EASA Part 66 - Module 4 - Electronic Fundamentals

JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

uk

engineering

ELECTRONIC FUNDAMENTALS

Index 1

SEMICONDUCTOR DEVICES................. DEVICES.... .......................... .......................... ........................ ........... 1-1 1.1 ................................ ....................... ....................... ..................... .......... 1-2 RECTIFIER DIODES ..................... 1.1.1 Circuit Circuit Symbols Symbols & Identificatio Identification n ..................... .............................. ......... 1-2 1.1.2 Operating Operating Characteristics............. Characteristics........................ ..................... ............... ..... 1-3 1.1.3 Parallel Parallel & Serial Serial Arrangement Arrangements s of Diodes Diodes .............. .............. 1-4 1.1.4 Rectification...... Rectification................. ...................... ...................... ...................... ..................... .......... 1-5 1.2 .................................. ...................... ...................... ...................... .............. ... 1-10 SIGNAL DIODES ....................... 1.3 ................................. ...................... ...................... ...................... ................ ..... 1-10 ZENER DIODES ...................... 1.4 ................................. ...................... ...................... .............. ... 1-11 LIGHT EMITTING DIODES ...................... 1.5 ................................. ...................... ...................... ...................... .................. ....... 1-11 PHOTOCELLS ...................... 1.5.1 Photocondu Photoconductive ctive Cells.......................... Cells..................................... .................. ....... 1-11 1.5.2 Photovoltaic Photovoltaic Cells...................... Cells................................. ...................... .................. ....... 1-12 1.6 ................................. ...................... ...................... ...................... ................ ..... 1-12 PHOTODIODES ...................... 1.7 ................................. ....................... ....................... ..................... .......... 1-12 VARACTOR DIODE ...................... 1.8 ................................ ...................... ............. .. 1-13 SILICON CONTROLLED RECTIFIER ..................... 1.9 ................................ ...................... ...................... ...................... .................. ....... 1-13 TRANSISTORS ..................... 1.9.1 NPN Transistor............. Transistor........................ ...................... ...................... .................... ......... 1-14 1.9.2 PNP Transistor Transistor.......... ..................... ...................... ...................... ...................... ............. 1-16 1.10 TESTING SEMICONDUCTOR DEVICES ...................... ................................. .................. ....... 1-18 1.10.1 1.10.1 Testing Testing Diodes Diodes ...................... .................................. ....................... ..................... .......... 1-18 1.10.2 1.10.2 Testing Testing Transistors.............. Transistors......................... ...................... ...................... ............. .. 1-19

2

OPERATIONAL AMPLIFIERS................... AMPLIFIERS...... .......................... ........................... ...................... ........ 2-1 2.1 ................................. ...................... ...................... ............. 2-1 THE PERFECT AMPLIFIER ...................... 2.2 ................................. ...................... ...................... .............. ... 2-1 OP AMP SPECIFICATION ...................... 2.3 ................................. ...................... ...................... .............. ... 2-2 POWER REQUIREMENTS ...................... 2.4 ................................ ...................... .................. ....... 2-2 PIN OUTS & CIRCUIT SYMBOL ..................... 2.5 .................................. ....................... ...................... ....................... ................... ....... 2-3 OPERATION ...................... 2.5.1 Negative Negative Feedback Feedback ..................... ................................ ...................... ................ ..... 2-3 2.6 ................................ ...................... ...................... ................ ..... 2-5 OP- AMP COMPARATOR ..................... 2.7 ................................ ...................... ...................... ................ ..... 2-6 OP AMP SUMMING AMP .....................

3

PRINTED CIRCUIT BOARDS.................. BOARDS..... .......................... .......................... ........................ ........... 3-1 3.1 ................................. ....................... ....................... ...................... ............. .. 3-2 BASE MATERIAL ...................... 3.2 ................................. ...................... ...................... .............. ... 3-2 CONDUCTOR MATERIAL ...................... 3.3 ................................. .................. ....... 3-2 BONDING OF CONDUCTOR MATERIAL ...................... 3.3.1 Inspections Inspections & Tests.......... Tests ..................... ...................... ...................... ................ ..... 3-3 3.4 ................................. ...................... ...................... .............. ... 3-4 MACHINING OF BOARDS ......................

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

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engineering 3.5 3.6

3.7

3.8 3.9 3.10 3.11 3.12 3.13 3.14

ELECTRONIC FUNDAMENTALS

................................ ....................... ....................... ..................... .......... 3-4 CIRCUIT ARTWORK ..................... ..................... ................................ ...................... ...................... ................ ..... 3-5 3.6.1 Etching Etching Process ..................... ................................ ...................... ..................... .......... 3-5 3.6.2 Additive Additive Process............ Process....................... ...................... ...................... ................... ........ 3-6 3.6.3 Inspection.................. Inspection............................. ...................... ...................... ...................... ............. 3-7 ................................. ...................... ...................... ................ ..... 3-7 SOLDERING METHODS ...................... 3.7.1 Hand Soldering Soldering ..................... ................................. ....................... ..................... .......... 3-7 3.7.2 Mass Soldering Soldering ..................... ................................. ....................... ..................... .......... 3-7 ................................. ...................... ...................... .............. ... 3-9 SOLDER SPECIFICATION ...................... ................................. ...................... ................ ..... 3-9 FLUXES & THEIR APPLICATION ...................... ................................. ....................... ...................... ...................... ............. 3-10 SOLDER RESISTS ..................... ................................ .............. .... 3-10 PLATING OF PRINTED WIRING CIRCUITS ...................... 3.11.1 3.11.1 Through-H Through-Hole ole Plating Plating ..................... ................................ ...................... ............. .. 3-10 ................................. ...................... ........... 3-11 ORGANIC PROTECTIVE COATINGS ...................... ................................. .................. ....... 3-11 FLEXIBLE PRINTED WIRING CIRCUITS ...................... ................................ ...................... ............... .... 3-12 HANDLING OF CIRCUIT BOARDS ..................... 3.14.1 3.14.1 Electrostatic Electrostatic Discharge Discharge Sensitive Sensitive Devices Devices ............. ............. 3-12 3.14.2 Removal & Installation of ESDS Printed Circuit Boards 3-15 3.14.3 Removal & Installation of Metal-Encased ESDS LRU's 3-16 PRINTING OF CIRCUITS

4

SYNCHRONOUS DATA TRANSMISSION............. TRANSMISSION ........................... ...................... ........ 4-1 4.1 .................................. ...................... ....................... ....................... ........... 4-1 DESYNN SYSTEM ....................... 4.1.1 The Basic Desynn Desynn ..................... ................................ ...................... .................. ....... 4-1 4.1.2 Slab Desynn............................ Desynn........................................ ....................... ................... ........ 4-4 4.2 ................................ ...................... ...................... .................... ......... 4-4 SYNCHRO SYSTEMS ..................... 4.2.1 Synchro Synchro Types Types ...................... .................................. ....................... ..................... .......... 4-5 4.2.2 Synchro Synchro Schematics......... Schematics.................... ...................... ...................... ................ ..... 4-7 4.2.3 XYZ Synchro Synchro system......................... system.................................... ..................... .......... 4-9 4.2.4 Synchro Synchro Supplies Supplies ...................... ................................. ...................... .................. ....... 4-9 4.2.5 Torque Torque Synchro Synchro System......................... System................................... ................ ...... 4-10 4.2.6 Electrical Electrical Zero ..................... ................................ ..................... ..................... .............. ... 4-13 4.2.7 Differential Torque Synchro System............... System ....................... ........ 4-14 4.2.8 Control Synchro System.................... System...... ........................... ..................... ........ 4-16 4.2.9 Differential Control Synchros................ Synchros... .......................... .................. ..... 4-20

5

SERVO SERVO SYSTEMS................ SYSTEMS........................... ...................... ...................... ...................... ....................... ............ 5-1 5.1 CATEGORIES OF SERVO SYSTEMS SYSTEMS ..................... ............................... .......... 5-1 5.1.1 open loop ..................... ................................. ....................... ...................... ................... ........ 5-1 5.1.2 closed loop ..................... ................................ ...................... ...................... .................. ....... 5-2 5.2 ..................... 5-3 REMOTE POSITION CONTROL SERVOMECHANISMS ..................... 5.2.1 Positional Positional Feedback Feedback ...................... ................................. ...................... .............. ... 5-3 5.3 TYPES OF INPUTS ....................... .................................. ...................... ....................... ..................... ......... 5-5 5.3.1 Step Input............................ Input....................................... ...................... ..................... ............. ... 5-5

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5.4 5.5

5.6

5.7

5.8

6

ELECTRONIC FUNDAMENTALS

5.3.2 Ramp Input.................. Input............................. ...................... ...................... ..................... .......... 5-5 5.3.3 Accelerating Input......................... Input............ .......................... .......................... ............. 5-5 SYSTEM RESPONSE ..................... ................................ ...................... ...................... .................... ......... 5-6 D AMPING ...................... ................................. ...................... ....................... ....................... ...................... ........... 5-7 5.5.1 Frictional Forces which Produce Damping ............. 5-7 5.5.2 Velocity Feedback Damping....................... Damping......... .......................... ............ 5-9 ................................. ........... 5-11 VELOCITY CONTROL SERVOMECHANISMS ...................... 5.6.1 Residual Residual Error ...................... ................................ ..................... ...................... ............. .. 5-11 5.6.2 Velocity Velocity Lag...................... Lag................................. ...................... ...................... ................ ..... 5-11 ................................ ................ ..... 5-12  A.C. SERVOMECHANISM COMPONENTS ..................... 5.7.1 E & I Bar Transducer................ Transducer... .......................... ........................... ................. ... 5-12 5.7.2 A.C. Tachogene Tachogenerators rators ...................... ................................. ...................... ........... 5-13 ................................. ...................... .................. ....... 5-15 PRACTICAL SERVO SYSTEMS ...................... 5.8.1 Direct Servo Current System................. System... ............................ ................. ... 5-15 5.8.2 Alternating Current Servo System .......................... .............. ............ 5-16

