. Some components require special care when soldering. Many must be placed the correct way round and a few are easily damaged by the heat from soldering. Appropriate warnings are given in the table below, together with other advice which may be useful when soldering. For more detail on specific components please see the Components page or click on the component name in the table. For most projects it is best to put the components onto the board in the order given below:

Components

1

IC Holders (DIL sockets)

2 Resistors

3

Small value capacitors (usually less than 1µF)

Pictures

Reminders and Warnings Connect the correct way round by making sure the notch is at the correct end. Do NOT put the ICs (chips) in yet. No special precautions are needed with resistors. These may be connected either way round.

4

Electrolytic capacitors (1µF and greater)

5 Diodes

6 LEDs

Take care with polystyrene capacitors because they are easily damaged by heat. Connect the correct way round. They will be marked with a + or near one lead. Connect the correct way round. Take care with germanium diodes (e.g. OA91) because they are easily damaged by heat. Connect the correct way round. The diagram may be labelled a or + for anode and k or for cathode; yes, it really is k, not c,

7 Transistors

8

Wire Links between points on the circuit board.

for cathode! The cathode is the short lead and there may be a slight flat on the body of round LEDs. Connect the correct way round. Transistors have 3 'legs' (leads) so extra care is needed to ensure the connections are correct. Easily damaged by heat. Use single core wire, this is one solid wire which is plasticcoated. If there is no danger of touching other parts single core wire you can use tinned copper wire, this has no plastic coating and looks just like solder but it is stiffer.

9

Battery clips, buzzers and other parts with their own wires

Wires to parts off the circuit board, 1 includingswitches, relays, variable resistors 0 andloudspeakers.

1 ICs (chips) 1

stranded wire

Connect the correct way round. You should use stranded wire which is flexible and plasticcoated. Do not use single core wire because this will break when it is repeatedly flexed. Connect the correct way round. Many ICs are static sensitive. Leave ICs in their antistatic packaging until you need them, then earth your hands by touching a metal water pipe or window frame before touching the ICs. Carefully insert ICs in their holders: make sure

all the pins are lined up with the socket then push down firmly with your thumb.

Vacuum tube .

Modern vacuum tubes, mostly miniature style

In electronics, a vacuum tube, electron tube (in North America), or thermionic valve (elsewhere, especially in Britain), reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum. Vacuum tubes may be used for rectification, amplification, switching, or similar processing or creation of electrical signals. Vacuum tubes rely onthermionic emission of electrons from a hot filament or cathode, that then travel through a vacuum toward the anode (commonly called the plate), which is held at a positive voltage relative to the cathode. Additional electrodes interposed between the cathode and anode can alter the current, giving the tube the ability to amplify and switch.

Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio communication and broadcasting, television, radar, sound reproduction, large telephone networks, analog and digital computers, and industrial process control. Although some of these applications had counterparts using earlier technologies, such as the spark gap transmitter or mechanical computers, it was the invention of the triodevacuum tube and its capability of electronic amplification that made these technologies widespread and practical. In most applications, vacuum tubes have been replaced by solid-

state devices such as transistors and other semiconductor devices. Solid-state devices last much longer, and are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes still find particular uses where solid-state devices have not been developed or are not practical. Tubes are still produced for such applications and to replace those used in existing equipment such as high-power radio transmitters.

OPERATIONAL AMPLIFIER

A Signetics μa741 operational amplifier, one of the most successful opamps.

An operational amplifier ("op-amp") is a DC-coupled highgain electronic voltageamplifier with a differential input and, usually, a singleended output.[1] An op-amp produces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals.[2] Operational amplifiers are important building blocks for a wide range of electronic circuits. They had their origins in analog computers where they were used in many linear, non-linear and frequency-dependent circuits. Their popularity in circuit design largely stems from the fact that characteristics of the final elements (such as theirgain) are set by external components with little dependence on temperature changes and manufacturing variations in the opamp itself. Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost over $100 US in small quantities. Opamps may be packaged as components, or used as elements of more complex integrated circuits. The op-amp is one type of differential amplifier. Other types of differential amplifier include the fully differential amplifier (similar to the opamp, but with two outputs), the instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to the instrumentation amplifier, but with tolerance to common-mode voltages that would destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps and a resistive feedback network).

Activity No.1 THE HALF-WAVE RECTIFIER OBJECTIVE To observe and measure the input voltage of a half-wave rectifier INTRODUCTION DC and AC voltages and currents serve the power requirements of a wide variety of electronic devices.Since ac power is more efficient and economical to transmit,it is generally distributed by utility companies.There is a necessity therefore for the rectification (conversion) of ac into dc voltages and currents.In this experiment we shall be concerned with electronic means of achieving rectification. Direct current is current which flows in only one direction.The semiconductor diode is well suited to accomplish rectification,since it permits current to flow in only one direction. Materials required:    

VOM Diode,1N4001 Capacitor,1000 uf/16v Transformer 12v

PROCEDURE Connect on the mounting board the circuit.Have your instructor check your circuit before proceeding.

Activity No.2 THE FULL-WAVE RECTIFIER OBJECTIVE To observe and measure the input and output voltages of a full-wave rectifier. INTRODUCTION It is possible to rectify both alternations of the input voltage by using two diodes.However,the power transformer which was used in the previous experimentwill have to be modified in order that in can be used for this purpose.The transformer needs to have a center-tapped secondary.In such a circuit,each diode receives one-half of the voltage across the secondary winding. Materials required:    

VOM 2pcs. 1N4001 1pc.1000uf/16v Transformer 12v x 12v

PROCEDURE Connect the circuit on the mounting board.Have your instructor check your circuit before proceeding.

Activity No.3 THE BRIDGE-TYPE RECTIFIER OBJECTIVE To observe and measure the input and output voltages of a bridge-type rectifier. INTRODUCTION Another form of full-wave rectification is the bridge-type rectifier.It uses four diodes and does not required a center-tapped transformer making it more widely used than the full-wave rectifier circuit used in the previous experiment.In the bridge configuration two diodes are conducting on each half of the ac input signal. Materials required:      

VOM 4pcs.,1N4001 1pc.,1000uf/16v LED 1k resistor Transformer 12v (secondary)

PROCEDURE Connect the circuit on the mounting board.Observe the proper polarity of each diode.Have your instructor check your circuit before proceeding.

Activity No.4 THE SPLIT-TYPE POWER SUPPLY OBJECTIVE To measure the input and output voltages of split type power supplies. INTRODUCTION Present electronic devices such as amplifiers or those using integrated circuits not only require single power supply but also dual symmetrical or split-type power supply.This type of power supply features at the same time two equal voltages but opposite in polarities which is an advantage over having two single power supplies built for one electronic device. Material required:    

VOM 4pcs,1N4001 2pcs.1000uf/16v Transformer 12v x12v (secondary)

PROCEDURE Connect the circuit on the mounting board.Observe the proper polarity of each diode.Have your instructor check your circuit before proceeding.

Activity no. 1 Schematic Diagram of a Half-Wave Rectifier

Activity Sheet 1

Activity no. 2 Schematic Diagram of a Full-Wave Rectifier

Activity Sheet 2

Activity no. 3 Schematic Diagram of a Bridge-Type Rectifier

Activity Sheet 3

Activity no. 4 Schematic Diagram of a Split-Type Rectifier

Activity Sheet 4

Printed Cicuit Board (PCB) Lay out and placement guide

D1

C1

R1

Q1

C3

R2

Dcv out +

D2 AC in D3 – D4

Gnd neg. C2 Materials needed ;    

D1 to D2= 1N4001 C1,C3= 1000 uf/16v C2-330 uf/16v Q1=C1061 NPN



R1=390 ohm

Z1

Led1

       

R2=1K ohm Z1= 12v Led1=Red 2x2 Pcb Permanent pen Masking tape Cutting knife 12v dc mini drill

For Etching Process;   

Ferric Chloride solution Plastic basin Clean water

Exercises; The symbols of the given names below symbol

name

symbol

name

Antenna

Zener diode

Resistor(fixed) Capacitor(fixed)