OTHER OTHER TRANSDUCE TRANSDUCERS RS ..................... ................................ ...................... ...................... .................. ....... 6-1 6.1 ....................... .. 6-1 LINEAR VARIABLE DIFFERENTIAL TRANSFORMER ..................... 6.2 ................................. ..................... .......... 6-2 ROTARY VARIABLE TRANSFORMER ...................... 6.3 .................................. ...................... ............. 6-2 INDUCTIVE TYPE TRANSDUCERS ....................... 6.3.1 Induced Induced EMF Type........... Type ...................... ...................... ...................... ................ ..... 6-2 6.3.2 A.C. Current Current Control................... Control.............................. ...................... ................. ...... 6-3

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ELECTRONIC FUNDAMENTALS

SEMICONDUCTOR DEVICES

The early discoveries in the field of electricity made by Volta, AmpHata! Amp Hata! Yer işareti tanımlanmamış.ere, Gains, Faraday, Hertz and others raised fundamental problems concerning the nature of matter. The first breakthrough came in 1897, when Sir J.J . Thompson discovered the electron, a discovery soon verified by other investigators. In 1913 Bohr Bohr evolved the basic theory of atomic structure, s tructure, and that theory has been developed to our present concept of the nature of matter. The electrical characteristics of an atom are determined by how tightly the nucleus holds on to its outer electrons. electrons. If the outer electrons are easily easily removed from the atom, the material will conduct conduct easily and is known as a conductor. If the outer electrons are difficult to dislodge from their orbits, the material is known as an insulator. The material used in diodes and transistors is known as 'semi-conductor' material. One of the attributes of this material is that the number of free electrons in any given area can be fixed during the manufacturing process. Interest in semi-conductors began in 1873, when it was discovered that the resistance of rods and wires of selenium selenium decreased as they were were heated. This was surprising because the resistance of metals normally increased with an increase in temperature. Furthermore, some lowering of resistance was noted when when the rods were exposed to light. Later investigations found found similar effects in other materials, but the change in resistance was so small that no practical applications could be found. By 1906 a number of crystalline semi-conductors were being used as radio signal detectors, but the introduction of thermionic valves put an end to them. The valves were more reliable and had the advantage of being able to amplify the signal as well as detect it. During the development of radar systems in WW ΙΙ, it was discovered that valve type mixers would not operate at the high frequencies frequencies being used. Research turned to semi-conductor type mixers, and and silicon proved the most successful. After the war, the peculiar properties of Germanium and Silicon were rigorously investigated, and a germanium diode detector was made and used extensively in radio and t elevision. During development of the Germanium detectors an important discovery was made. It was found that when two very close c lose contacts are made with a piece of germanium, the current flow through one of the contacts affects the amount of current flow through the other.

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ELECTRONIC FUNDAMENTALS

Bell Telephone Laboratories latched onto this phenomena and eventually in 1948 they announced the manufacture of the first solid-state amplifying device, the transistor. This triggered renewed interest in semi-conductor diodes, resulting in the development of a huge variety of semi-conductor devices that we now take for granted. 1.1

RECTIFIER DIODES

 A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductor device which allows current to flow f low in one direction but not in the other. When conducting, the diode is said to be 'forward biased'. biased'. Under these conditions the diode offers little resistance to current flow. When opposing current flow, the diode is said t o be 'reverse biased'. biased'. Under reverse biased conditions the diode has a high resistance. 1.1.1 CIRCUIT SYMBOLS & IDENTIFICATION

The various symbols used for diodes are shown below.

Whether the triangles are filled or unfilled depends only on t he drawing office preference. Where it is considered necessary, it is possible possible to show that one of the the electrodes is connected to the case of the device by adding a dot to t o the symbol, but this is not often used. In every symbol, the arrow indicates the direction of conventional current flow. flow.

The base of the triangle is the end where conventional current enters the diode, this end is called the anode. anode. The end through through which current leaves the diode diode is the cathode. In some cases the arrow symbol is marked on the diode, where it is not, the cathode is identified by a band or distinctive shape as shown above. Two identification codes are used for diodes. In the American system the code always starts with 1N and is followed followed by a serial number, number, i.e. 1N4001. In the continental system, the first letter gives the semiconductor material; A for germanium; B for silicon, and the second letter identifies the use; A - signal diode; Y - rectifier diode and Z for zener diode. To complicate the situation some manufacturers have their own codes. Issue 1 - 02 October 2002

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ELECTRONIC FUNDAMENTALS

1.1.2 OPERATING CHARACTERISTICS CHARACTERISTICS

Most semiconductor diodes are made from silicon or germanium, these two materials have different operating characteristics, although the principle of operation and circuit symbols are both the same. 1.1.2.1

Biasing

 A diode is said to be 'biased' when a voltage is applied between the the terminals such that the diode operates as required.  An external voltage applied so that the anode is positive and the cathode cathode negative is called 'forward 'forward bias'. bias'. There are many ways of achieving this, for example: •

Connect the anode to +3V and the cathode to 0V. 0V.

•

Connect the anode to +1V and the cathode to -1V. -1V.

•

Connect the anode to -50V -50V and the cathode to -52V.

So far as the diode is concerned, it is the voltage of the anode with respect to the cathode which determines the bias. If the voltage is applied so that the anode is negative with respect to the cathode, the diode is ‘reverse ‘reverse biased’, biased’, again, there are many ways of achieving this. The forward voltage required to make the diode conduct depends on the material it is made from. Germanium diodes require require a voltage of approximately approximately 0.1 to 0.2 volts and silicon diodes 0.6 to 0.7 volts. 1.1.2.2

Forward Voltage Drop

Ideally a diode should have zero resistance when conducting and should cause no voltage drop, unfortunately this does does not happen. Germanium diodes create a voltage drop of approximately 0.6V and silicon diodes a drop of approximately 1.1V. This needs to be taken into account when doing circ uit calculations. 1.1.2.3

Reverse Leakage Current

When a diode is reverse biased, it should ideally have infinite resistance and no current should flow. Unfortunately when a diode is reverse biased, a small small current called 'reverse leakage current' flows, generally this is too small to be of significance, however, it should be noted that t he value of this current increases with an increase in diode temperature. The reverse current of silicon diodes diodes is much smaller than that of germanium diodes, (approx. one thousandth), therefore silicon diodes can be used more successfully at high temperatures (150º - 200ºC) than germanium diodes (80º - 100ºC).

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

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engineering 1.1.2.4

ELECTRONIC FUNDAMENTALS

Reverse Breakdown Voltage

If the reverse bias voltage v oltage is increased, eventually the diode breaks down and current flows in the wrong direction direction through the diode. diode. This causes permanent damage and the diode has to be replaced. The breakdown voltage can have any value from a few volts , up to 1000V for silicon diodes and 100V for germanium, depending on the construction and forms of material used. The maximum reverse reverse voltage is an an important diode characteristic. Under normal conditions this value should not be exceeded. exc eeded. 1.1.2.5

Graphical Representation

Shown below is a graphical representation of the operating characteristics of a typical silicon and germanium diode.

1.1.3 PARALLEL & SERIAL ARRANGEMENTS ARRANGEMENTS OF DIODES

It is possible to operate silicon rectifier diodes in parallel or in series to provide respectively, higher current or higher voltage capabilities. 1.1.3.1

Parallel Arrangements Arrangements

In parallel arrangements used for higher currents, some method must be used to ensure that the current divides equally equally through the individual individual diodes. This is difficult to do.

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ELECTRONIC FUNDAMENTALS

Series Arrangements

Series arrangements can be used if the applied voltage is greater than the maximum rated value of a single diode. Some method must be used to ensure the applied voltage divides equally among among the individual diodes. Resistors or capacitors in parallel can be used in an effort to achieve this. 1.1.4 RECTIFICATION

Rectifier diodes are designed to convert ac to dc. To do this effectively and efficiently they must have: •

Low resistance to current flow in the forward direction.

•

High resistance to current flow in the reverse direction.

 Almost all semiconductor rectifier diodes are silicon, junction types. The symbol used in circuit diagrams can be any of those shown earlier in the notes. 1.1.4.1

Basic Rectifier Circuit

 A basic rectifier circuit is shown below. The diode is inserted in series between the a.c. supply and the load.

The diode only passes passes current when forward biased. biased. Thus when an a.c. a.c. signal is applied, pulses of uni-directional (d.c.) v oltage are developed across the output load resistance. Note from the diagrams that the d.c. polarity can be reversed by reversing the diode connections. If the average value of ½ wave rectified a.c. is calculated it i t will be found to be 32% of the peak value of the output voltage.

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

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engineering 1.1.4.2

ELECTRONIC FUNDAMENTALS

Centre Tap Full Wave Rectifier

In full wave rectification, both halves of every cycle of input voltage produce current pulses through the load resistor.

In the circuit shown above, two diodes D1 and D 2 and a transformer with a centretapped secondary are used. During the positive half cycle of the input waveform, A is positive with respect to O and D1 conducts, the current flowing top to bottom through the load resistor. During this time diode D2 is reversed biased and does not conduct. During the negative half cycle of the input waveform, B is positive with respect to O and D2 conducts, the current again flowing top to bottom through the load resistor. During this time diode D 1 is reverse biased and does not conduct. In effect, the circuit consists of two half wave rectifiers working into the same load on alternate half cycles of the input. The current through R is in the same direction during both half cycles and a fluctuating d.c. is created across R. The average value of this full wave rectified a.c. is 64% of the peak value of the voltage across the load resistor R. The output frequency is double that of the input frequency.

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ELECTRONIC FUNDAMENTALS

Full Wave Bridge Rectifier

The circuit of a Full Wave Bridge Bridge rectifier is shown below. The rectifier has 4 diodes diodes as opposed to 2 and does not have a centre c entre tapped transformer.

During the positive half cycle diodes D1 and D2 conduct, the current flowing top to bottom through the load R L. During the negative negative half cycle D3 and D4 conduct, conduct, the current again flowing top to bottom through the load. The output from this rectifier is the same as that obtained obtained from the centred tapped transformer type. The average value again being 64% of the peak voltage across the load resistor. It should be noted that in this rectifier, the peak voltage across R L is equal to the whole of the secondary transformer output voltage, whereas in t he previous rectifier, the peak voltage across RL is only half the transformer secondary voltage. 1.1.4.4

Smoothing

The rectifier circuits previously discussed produce pulsating d.c. outputs. A smoothing circuit changes these outputs into a s teady d.c. voltage level. 1.1.4.4.1

Half Wave Rectifier

The diagram below shows a simple half wave rectifier with a reservoir capacitor, C, connected in parallel with the load R L. The capacitor capacitor charges towards the peak value of the input voltage whenever the input voltage is greater than V C and the diode is conducting. conducting. When the input input voltage is less less than VC the diode cuts-off and the capacitor discharges through the load.

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

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ELECTRONIC FUNDAMENTALS

This results in a mean d.c. output level less than the peak of the input, with a ripple superimposed at the input frequency.

1.1.4.4.2

Full Wave Rectifier

The diagram above shows a centre tapped full wave rectif ier with a reservoir capacitor. The charge is now topped up twice during during each cycle of the input input waveform which results in: •

A lower amplitude ripple, at twice the frequency of that from the half half wave rectifier.

•

A higher mean d.c. d.c. output than that that from a similarly loaded half wave rectifier.

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

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ELECTRONIC FUNDAMENTALS

Ripple Factor

 A measure of the amount of ripple present at the output of a d.c. supply is given by the ripple factor, which is usually expressed as a percentage and defined as: Ripple factor = Hata! ×  100% 1.1.4.6

Peak Inverse Voltage

The peak voltage across a rectifier diode in the reverse direction is known as the 'peak inverse voltage'. In a half wave rectifier with a reservoir capacitor, the peak inverse voltage is twice the amplitude amplitude of the peak voltage across the load. i.e. mean d.c. level to maximum negative peak. peak. The diode must be be able to withstand this this voltage without breaking down. 1.1.4.7

Voltage Regulation

Voltage regulation is a measure of the ability of a power supply to provide an increased load without a fall in output voltage. Regulation = Hata! ×  100% 1.1.4.8

Filter Circuits

Smaller ripple factors and improved voltage regulation is obtained by using R-C and L-C filter circuits across the output of the rectifier. 1.1.4.9

Ripple Frequency

The ripple frequency on the d.c. output from a half wave rectifier is equal to the supply frequency. For a full wave rectifier, the ripple ripple frequency is double the supply frequency.

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ELECTRONIC FUNDAMENTALS

SIGNAL DIODES

Signal diodes are used to detect radio signals (a process similar to rectification in which radio frequency a.c. is converted to d.c.), because of their very low capacitance. A capacitor passes a.c. The higher higher the frequency of the a.c. and the greater the capacitance, the less less opposition it offers. At radio frequencies, a normal normal diode would be of little use as a detector because because of its large junction area. area. The large junction area resulting in a large capacitance v alue and little opposition to current flow.  A point diode type signal diode has a very small junction area resulting in a low value of capacitance and a large opposition to current f low. Germanium is used for signal diodes since it has a lower 'turn-on' voltage than silicon, and so lower signal voltages start it conducting in the forward f orward direction. 1.3

ZENER DIODES

In an ordinary diode, if the reverse bias is increased, the diode breaks down and the diode suffers permanent damage. damage. A zener diode is designed designed to be used in the the breakdown region. The zener diode looks like a rectifier diode, the cathode cathode often being marked by a band. band. Its symbol is shown above. From the characteristic graph, it can be seen that the reverse current is negligible as the reverse bias is increased until the breakdown voltage is reached, then it suddenly increases. The breakdown voltage is called the the zener or reference reference voltage.

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ELECTRONIC FUNDAMENTALS

The important thing is that the voltage across the diode remains almost constant over a wide range of of reverse currents. It is this property of a zener diode diode that makes it useful in stabilised power supplies. To limit the reverse current at breakdown and prevent overheating, the power rating of the diode must must not be exceeded. This is achieved by using a resistor in series with the diode. 1.4

LIGHT EMITTING DIODES

 A light emitting diode is a specially constructed and doped diode type device which emits light when operated in the forward bias condition. condition. The colour of light emitted depends on the semi-conductor material used. Gallium arsenide phosphide - red light Gallium phosphide - green light Symbols used are similar to the photodiode.

Unless an LED is of the t he constant current type, which incorporates an integrated circuit regulator, it must have an external resistor connected in series to limit the forward current which typically may only be 10mA. The voltage drop across across a conducting LED is about 1 to7 volts. In seven segment LED displays, each segment is a separate LED and depending on which segments are energised, energised, the display lights up the number 0 to 9. Such displays are usually designed to operate from a 5V supply - each segment needs a separate current limiting resistor and all the cathodes or anodes are joined together to form a common connection. 1.5 PHOTOCELLS Photocells change light into electrical signals. signals. There are two basic types, Photoconductive cells and Photovoltaic cells. 1.5.1 PHOTOCONDUCTIVE PHOTOCONDUCTIVE CELLS

The resistance of certain semiconductors decreases as the intensity of light falling on on them increases. They are therefore therefore light sensitive resistors and sometimes referred to as light dependent resistors.

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ELECTRONIC FUNDAMENTALS

1.5.2 PHOTOVOLTAIC CELLS

When illuminated, a photovoltaic cell produces produces a voltage. If an external circuit is connected to the cell, current current flows through it. The source of energy is the light.

The voltage available depends on the material used, the intensity of the light and the amount of current drawn from the cell. For a silicon cell in full sunlight sunlight the voltage on open circuit is 0.45V. With a maximum current of 35mA 35mA for each square cm of of cell. Only about 10% 10% of the light is turned into electrical energy. 1.6 PHOTODIODES Photodiodes are operated under under reverse bias conditions. conditions. The leakage current increasing in proportion to the amount of light falling on the device. device. Photodiodes are used as fast counters counters and light meters.

1.7

VARACTOR DIODE

 A varactor diode is a special type of diode constructed to act as a voltage controlled capacitor. It is also known as a varicap diode. The diode is operated under reverse bias conditions, with an increase in bias decreasing the value of capacitance. The circuit symbols are as shown below.

There are 3 main uses for varactor diodes: •

As remotely controlled capacitors in RF tuned circuits.

•

As variable capacitors in amplifiers.

•

As variable capacitors in frequency modulator circuits.

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ELECTRONIC FUNDAMENTALS

SILICON CONTROLLED RECTIFIER

Silicon controlled rectifiers (SCR's) are now more commonly known as thyristors. They are semiconductor devices which rectify a.c. and control the power supplied to a load in a way way that wastes very little energy. They are commonly used in household lighting dimmer switches. The general symbol is shown below, below, together with the symbol for 'P' and 'N' types.

SCR's normally block the flow of current in both directions, but can be triggered t riggered so as to allow current to flow f low in the forward direction as in a normal diode, whilst still blocking current flow in the reverse direction. In the triggered triggered condition the characteristics are similar to rectifier diodes.  An SCR will continue to conduct until the load current is reduced to zero, or until it is reverse biased, when it automatically returns to the blocking state. The SCR is triggered by applying a pulse to a third terminal called the gate. The duration of the pulses can be extremely short. 1.9 TRANSISTORS Transistors are the most important device in electronics today. Not only are they made as discrete components, but integrated circuits may contain several thousands on a tiny slice of of silicon. They are 3 terminal devices used as amplifiers and as switches, and are classed as active devices. Hundreds of different different transistors are available. The same identification identification code is used used as for diodes, but in the American system transistors always start with 2N followed by a number. number. In the continental continental system the first letter gives gives the semiconductor material and the second letter gives the use: • C indicates an audio frequency device. • F a radio frequency device. • S a switching transistor.

 An example being BC108, a silicon audio frequency frequency amplifier device.

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The two basic types of transistors are: bipolar or junction transistor. • The bipolar unipolar or field effect transistor. • The unipolar In this element of the course we concentrate on bipolar transistors, of which there are two basic types. The NPN and the PNP, both of which which are active devices having three terminals labelled; Base, Collector and Emitter. 1.9.1 NPN TRANSISTOR

NPN transistors are made from 3 pieces of semi-conductor material joined together in a manner similar to two diodes, as shown in the diagram below. Also shown is the circuit diagram with each terminal t erminal identified.

If the base is made positive with respect to the collector, the diode, or junction or  junction as  as it is called, is forward f orward biased and current flows (conventional current flows fr om base to collector). If the base is made positive with respect to the emitter, again the junction (diode) is forward biased and conventional current flows from base to emitter. If the collector is made positive with respect to the emitter, or the emitter is made positive with respect to the collector no current will flow, because in either direction one of the junctions (diodes) is reverse biased and will prevent current flow. The last three paragraphs should be noted, as their contents is invaluable when it comes to determining determining the terminals and testing transistors. This will be discussed later.

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NPN Transistor as a switch

If the NPN transistor is connected as shown, it can be used as a switch. In this diagram the transistor is being used to turn on a lamp, it could however be used to operate any type of d.c. device such as a relay, solenoid, solenoid, another transistor or an LED. When the base is made positive with respect to the emitter, the junction is forward f orward biased and current flows in through the base and and out of the emitter. The flow of current from base to emitter creates a reaction in the transistor that causes the reverse biased collector/base junction to break break down and conduct. conduct. Current can then flow from the battery positive terminal, through the lamp, through the reverse biased collector base junction, through the forward biased base emitter junction and back to the battery, illuminating the lamp. When the base is made sufficiently positive with respect to the emitter (approx. ( approx. 0.6V for silicon, 0.2V for germanium) so that current flows from collector to emitter through the transistor, the transistor is said to be switched or turned 'ON'. If the base / emitter potential is reduced below the switch 'ON' potential, or removed totally, the collector / base junction will return to its reverse bias condition and will prevent current flowing around the circuit through the lamp. lamp. Under these conditions the transistor is said to be switched or turned 'OFF'. If should be noted that it may be necessary to limit the t he current through the transistor when it is switched on, this can be achieved by a series resistor as in the LED circuit. 1.9.1.2

NPN Transistor as an amplifier

When the base is made positive with respect to the emitter so that the transistor is switched 'ON', the amount of base base emitter current required is very very small. If the base / emitter current is increased slightly, by increasing the base emitter voltage, the transistor will turn 'ON' more, its effective resistance will decrease and the collector / emitter current will increase.