Light emitting diode(LED) Tunnel diode

Inductor(fixed)

Capacitive diode

Ground

Ohmmeter

Fuse

Ammeter

Speaker

Voltmeter

Microphone

Galvanometer

Plug

Resistor(variable)

Thermistor

Capacitor(variable)

Shielded wire

Inductor(variable)

Amplifier

I.F. transformer

Wire connected

Single pole/ Single throw Single pole/ Double throw Double pole/ Single throw Double pole/ Double throw

Wire not connected Crystal Rectifier diode

Exercises Color coding 1.)brown,red,blue,black=_____

11.)brown,blue,silver,silver=_______

2.)red,red,brown,black=____

12.)orange,orange,black,black=_______

3.)green,blue,black,black=____

13.)gray,black,gold,gold=______

4.)gray,black,red,gold=_______

14.)blue,black,silver,silver=_______

5.)orange,white,brown,gold=___

15.)violet,red,black,black=______

6.)yellow,violet,black,gold=_____

16.)green,green,green,black=______

7.)blue,black,gold,gold=_______

17.)white,black,blue,gold=________

8.)white,white,gold,gold=______

18.)gray,gray,violet,gold=________

9.)violet,red,gold,gold=________

19.)brown,black,blue,gold=_______

10.)green,green,silver,silver=____

20.)red,red,yellow,gold=_______

Exercises: De-coding 1.)200Ω + or – 5%=_________ 11.)0.47Ω=______________ 2.)5,600Ω + or –10%=_______

12.)0.2Ω=_______________

3.)100,000Ω=__________

13.)10kΩ=_______________

4.)33Ω=________________

14.) 4.7M=______________

5.)470Ω=_________

15.)2MΩ=_______________

6.)5Ω=___________

16.)500kΩ=______________

7.)20Ω=__________

17.)1.2kΩ=_______________

8.)2,500,000Ω=_______

18.)2.2MΩ=______________

9.)40,000Ω=__________

19.)1GΩ=_________________

10.)6.5Ω=________

20.)10.7Ω

.

=_______________

Exercises:

Convert the following ; 1.) 20,000Ω =_____________KΩ

11.) 5MΩ=__________________Ω

2.) 100,000Ω =____________KΩ

12.) 6.8MΩ=________________ Ω

3.) 3,500,000Ω=___________KΩ

13.) 10MΩ=________________ Ω

4.) 5,000Ω=______________ KΩ

14.) 6GΩ=_________________ Ω

5.) 20Ω=_________________ KΩ

15.) 2TΩ=__________________Ω

6.) 10kΩ=_________________Ω

16.)6,000,000Ω=___________MΩ

7.) 4.7kΩ=________________ Ω

17.)2,000 Ω=______________MΩ

8.) 500kΩ=________________ Ω

18.) 4,700 Ω=______________MΩ

9.) 8kΩ= __________________Ω

19.) 500 =________________ MΩ

10.)2kΩ=_________________ Ω

20.) 10 Ω =______________ MΩ

Example ; 1 0 3 = 10 000 pf = 0.01 uf

Exercises : Convert the following from pf to uf

1. 2. 3. 4. 5. 6. 7. 8. 9.

203 104 222 223 224 332 102 503 204

= = = = = = = = =

_____________ _____________ _____________ _____________ _____________ _____________ _____________ _____________ _____________

pf pf pf pf pf pf pf pf pf

= = = = = = = = =

______________ ______________ ______________ ______________ ______________ ______________ ______________ ______________ ______________

uf uf uf uf uf uf uf uf uf

10. 602 = _____________ pf = ______________ uf

Definition of Terms antenna

=intercept modulated resignal transmitted by the transmitter of a station

resistor

=regulate the flow of electric currnet in the circuit to a desired amount.

Capacitor

=it is an electronic device or component that is used to store elecrical charges or voltage.

Ground

=part of a circuit that is zero potential with respect to the earth or conducting body such as the chasis of a radio set,at general ckt.

Speaker

=it is a device that convert the electrical inpulse into soundwaves.

Amplifier

=it is a device used to enlarge, or amplify signal.