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If the base / emitter current is decreased slightly by reducing the base / emitter voltage, the transistor will turn 'OFF' more, it's effective resistance will increase and the collector / emitter current will decrease.

The transistor can therefore be be likened to a variable resistor. resistor. As the base / emitter bias increases, the resistance of the transistor effectively decreases and more current flows from collector to emitter. The change in current and resistance causes the output voltage to decrease.  As the base / emitter bias decreases, the effective resistance of the transistor increases and less current flows from collector to emitter. The change in current and resistance now causes the output voltage to increase. When set up correctly, millivolt changes across the base / emitter junction produce changes at the output of 10's or even 100's of volts, depending on the collector voltage. If a small sinusoidal a.c. signal is applied to the base / emitter junction, the bias will vary sinusoidally as will the resistance of the transistor and the output voltage, however the output voltage will vary sinusoidally 10's of volts for millivolt changes in the input signal. signal. (Using the example example voltage in the diagram). It should be noted, that although the changes in output voltage are much greater than the changes in input voltage, the bipolar transistor is a current device. device. Small changes in base / emitter current result in large changes in collector / emitter current. It is these changes in collect / emitter emitter current that produce the large large output voltage swings. 1.9.2 PNP TRANSISTOR

PNP transistors are made in a similar manner to NPN transistors, except the direction of the junctions is reversed.

If the base is made negative with respect to the collector, the diode, or junction is forward biased and current flows. Issue 1 - 02 October 2002

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If the base is made negative with respect to the emitter, the junction is forward biased and current flows. Current cannot flow between collector and emitter, because irrespective of the bias applied, one junction will be reverse biased.  Again these three statements are worth remembering when when it comes to determining the terminals and testing transistors. 1.9.2.1

PNP Transistor as a switch

When connected as shown, the PNP transistor can also be used as a switch, however, for the transistor to be tuned 'ON', the base must be made negative with respect to the emitter. For a silicon transistor the base needs to be about 0.6V negative with respect to the emitter, for a germanium transistor 0.2V negative. Once turned 'ON', conventional current flows from the emitter to the collector, which is in the opposite direction to that in the NPN transistor. 1.9.2.2

PNP Transistor as an amplifier

The PNP transistor can also also be used an amplifier. amplifier. It operates in a similar manner to the NPN transistor except the transistor t ransistor must be turned 'ON' by making the base negative with respect to the emitter, as seen above. If the base / emitter potential potential is increased by making the base more negative with r espect to the emitter, the transistor turns 'ON' more, its effective resistance decreases and more emitter / collector current flows. If the bias potential is decreased, by making the base less negative with respect to the emitter, the transistor turns 'OFF' slightly, the effective resistance increases and less emitter / collector current flows.  A small sinusoidal signal applied to the base will vary the effective resistance of the transistor and produce much larger changes in the output voltage as with the NPN transistor. Again it must be realised realised that the transistor is a current device. The small changes in base emitter bias potential cr eated by the input signal results in small changes in base emitter current, resulting in large changes in collector / emitter current.

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1.10 TESTING SEMICONDUCTOR SEMICONDUCTOR DEVICES 1.10.1 TESTING DIODES

Diodes only conduct in one direction, it is therefore relatively easy to determine the terminals and serviceability using a multimeter, however, 2 points need noting: •

When AVO's AVO's are used on a resistance range, the black terminal is positive with respect to the red terminal.

•

The potential potential difference between the red and black black terminals of a digital meter may be insufficient to forward bias a silicon diode (remember: requires 0.6V). This would indicate that the diode was non conducting in both direction leading to the false assumption that the diode is unservic eable.

1.10.1.1

Determining the Terminals

When forward biased, a diode has a resistance of approx. 1k Ω. When reverse biased the resistance is in the order of megohms. To determine the terminals terminals of a diode, it is simply a matter of connecting the meter across the diode to see if it will conduct, if it will not, the terminals should be reversed to confirm conduction and serviceability. When conducting, the black black terminal of an AVO, or or the red terminal of a digital meter, is connected to the anode (flat end of symbol). 1.10.1.2

Confirming Serviceability

The serviceability of a diode is determined by ensuring it has a resistance in the order of 1KΩ in one direction and a resistance in i n the order of megohms in the opposite direction. Remember the points made about the two types of meter.

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1.10.2 TESTING TRANSISTORS

 As we have seen, transistors basically comprise 2 back-to-back diodes, therefore the process of confirming the serviceability and determining the terminals is similar to that used for diodes. 1.10.2.1

Determining the Base

The base of the transistor can be found by considering the transistor as two back-toback diodes, and using a multimeter set on ohms.

1.10.2.1.1 NPN Transistors

Connect the positive terminal of the meter to one of the three transistor terminals. Measure the resistance between between this terminal and the the other two. If both indicate a low resistance then the positive positive terminal is connected to the base. base. If the resistance to the other two terminals is not low, the positive terminal is not connected to the base. Connect the positive terminal of the meter to another terminal and and repeat the process until the base is determined. 1.10.2.1.2 PNP Transistors

The procedure used to identify the base of a PNP transistor is the same s ame as that used to determined the base of the NPN transistor, except that the negative terminal of the meter is connected to each transistor terminal in turn, and it is this negative terminal that indicated the base. 1.10.2.2

Confirming the Serviceability

Both types of transistor are serviceability tested by confirming that each forward biased junction (Diode) has a low resistance, and each reverse biased junction a high resistance. The high resistance between between collector and emitter should also be confirmed. Remember the points points made about AVO's AVO's and Digital meters, otherwise incorrect conclusions may be drawn from the observations.

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PNP

ELECTRONIC FUNDAMENTALS

Base to Emitter

- forward biased - low resistance

Base to Collector

- forward biased - low resistance

Emitter to Collector

- reverse biased - high resistance

Emitter to Base

- forward biased - low resistance

Collector to Base

- forward biased - low resistance

Emitter to Collector

- reverse biased - high resistance

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ELECTRONIC FUNDAMENTALS

OPERATIONAL AMPLIFIERS

Operational amplifiers are integrated circuit devices designed to be a close approximation to the perfect amplifier. 2.1

THE PERFECT AMPLIFIER

 Although a theoretical device, the specification of a perfect amplifier would be as follows: •

Gain infinitely Gain infinitely high. This has to be controlled in some way otherwise the smallest input would result in maximum output.

•

Input impedance. impedance. Infinitely high so as not to load the source.

•

Output impedance. impedance. Zero, so that the amplifier can be connected to any load without the output voltage being affected.

•

Bandwidth. Bandwidth. Infinite, so that signals from d.c. to infinite frequency are all amplified by the same amount.

•

should be unaffected unaffected by variations in the power Supply voltage. voltage. The amplifier should supply voltage.

2.2

OP AMP SPECIFICATION

The following specification is for a SN72741 operational operational amplifier. This is a very popular operational amplifier generally simply referred to as a 741 op-amp. •

approx.) Gain - 200 000 voltage gain (106db approx.)

•

Input impedance - 2MΩ.

•

Output impedance - 75Ω

•

Bandwidth - d.c. to 1MHz.

•

supply of plus and minus 5 to Supply voltage - The op-amp will operate with a supply 15 volts, and take a quiescent quiescent current of about 2mA. 2mA. The output voltage will change less than 150 μV per volt change in supply voltage.

It can be seen that the 741 Op Amp approximates the specification of a perfect amplifier.

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POWER REQUIREMENTS

Operation is most convenient from a dual balanced d.c. power supply giving equal positive and negative voltages voltages (+ Vs) in the range +5V to +15V. The centre point of the power supply, i.e. 0V is common to input and output and is taken as t heir voltage reference. The input signs on the circuit symbol for an Op Amp should not be confused with those for the supply polarities.  An op-amp can be operated from a single power power supply. The voltage difference available from, for example, a 0V to 18V supply is the same as that from a +9V to 0V to -9V one, however, if a single power supply is used, extra components are required. 2.4

PIN OUTS & CIRCUIT SYMBOL

The circuit symbol and pin outs of a typical operation amplifier are shown below.

Most of the terminals are self-explanatory or will be explained in the course of these notes. Terminals 1 and 5, the offset null terminals however require require further explanation.

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If the same input signal is applied to the input terminals 2 and 3 the output (terminal 6) should be zero, in practice it is not. For d.c. amplification amplification this not not acceptable. acceptable. The output is zeroed by connecting a resistor between terminals 1 & 5 as shown, and and adjusting it until the output output falls to zero. For a.c. amplification a coupling capacitor in series with the output removes any unwanted d.c. offset. 2.5 OPERATION  An operational amplifier has one output and two inputs inputs as seen on the circuit and pin-out diagrams. The two inputs are referred referred to as, the non-inverting input, input, marked with a +, and the inverting input, marked with a -. If the voltage applied to the non-inverting input (+) is positive relative to the other input, the output voltage is positive. If the voltage applied to the non-inverting input is negative relative to the other input, input, the output voltage is negative. That is, the non-inverting input and the output are in-phase. If the voltage applied to the inverting i nput (-) is positive relative to the other input, the output voltage is negative. negative. If the voltage applied to the inverting input (-) is negative relative to the other input, the output voltage is positive. positive. That is, the inverting input, and the output are anti-phase. Basically an op-amp is a differential differential amplifier. It amplifies the difference between between the two input voltages. There are 3 cases: • If

V+ > V-

the output is positive

• If

V+ < V-

the output is negative

• If

V+ = V-

the output is zero

In general to output is given by by V0 = A0 × ((V+) - (V-)) where A0 is the gain. 2.5.1 NEGATIVE FEEDBACK

 As already mentioned, and as can be seen from the transfer characteristic to the left. There is only a very small range of input values giving an output that is directly proportional (A to B). It takes very little input to drive the amplifier into saturation due to its extremely high gain.

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 Assuming a gain of 105, the maximum input voltage swing (for linear amplification) is 5 ±9V/10 = ±90μV. This is of little practical use. To reduce this gain and allow larger input signals, requires the use of negative feedback. Part of the output is fed back to the input in such a way that it produces a voltage at the output that opposes the one from which it was taken. This basically means taking part of the output and feeding feeding it back to the inverting input. input. (Feedback applied to the non-inverting input would be positive and would increase the output). The application of negative feedback also gives greater stability, less distortion and increased bandwidth, it also becomes possible to exactly predict the gain of the amplifier. The relatively small loss in gain is far outweighed outweighed by the advantages advantages obtained.  A simple feedback network is shown in the diagram of an inverting amplifier below.

The signal to be amplified is applied to the inverting input via the resistor, the output is therefore antiphase with respect to the input. The non-inverting input input is connected to ground. ground. Negative feedback feedback is provided by resistor Rf, called the 'feedback resistor', it feeds back a certain amount of output voltage to the inverting input. Using this arrangement the gain can be calculated from; -Rf/R1

if Rf = 1MΩ  and R1 = 10k Ω

the gain

A = Hata!  Hata!  = -100 and,

an input of 0.01V will cause an output change of 1.0V. It should be noted that the gain depends entirely on the values of resistors Rf and R1, and is totally independent of the parameters of the operational amplifier.

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OP-AMP COMPARATOR

If both inputs of an Op Amp are used together, then V 0, the output voltage, is given by: V0 = A0 × (V2 - V1) Where V1 is the inverting input and V2 the non-inverting input. (Note: no feedback is used). The difference in voltage at the input terminals is amplified and appears at the output, however the gain is so large, that about 90 μV difference in the two inputs will cause the output to fall or rise to the +ve or -ve supply voltage. When V1 > V2 the output output is almost -Vs, when V1 < V2 the output is almost +Vs. The op-amp basically behaves like a two state switch, switching 'high' or 'low' ' low' depending on the difference in the inputs. By connecting a reference voltage to the inverting input and a signal to the noninverting input, the output will swing to +Vs when the signal is greater than the reference voltage and to -V s when the signal is s maller than the reference signal.

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ELECTRONIC FUNDAMENTALS

OP AMP SUMMING AMP

When connected as multi-input inverting amplifier (see previous topic on feedback), an op amp can be used to add a number of v oltages, either a.c. or d.c.

In the above circuit, 3 input voltages, Vin 1, Vin 2 and Vin 3 are applied through resistors R1, R2 and R3 respectively. Hence:

Hata! = Hata! + Hata! + Hata!

V0ut

= - Hata!

Thus the input voltages are added and amplified if Rf is greater than each of the input resistors. If R1 = R2 = R3 R3 = Rin, Rin, the input voltages are amplified equally and

V0ut

= Hata! (Vin Hata! (Vin 1 + Vin 2 + Vin 3)

If R1 = R2 = R3 = Rin = Rf then

V0ut

= (Vin 1 + Vin 2 + Vin 3)

The output voltage is the sum of the input voltages but is of opposite polarity. This device can be used as a digital to analog converter by making R2 twice the size of R1, and R3 twice the size of R2. If a 3 bit digital word word is then be applied applied to the resistors, with the least significant bit applied to R1 and the most significant bit applied to R3, the output will be the analogue equivalent of the binary word. Issue 1 - 02 October 2002

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ELECTRONIC FUNDAMENTALS

PRINTED CIRCUIT BOARDS

The assembly of the various circuits which form part of the units employed in aircraft electronic systems, necessitates the t he interconnection of many components by means of electrical conductors. Before the introduction introduction of printed wiring, these conductors conductors were formed by wires which connected to the components either by soldering, or by screw and crimped terminal methods. In the development of circuit technology, micro-miniaturisation, rationalisation of component layout and mounting, weight saving, and the simplification of installation and maintenance become essential factors; and as a result, the technique of printing the required circuits was adopted.

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In this technique, a metallic foil f oil is first bonded to a base board made from an insulating material, and a pattern is then printed and etched on the foil to form a series of current conducting paths, the pattern replacing the old method or wiring. Connecting points and mounting pads, for the soldering of components appropriate to the circuit, are also formed on the board, so that, as a single assembly, the board satisfies the structural and electrical requirements of the unit which it forms a part. If the circuit is a simple one, the wiring may be formed on one side of a board, but, where a more complex circuit is required, wiring is continued on to the reverse side, which also serves as the mounting for components. In addition, complex complex circuits may be incorporated in multi-layer assemblies. 3.1

BASE MATERIAL

The base material, or laminate as it is sometimes called, is the insulating material to which the conducting material material is bonded. The base material also serves as a mounting for the the components which which comprise the circuit. The base material material is commonly made up either of layers of phenolic resin impregnated paper, or of epoxy resin impregnated fibre glass cloth which has been bonded to form a rigid sheet, which can be readily sawn, sawn, cut, punched or drilled. The thickness of the base material depends on the strength and stiffness requirements of the finished board, which, in turn are dictated by the t he weight of the components to be carried, and by the size of the printed conductor area. 3.2

CONDUCTOR MATERIAL

The most commonly used conducting material is copper foil, the minimum purity value of which is 99.5%. 3.3

BONDING OF CONDUCTOR MATERIAL

For the manufacture of a typical circuit c ircuit board, the base material and copper foil are cut into sheets, and are then inspected and assembled inside a clean room in alternate layers with stainless steel separator plates (known as cauls) interposed between the layers, as shown shown below. The steel plates, which are accurate in thickness to within 0.001 inch, are very hard, and have a delicately grained surface which is imparted to the finished boards.

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The layered sheets (the assembly) are then passed out of the clean room to be bonded in a hot hot press. During the pressing operation, operation, the heat melts the resin resin in the base material , so that it flows and fully wets out the material and the copper foil. The pressure applied is adjusted so as to exclude all air and vapour from any residual volatiles. As polymerisation of the resin mix proceeds, proceeds, each layer of the base material reaches the fully cured state s tate with the copper foil firmly bonded to it.

 After cooling has taken place, the individual copper-clad boards are trimmed to the required size, inspected, and packed in sealed polythene bags. 3.3.1 INSPECTIONS & TESTS

 After manufacture, all boards are inspected, and tests are carried out on selected samples, in accordance accordance with the relevant specifications. Tests will include: of appearance appearance • Inspection of • Checks on thickness • Measurement of bow and twist

of the foil • Measuring the peel strength of • Checking the heat resistance by solder • Measurement of pull-off strength •  Electrical tests

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MACHINING OF BOARDS

 All boards require machining, e.g. guillotining, sawing, punching, punching, and drilling during the various stages of production. Guillotining is Guillotining is one of the quickest and most economical methods of cutting sheets of copper-clad laminates into strips and panels, and it is frequently employed in conjunction with subsequent subsequent punching operations. Correctly performed guillotining results in a clean, burr-free burr-fr ee edge, with no wastage of stock. Cutting with Cutting with a circular saw is superior to guillotining as it gives a cleaner edge, especially so as the thickness of the laminate laminate increases. Wood-cutting machinery is satisfactory for laminates. The type of resin, base material and the degree of cure, are the main factors affecting the drilling characteristics drilling characteristics of a laminate. All laminates are abrasive particularly those with glass fibre base material, and drilling techniques should be adapted to suit. Where large quantities of laminates are required, and cost of tools is acceptable, punched parts can be produced by conventional pierce and blank methods, such methods are most commonly adopted for copper-clad phenolic / paper base laminates. 3.5

CIRCUIT ARTWORK

The quality of a printed wiring board is, in the fist instance, dependent on the production of master artwork which must show precisely the circuit conductor pattern required, where components are to be located, circuit module designations and other essential essential references. Artwork production requires requires the use of dimensionally stable base materials, and the application of s killed drafting techniques, because, unlike conventional electrical drawings, which are used as a guide to the build-up of an assembly of wiring and connections, a printed wiring board is an actual reproduction of the original artwork produced for it. Human error in drafting can be reduced, and, in c ertain cases, eliminated, by the use of numerically-controlled drafting machines. These are accurate X and Y coordinate plotting machines which are capable of automatically plotting a point, or line, on a surface whether it be on a film or glass base.

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ELECTRONIC FUNDAMENTALS

PRINTING OF CIRCUITS

The printing of circuits is carried out using either an etching process or an additive process. Both of these processes, are briefly described described in the following following paragraphs. 3.6.1 ETCHING PROCESS

In this process the copper foil is first cleaned, either chemically or mechanically, and is then coated with a photo-sensitive solution known as a 'resist', which has the property of becoming soluble when exposed to strong light.  A photographic positive of the circuit artwork is then placed over the sensitised board and time-exposed in a special printing machine. machine. After exposure, the resist is washed away to leave unprotected areas of copper copper around the circuit pattern. The board is dried by a clean, oil and water free air blast. The complete board is then inspected to ensure that no resist has been removed from any part of the conductor pattern itself, and that no resist particles are present in areas which are to be etched away. The board is then placed in a bath which which contains an etching solution, solution, such as ferric chloride or ammonium persulphate, which etches away all the unprotected copper.

When the etching process has been satisfactorily completed, the board is thoroughly washed in water in order to remove all traces of etching solution, and is then dried and given a final inspection.  As printed circuit boards with the same circuit pattern are often required in large numbers, the simple 'print and etch' process is generally superseded by a screen printing process.

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3.6.2 ADDITIVE PROCESS

In this process, copper is deposited only in the areas where conductors are required. To achieve this the base material is pre-coated with a suitable adhesive, the circuit holes are pre-fabricated, and the board is sensitised with a photo-resist solution. A negative of the circuit pattern is then screen printed onto the board so that the exposed areas define the conductor network. network. These exposed areas areas are chemically activated, and the board is then immersed in an electrolyses copper plating solution. After a period of time consistent with with the deposition of the the required thickness of copper, the board is removed from the bath. The major advantages advantages of the additive process are: no chemical etching takes place, thereby eliminating wastage of copper, the thickness of the deposited copper can be reduced and made more uniform, the conductor widths and spacing are less restricted, and the hole diameter can be reduced, thereby increasing the board area available for routing of conductors.