Plug

=for connecting an electric cord to a source of electricity.

Direct current = an electric current of constant direction (ex.battery ) Alternating

= current (˜) = one that continuosly varies in magnitude and periodically reverse in polarity.

Fuse =it is used to protect the circuit in shortaged when excessive current passed to a circuit. Microphone = converts sound waves into electrical impulses. Ammeter

= an instrument that measures electric cuurent

Voltmeter

= for measuring potential difference.

Galvanometer =sensitive instrument for mesuring very small values of current. Transformer

=an electromagnetic device used to increase or decrease an AC voltage.

“IF”transformer=a transformer that couples signal between two amplifiers.

Headphone

=contains two small dynamic speakers with separate leads for stereo, thepurpose of headphone is to concentrate sound at the ears while blocking extraneous background sounds.

Diode(rectifier)=an electronic device which is used to change alternating current to direct current. Tunnel diode =is heavily poped germanium or galium or arsenic diode with radically different characteristics in a negative tesistance. The current decrease for an increase of applied voltage. Zener diode =it is used as a voltage regulator Conductors =substance that has low resistance materials that let free electrons move freely from atom to atom(ex.gold,silver,copper) The purpose of using conductors is to allow electric current to flow with minimum position Insulators

=substance that have a property of conductors or insulators

(ex.transistor,diode) Semi-conductors =substance that have a property of conductors and insulators.(ex.transistor,diode) Lightning =is a bright flash of light which is often seen during storms, the flash is actually a discharge of electricity in the air. Thunder is a familiar result of discharge. Circuit = is the path of electron or current flow.   

Close circuit=complete path of current flow. It is a flow of electron from one point of the circuit to the other. Open circuit=incomplete path of current flow. Short circuit=complete path of current flow but without flowing to the resistance load.

Battery - provides voltage. Switch - a device used to break or complete the current path. Relay - a switch that is designed to trigger another switch. Circuit breaker - a switch designed to protect an electrical circuit from overload. When the circuit breaker senses an overload in voltage or current it

switches off to discontinue electrical flow to the rest of the circuit. Potentiometer/Variable Resistor - a resistor whose resistance value is not fixed and can be adjusted.. Variable Capacitor - a capacitor whose charging capacity is not fixed and can be adjusted. Inductor - stores energy in a magnetic field and opposes change in current. Transistor - amplifies signal. Comparator - a device that compares two voltages or currents and switches its output to indicate which is larger. Operational Amplifier - amplifies output voltage. NE555 - can be used as a timer, produce pulse or oscillation. Diode - allows current to pass in one direction while blocking it from passing the opposite direction. LED (light emitting diode) - a device that produces light. Varactor Diode - variable capacitor which stores energy and controlled by voltage. Zener Diode - allows current to pass in the opposite direction after the circuit reaches a certain threshold (for example, it can conduct after voltage reaches 5v). Photoresistor - light sensing resistor whose resistance decreases as light intensity increases. Thermistor - a temperature sensing resistor whose resistance increases or decreases based on changes in temperature. Thryistor - acts as a switch, conducting when it receives current trigger and continues to conduct while it is forward biased. Darlington Pair Transistors - two transistors connected at the base, which amplifies current twice as much. Voltage Divider - a strategically placed resistor to decrease the voltage at a certain point of the circuit. Bridge Rectifier - an arrangement of four or more diodes that can convert AC to DC.

Breadboard - a board that can be used to construct a circuit which does not require soldering. It has holes where components can be pushed in. Printed Circuit Board (PCB) - a board that can be used to construct a circuit which requires solodering Diagram a figure, usually consisting of a line drawing, made to accompany and illustrate a geometrical theorem, mathematical demonstration, etc. Block diagrams are used to understand (and design) complete circuits by breaking them down into smaller sections or blocks. Each block performs a particular function and the block diagram shows how they are connected together. No attempt is made to show the components used within a block, only the inputs and outputs are shown. This way of looking at circuits is called the systems approach. schematic, or schematic diagram, is a representation of the elements of a system using abstract, graphic symbols rather than realistic pictures.

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