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3.6.3 INSPECTION

 After printing, circuit patterns are inspected with particular attention being paid to the following: Condition of the Edges of Conductors Conductors • Dimensional Accuracy and Condition • Condition of the Pattern Surfaces • Particles of Copper in Unwanted Areas •  Insulation Areas • Lack of Resin Bond in etched Areas

3.7

SOLDERING METHODS

There are two main methods of soldering employed in connection with printed circuits boards, (a) hand soldering and (b) mass soldering. 3.7.1 HAND SOLDERING

This method is used for soldering joints separately, e.g. in limited batch production, and when a component or a wire is replaced after a test or a repair has been carried out. This method involves the use use either of electrically heated hand hand irons, or of resistance type hand tools when the use of these is permitted. 3.7.2 MASS SOLDERING

In this method, all joints of a finally assembled board are soldered simultaneously, by bringing the board into contact with an oxide-free s urface of molten solder, which is contained in a special type of bath. Mass soldering may be carried carried out in any one of five different ways: Flat or Static Dipping Dipping - one edge of the board is first lowered on to the solder and the other edge is then lowered slowly to allow f lux and solvent vapour to escape. t he bottom of the solder bath through a Wave Soldering – Soldering – solder is pumped from the narrow slot, so that a symmetrical 'standing wave' of solder is produced across the width of the bath. The circuit board after being being fluxed, is then either manually manually or automatically passed against the crest of the solder wave by a conveyor.

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Weir and Cascade Soldering Cascade Soldering systems are of the moving solder type, the solder flowing down a trough by gravity, and then being returned to the main bath by a pump. In weir-soldering (diagram (a)) a circuit board is lowered on to the solder; while in cascade soldering (diagram (b)) a board is conveyed across the crests of solder waves in a direction opposite to the solder flow.

als o known as 'heat cushion' soldering. Reflow soldering is soldering is an automated process also It is applied particularly to circuit boards on which microcircuits and associated devices are to be be assembled. These efficient but but costly components require require a special soldering technique, so that their f ull potential as surface-mounted devices can be realised. The reflow technique is generally recognised as the best method, since the soldered joints are easier to t o inspect and to remake when a faulty component has to be replaced. In addition, soldering times and the risk of overheating sensitive components are reduced, and distortion of leads is prevented. The sequence of reflow soldering is shown in the diagram on the following page. The leads of the circuit or component and the relevant lands on the circuit board, which have been pre-tinned by such methods as wave s oldering or dip soldering, are first brought into contact with each other and accurately aligned. aligned. The sequence is then initiated by lowering the electrode on the lead to be soldered.

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Shortly before the electrode makes contact with t he lead, the pre-set heating power is automatically switched on. The electrode is then pressed on to the lead lead under a load which gradually increases until until the pre-selected value is reached. reached. The solder melts, and in reflowing, it forms f orms a 'cushion' through which the lead is pressed against its corresponding land of the circuit board. As soon as the cushion is formed, the timing device cuts off off the heating supply. After a 0.75 second delay, delay, an air blast is delivered to cool the soldered joint, this accelerates the completion of the soldering process, and also improves the quality of the joint. At the end of the cooling period, the load is relieved, and the electrode is automatically raised ready for the next operation.

3.8

SOLDER SPECIFICATION

For the mass-soldering of printed wiring boards, solder complying with BS 219 Grade K (60/40 tin/lead) is the one most commonly used, since it has a free-flow characteristic which permits good joint formation in the short period during which boards are in contact with the solder. The solder temperature is chosen for each individual combination of board and types of material being processed, but it should normally be within the range 220 °C to 260°C. 3.9

FLUXES & THEIR APPLICATION

To assist in the wetting of surfaces by molten solder, a flux must be used both to prevent oxidation during joint formation, and to dissolve the thin oxide films which may already be present on the surfaces which are to be joined, and on the solder itself.

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3.10 SOLDER RESISTS There are organic coatings which are designed for use on both rigid and flexible printed circuit, to mask mask off those areas where soldering is not required. Some important advantages of the use of solder resists are as follows: •

Elimination of bridging and icicling between between closely spaced conductors and mountings.

•

Protection is afforded against corrosion and contamination during storage, handling subsequent life of the circuit.

•

Flexibility of circuit patterns is maintained maintained since a resist flexes with the conducting material.

•

The surface resistance values of the circuit patterns patterns are improved.

•

Minimising of solder contamination from large surfaces of copper and other plated materials, thereby maintaining a high level of solder purity and an extension of bath life.

•

Heat distortion is minimised, minimised, since a resist acts as a heat barrier.

3.11 PLATING OF PRINTED PRINTED WIRING CIRCUITS Plating finishes for printed wiring circuits circ uits are used as aids to the performance of circuits under specific conditions of use, and are are not intended to be decorative. The choice of finish is, therefore, governed strictly by the functional and environmental conditions in which the circuit will be used. In many cases, the different parts of a circuit may be subjected to different conditions of use, and provided there is cleat demarcation between these parts, they can be plated with the appropriate finishes.  A typical example of this differential plating method, is a circuit that is tin/lead plated for solderability over the component area, and nickel/gold plated for durability on edge-connector finger contacts. 3.11.1 THROUGH-HOLE PLATING

Through-hole plating is a process which is widely employed to provide a conducting surface in the holes of single-sided si ngle-sided and double-sided boards, and also to provide a land or pad for the connection of components.

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3.12 ORGANIC PROTECTIVE PROTECTIVE COATINGS  After printed wiring boards have been manufactured, manufactured, organic coatings are applied to their surfaces, to protect them from oxidation and and contamination. contamination. The coatings coatings vary, depending on whether temporary protection is required, e.g. for maintaining clean copper surfaces during normal handling prior to soldering, or, whether permanent protection is to be applied after soldering for protecting the circuit and components form subsequent environmental contaminants. For temporary protection the coating is usually of a resin-based type which does not require removal before before soldering, since it also serves as a flux. Permanent protective coatings are usually epoxide or polyurethane-based resin, having exceptionally low oxygen absorption, high humidity resistance, and resistance to cracking and discoloration. 3.13 FLEXIBLE PRINTED WIRING CIRCUITS Unlike rigid printed wiring boards, flexible circuits serve only as a means of interconnecting units, particularly those which r equire to be moved relative to each other, and those which may may be mounted in different different planes. Flexible circuits also permit easier assembly and higher density packaging of units. Flexible circuits are laminated form, consisting of a flexible base insulation material (e.g. polyester, epoxy glass cloth and polyimide) copper foil, and an insulating coverlay of the same material as the base.

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3.14 HANDLING OF CIRCUIT BOARDS 3.14.1 ELECTROSTATIC DISCHARGE SENSITIVE SENSITIVE DEVICES

Many electronic Line Replaceable Units (LRU's) on aircraft contain printed circuit boards containing components that are susceptible to damage from electrostatic discharges. Such components components are referred to as electrostatic discharge sensitive sensitive (ESDS) devices. Decals installed on ESDS LRU's, indicate that special handling is required. Some decals are shown below, the lower four four are typical Boeing ESDS decals. 3.14.1.1

Static Electricity & Electrostatic Discharge

The most common conception of static electrici ty and its accompanying discharge, is the miniature lighting shock you receive when you touch a metal door handle having walked across a nylon carpet. If the door handle is touched with a key first, the discharge will be seen but not felt. f elt. The discharge occurs because different materials receive different levels of charge as materials are rubbed together or pulled apart. apart. The different charge levels levels create potential differences between the different materials, and when materials of different electrical potential are brought into close proximity with each other, a discharge occurs as the potentials equalise. The different levels of charge with respect to cotton (the reference material) are shown on the following page, in what is known as the Triboelectric Series. The further up or down, the greater the charge and hence the greater the discharge when the two materials are brought together. Issue 1 - 02 October 2002

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Triboelectric Series Material  Air Human Hands  Asbestos Rabbit Fur Glass Mica Human Hands Nylon Wool Fur Lead Silk  Aluminium Paper Cotton Steel Wood  Amber Sealing Wax Hard Rubber Nickel Copper Brass Silver Gold Platinum Sulphur  Acetate Rayon Polyester Celluloid Orion Saran Polyurethane Polyethylene Polypropylene PVC (vinyl) Kelf (ctfe) Silicon Teflon Issue 1 - 02 October 2002

Increasingly Positive

Increasingly Negative

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The typical voltages that can occur are shown in the table below, note the importance of humidity. Electrostatic Voltages Means of Static Generation

10 to 20 Percent Relative Humidity

65 to 90 Percent Relative Humidity

Walking across carpet

35,000

1,500

Walking over vinyl floor

12,000

250

Worker at bench

6,000

100

Vinyl envelopes for work instructions

7,000

600

Common poly bag picked up from bench

20,000

1,200

Work chair padded with polyurethane form

18,000

1,500

The last table shows a list of static sensitive devices and the voltages that can cause damage. The damage may vary from a slight degradation of performance, giving rise to intermittent and spurious indications, to complete destruction, giving rise to total system failure. The amount of damage varies with the amount of energy that strikes the component. The less obvious damage can cause considerable and expensive maintenance headaches which may lead to lack of confidence in the equipment.

Static Sensitive Device

Sensitivity Range where damage can occur

Field Effect Transistor (MOS / FET)

150 - 1000 volts

CMOS

250 - 1000 volts

Bipolar Transistors

4,000 - 15000 volts

Silicon-Controlled Rectificers (SCR)

4,000 - 15000 volts

Thin-Film Resistors

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150 - 1000 volts

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3.14.2 REMOVAL & INSTALLATION INSTALLATION OF ESDS PRINTED CIRCUIT BOARDS BOARDS

Equipment and Material •

Conductive bags

•

Wrist straps

•

100% cotton twine - commercially available

•

ESDS Labels

Removal of Boards 1. Remove system electrical power.

2. 3. 4. 5.

Warning: use only wrist straps straps with a grounding grounding lead resistance resistance of greater than 1 megohm. Inadvertent contact contact between a low resistance resistance wrist strap strap and a high voltage, is a shock hazard to personnel. Connect wrist strap assembly to a convenient ground on component containing PC board and to skin of person removing PC board. Gain access to printed circuit board. Remove printed circuit board using extractors provided. Immediately insert static sensitive board into a conductive bag and identify with an ESD label. Use an ESDS label label or 100% cotton twine to close the conductive bag.

Caution: Do not use staples staples or adhesive adhesive tape to close conductive bag. Damage to bag will expose contents to electrostatic discharge. 6. Close and secure unit unless replacement card is to be installed immediately. 7. Disconnect wrist strap from ground and operator. 8. Place bagged printed circuit card in a rigid container to maintain integrity of conductive bag during transportation. Installation of Boards 1. Check that system electrical power is off. Warning: use only wrist straps with a grounding lead resistance of greater than 1 megohm. Inadvertent contact contact between a low resistance resistance wrist strap strap and a high voltage, is a shock hazard to personnel. 2. Connect wrist strap assembly to a convenient ground on unit where the printed circuit board is to be installed and to skin of person installing PC board. 3. Gain access to receptacle that PC board is to be installed into. 4. Remove static sensitive PC board from conductive bag.

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Caution: Do not touch connector connector pins or other exposed conductors. Damage to components can result. provided. Lock extractors. 5. Install PC card in position using extractors provided. 6. Close and secure unit. 7. Disconnect wrist strap. 3.14.3 REMOVAL & INSTALLATION OF METAL-ENCASED METAL-ENCASED ESDS LRU'S LRU'S

General 8. Metal-encases ESDS units can be either rack mounted, panel mounted or bolted on. Equipment and Material 9. Dust caps. or anti-static dust caps should be used when available. If Note: Conductive or conductive or anti-static dust caps caps are not available, non-conductive dust caps may be used but with caution, since si nce they do not provide complete ESDS protection during handling. Remove metal encased LRU's with ESDS labels 10. Remove system electrical power. 11. Remove ESDS labelled unit from rack, panel, or mounted position. Caution: Do not touch connector connector pins or other exposed conductors. Damage to components may result. 12. Install dust caps on all connectors. Do not touch electrical pins in connectors. on the unit being Note: Dust caps from unit being installed may be used on removed. 13. Transport unit per standard practices with dust caps installed. Install metal encased LRU's with ESDS labels 14. Check that system electrical power is off. Caution: Do not touch connector connector pins or other exposed conductors. Damage to components may result. 15. Remove all dust caps from connectors of of unit being installed. Do not touch electrical pins in connectors. 16. Place unit in position and secure.

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SYNCHRONOUS DATA TRANSMISSION

Synchronous data transmission systems are designed to indicate the position of a component or control control surface that cannot cannot be directly observed. observed. The systems fall into one of two categories; d.c. systems called 'Desynn Systems' and a.c. systems which are generally grouped under the heading of 'Synchro Systems'. Both a.c. and d.c. systems sys tems comprise two main components, a transmitting element and a receiving element. element. The two being interconnected by wiring that provides the the signal path. The word 'synchronous' means means 'happening at the same time', which infers that when the transmitter is moved, the receiving element, normally an indicator, will follow that movement instantly. 4.1

DESYNN SYSTEM

There are a variety of different types of Desynn systems available: The Basic Desynn is Desynn is generally operated by a rotary motion, however linear versions are also found. The conversion from linear to rotary motion being achieved achieved by a push rod and gear wheel. The Micro Desynn was s mall movement obtained by such Desynn was designed to magnify the small items as pressure measuring measuring devices. They are operated operated by linear motion. The Slab The Slab Desynn was Desynn was designed to overcome signally errors inherent in the basic Desynn system. In the vast majority of instances the the errors in the basic Desynn could be considered insignificant. 4.1.1 THE BASIC DESYNN 4.1.1.1

Construction

In the basic Desynn system sys tem the transmitter comprises an endless resist ance wound on a circular former, this arrangement being referred to as a 'Toroidal Resistance'. Equally spaced at 120 ° intervals around the resistor are 3 tappings, it is to these that the signal wires are connected. connected. Running on the resistor are two wiper arm type contacts that are spaced apart by 180° and insulated from one another, it is to these that system power is applied.

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The indicator comprises a two pole permanent magnet rotor, pivoted to rotate inside a soft iron stator, the pointer being attached to the spindle. The stator carries three star connected windings that are connected to the t hree wires coming from the tappings of the transmitter.

4.1.1.2

Operation

When dc power, is applied to the wiper arms of the transmitter, current will enter the positive wiper arm and divide to flow in both directions, left and right, around the torroidal resistor. Both halves of the resistor have the same resistance, therefore the current in each path will be equal. The resistance of the resister varies linearly. linearly. That is, the change change in resistance for for every degree of movement around it will be the s ame, therefore when 28 volts is applied to the system as shown in the diagram, the voltage at tapping 2 will be approx. 9.3 volts, as will the voltage at tapping 3. The voltage at tapping tapping 1 will be 28 volts. Issue 1 - 02 October 2002

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The differences in potential at the three tappings cause c urrents to flow in the wires that connect to the receiver. The flow of current creates magnetic magnetic fields around the 3 stator windings in the receiver, which combine to produce a resultant field across the stator. The permanent magnet aligns with the resultant resultant stator field, in turn aligning with the wiper arms of the transmitter. tr ansmitter. If, for ease of explanation, the transmitter tr ansmitter wiper arms are rotated by 120 ° clockwise, the potential at tapping 2 will increase to 28 volts, the potential at tapping 3 will remain the same at 9.3 9.3 volts and that at tapping tapping 1 will decrease to 9.3 9.3 volts. Current will now flow out of the transmitter tr ansmitter at tapping 2 into the indicator at terminal 2, through the first winding where it will div ide equally, half returning to the transmitter tr ansmitter via terminals 1, the other half half via terminals 3. The resultant field now produced across the stator will be in line with stator coil 2, this will cause the permanent permanent magnet rotor, band pointer, to swing around 120 ° clockwise to once again align with the wiper arms of the transmitter. Irrespective of the position of the wiper arms in the transmitter, the current flow between transmitter and receiver will always create a field across the stator that aligns with their position. 4.1.1.3

Fail Safe Devices

 A problem with the Desynn as shown, is that should the d.c. power to the system fail, the pointer will remain in its last position. This is not a satisfactory situation, the instrument should 'fail safe', that is it should respond in such a way that the fault will be identified. This is achieved by fitting a small permanent permanent magnet in the indicator. Under normal operation, the field of the permanent magnet is weak in comparison to the fields produced by the coils and therefore has no no effect. When power is removed, the small permanent magnet attracts the permanent magnet rotor, moving the pointer off scale.

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4.1.2 SLAB DESYNN

If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360 °, it will be seen that they produce a sawtooth waveform as opposed to a sinewave. This results in the pointer pointer of the indicator not following the transmitter exactly. In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.

The solution is to use a modified Desynn transmitter called called a 'slab Desynn'. In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120 °. The output from this device is a sinewave. It can be connected to the same type of indicator and and operates in the same way as the basic Desynn. 4.2

SYNCHRO SYSTEMS

Synchro's are electromagnetic devices used to transmit positional data electrically from one location to another. another. They have an advantage advantage over Desynn's in that they can also be used to compute the sum of two rotations, or the difference in angle between them. They are used in applications applications requiring low output output torques. Servo systems, which will be examined in the next section, employ synchros in conjunction with an amplifier and a controlling motor to pr ovide an automatic control mechanism. They are used in applications requiring output torque's torque's greater than those which can be produced by a synchro.

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4.2.1 SYNCHRO TYPES

Synchro types may be classified as follows: •  Torque transmitter •  Torque receiver • Torque differential receiver • Torque differential transmitter •  Control transmitter •  Control transformer • Control differential transmitter •  Resolver 4.2.1.1

Torque Transmitter - TX

Used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and and the stator kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. The TX is generally used as the transmitting element in a remote position indicating system. 4.2.1.2

Torque Receiver - TR

The rotor of a torque receiver, which is i s free to turn, moves to a position dependent on the electrical angular information received from its connected torque transmitter or torque differential transmitter. The TR is generally used as the receiving element (indicator) in a remote position indicating system. 4.2.1.3

Torque Differential Receiver - TDR

The torque differential receiver is electrically connected to two torque transmitters. The rotor of the TDR, TDR, which is free to move, aligns with the stator field. The position of the stator field depends on the inputs from the two transmitters, and the way in which they are interconnected. By suitable connection, the TDR can be made made to indicate the sum of the transmitter inputs, or the difference between them.

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Torque Differential Transmitter - TDX

The torque differential transmitter has a stator that receives electrical positional information from a torque transmitter, and a rotor which is mechanically positioned. This enables it to transmit electrical electr ical information corresponding to the sum, or difference, between the electrical input and its own rotor angle. 4.2.1.5

Control Transmitter - CX

Used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and and the stator kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. The TX is generally used as the position transmitting element in a remote position control system. 4.2.1.6

Control Transformer - CT

 A CT is electrically connected to a CX and is used to produce an electrical signal for driving a servo system. The electrical signal produced, is an a.c. voltage with an an amplitude and phase dependent on the position of the rotor relative to the stator. 4.2.1.7

Control Differential Transmitter - CTX

 A CTX receives electrical information from a CX and has a rotor which can be mechanically moved. This enables it to transmit an electrical signal proportional to the sum or difference in angle between the electrical i nput and its own rotor position. 4.2.1.8

Resolver

 A resolver has two mutually perpendicular windings on the rotor and another another two on the stator (4 windings in total). total). It can resolve an input signal signal into its sine and cosine components, perform the operations of vector addition and subtraction or convert polar to cartesian co-ordinates and vice versa.

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4.2.2 SYNCHRO SCHEMATICS

 All synchros are represented by the same basic schematic symbol which may be drawn in any one of three different ways: This is the simplest and possibly the most commonly used representation in maintenance manuals. The code code letters are inserted in the centre circle to identify the type and function of the synchro.

Used when an explanation is given of the operation of a synchro. The schematic schematic shows the rotor in the zero degree position.

This is now commonly used when an explanation is given of the operation of a synchro.

Note: By convention, the vertical winding in the last 2 schematics is identified as S 2, the lower right as S1 and the lower left as S3.

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Differential transmitter and receiver synchros can be represented schematically by any one of the following symbols.

The resolver synchro can be represented schematically by any one of the following symbols.

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4.2.3 XYZ SYNCHRO SYSTEM

Synchros often appear in aircraft wiring and sc hematic diagrams with the letters X, Y, Z indicating the free end of each stator winding and the letters H and C indicating the ends of the rotor. S1 ----- X S2 ----- Z S3 ----- Y R1 ----- H R2 ----- C When connections to earth are required, the stator wir e designated S2 or Z is earthed and the C end of the rotor winding is earthed. 4.2.4 SYNCHRO SUPPLIES SUPPLIES

Synchros used in aircraft data transmission systems are operated from either 115V 400Hz or 26V 400Hz alternating current supplies. Radio systems commonly employ 26V 400Hz.

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4.2.5 TORQUE SYNCHRO SYSTEM

Torque synchro systems are used where the turning t urning force or torque required is very small. The system only produces sufficient torque torque to move a pointer over a scale, or to operate a micro switch, because of this they are limited to indicating systems. 4.2.5.1

Construction

The torque synchro system comprises a Torque transmitter (TX) and a Torque Receiver (TR) interconnected as shown below.

In practice: •

R2 and S2 will be connected to earth.

•

The rotor of the TX will be mechanically mechanically rotated by suitable means appropriate appropriate to the system whose positional information has t o be transmitted.

•

A pointer, pointer, which will indicate the transmitted transmitted data, is normally attached to the rotor of the TR.

The ac power supply is connected to both rotors, the rotors being connected in parallel.

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Operation

With supply current flowing, voltages are induced is the stator winding of both the TX and TR by transformer transformer action. With the rotors in the same same angular position, as shown in the diagram, the voltages in the TX and TR will be equal and opposite, hence no current will flow in the stator coils and interconnecting interconnecting wires. The system is said to be balanced or balanced or nulled. nulled. The voltage induced in the stator coils will depend on: •

The ratio of the number of turns on the rotor to the number of turns on the stator and,

•

the angular position of the rotor with respect to the stators.

For the position of the rotors shown s hown in the diagram, the voltages induced in the stators of both transmitter and receiver would be: •

S1

•  S2 •

S3

half maximum voltage maximum voltage half maximum voltage

If the transmitter rotor is i s rotated through any angle, the voltages induced in the stator coils of the TX will change. change. The voltages induced in the stator coils of the TR will remain unchanged. unchanged. This creates potential potential differences across the interconnecting wires, and current flow in them. The current flows produce magnetic fields around the stator windings which combine to f orm a resultant field across the t he stator of both the TX and TR.  A torque reaction will now exist between the resultant stator field and the field that exists around the rotor. This torque reaction will exist at both both the TX and TR. TR. The rotor of the TX is held by the system whose positional information has to be transmitted and cannot move. move. The rotor of the TR is however free to rotate and and moves around in response to the torque. Once the TR rotor is in the same angular position as the transmitter rotor, the voltages induced in the stators will again be equal and opposite, current will cease to flow and the system will once again be balanced. To ensure accuracy of the system there must be sufficient current flow to produce a torque even for small changes changes in transmitter position. This requires the impedance impedance of the windings to be be very small. Under normal operating operating conditions this is of no concern, however, should the receiver pointer jam then a large potential difference would exist between the TX and TR with with resulting high currents. This can easily lead to one or both of the synchros s ynchros burning out.

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Synchro System Faults

 A loss of supply to the TR rotor will result in Low Torque operation with possible 180° error.  A loss of supply to the TX rotor will result in no operation of the synchro.  An open circuit on one stator line will result in the receiver oscillating between 2 points approximately 75 ° apart.  A short circuit between 2 stator lines will result in the receiver being displaced by 0 °, 60°, 120°, 180°, 240° or 300° and movement in 180 ° steps. The table below shows the results or effects of a number of cross connections. c onnections.

Cross Connections

Fault Symptoms

S1 and S2 Reversed.

Receiver indicates 120 ° and rotates in opposite direction to transmitter.

S2 and S3 Reversed.

Receiver indicates 240 ° and rotates in opposites direction to transmitter.

S1 and S3 Reversed

Receiver indicates correctly but rotates in opposite direction to transmitter.

R1 and R2 Reversed

Receiver indicated 180° error but rotates in same direction as the transmitter.

R1 and R2 Reversed and S1 and S2 Reversed or R1 and R2 Reversed and

Receiver indicates 60 ° error and rotates in opposite direction to transmitter.

S2 and S3 Reversed R1 and R2 Reversed and S1 and S3 Reversed

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Receiver indicates 180 ° error and rotates in opposite direction to transmitter.

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4.2.6 ELECTRICAL ZERO

The electrical zero setting of a synchros provides a standard means of aligning synchros units so they will all have the same position at the same instant. This setting provides a common reference point at which all synchros are set before being installed.

Electrical zero is defined as the position of the rotor with respect to its stator when the voltage between S1 and S3 is zero and the voltage at S2 with respect to S1 or S3 is in phase with that that of R1 with respect to R2. R2. It simply means that the rotor is parallel to S2 and that R1 is at the top. By connecting the voltmeters as shown shown electrical zero can be determined. V1 should indicate zero and V2 should indicate indicate a value less than the supply supply voltage. Remember that if R2 were at the top V1 V1 would still indicate zero but, if the voltage between R1 and S2 would be antiphase and V2 would indicate a value greater than the supply voltage.

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4.2.7 DIFFERENTIAL TORQUE SYNCHRO SYNCHRO SYSTEM

 A differential synchro system consists of a differential synchro used in conjunction with a synchro transmitter transmitter and receiver. It is electrically connected as shown in the diagram below.

The first thing to notice is that the rotor of the differential synchro has three equally spaced windings and is connected to the transmitter and and receiver stators. When connected as shown it will provide an output which is the difference between the two inputs from the mechanical drives. It can also be wired to produce an addition addition of the two inputs. There is no connection between between the differential synchros and the supply. 4.2.7.1

Operation

Consider the differential synchro to be three 1:1 transformers between the three stator windings of the transmitter and the t he three stator windings of the receiver.

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When the system is set s et as shown in the diagram above, (the interconnecting wires have been removed for clarity), the induced voltages in the stators and across the transformers will be equal and no current will flow in any of the interconnecting wires. If the transmitter (on the left) is turned by 60º, the TX stator voltages will change and current will flow around the stator windings. Resultant fields will be set up and the TR rotor will feel torque, so the rotor will turn until, again, the voltages are equal and current stops flowing.  An important thing to remember is that all three all three components feel the torque reaction created by the interaction of rotor and stator st ator fields, but because the transmitter rotors are mechanically connected to other systems they will not be free to move. Only the receiver rotor (on the right) is free to respond. If the TX is left stationary and the TDX is rotated by 15º the voltages will be different and current will flow around around the stator windings. A torque reaction will occur and and the rotor on the receiver will turn until the voltages are equal and current s tops flowing. It should be noted that when the TDX is wired as shown, clockwise rotation of the TDX results in anticlockwise rotation of the TR.

If both the TX and the TDX were rotated then the TR would show the difference between the two movements.

The differential synchro need not not always be a transmitting device. The system could be arranged with two transmitting synchros and a TDR with a pointer attached. Under these conditions, conditions, the torque differential receiver (TDR) is the receiving element, but the system will respond as previously described to show the difference in the two inputs. Issue 1 - 02 October 2002

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Sum of Two Inputs

If the S1 and S3 connections between the TX and TDX are crossed and the S1 and S3 connections between the TDX and TR are also crossed, the system will algebraically add the two mechanical inputs. i.e.

TX 30° clockwise TDX 15° clockwise moves the TR 45 ° clockwise. TX 30° anticlockwise TDX 15° clockwise moves the TR 15° anticlockwise.

 A similar arrangement of cross connections could be used on a TDR type system to make the TDR show the sum of two inputs. 4.2.8 CONTROL SYNCHRO SYSTEM

Control synchros are used in electromechanical servo and shaft positioning systems. They only produce produce a signal representative of the position of the transmitter. This signal can then be amplified many times to power power very large motors that can move very large loads to a desired position. 4.2.8.1

Construction

In construction, control synchros are similar to torque synchros but because they do not have to handle any motive power for driv ing a load they may be of lighter construction. Also, because the signal from the receiver is going to be amplified to drive an output, the impedance of the windings can be made much higher and there is no danger of the system burning out. The control synchro system is the most common of all synchros and has extensive use in aircraft instrument and navigational systems.

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Operation

In a control synchro system sy stem the ac power supply is only connected to the rotor of the transmitter, the CX. The signal representing representing the position position of the transmitter is obtained from the rotor of the receiving element, the CT. Note that in the balanced balanced or nulled position, the rotors of the CX and CT are at 90 ° to each other. When the rotor of the CX is in the position shown, maximum voltage is induced in stator S2 and half maximum maximum voltage is induced in stator stator windings S1 and S3. S3. No emf's are induced in the stator windings of the CT, therefore a potential difference exists between each stator winding of the CX and CT and currents flow in the transmission wires. The current flowing in the CT stator windings produce magnetic fields that combine to form a magnetic field across the stator. This alternating field cut's the rotor winding. The emf induced in the CT rotor winding depends on the position position of the rotor relative to the resultant field. When the rotor winding is parallel to the resultant field, maximum voltage is induced in it, when the rotor is at 90 ° to the resultant field, zero emf is induced in it. The amplitude of the induced emf is proportional to the s ine of the angle between the rotor and resultant field. The phase of the induced emf depends depends on whether the rotor is clockwise or anticlockwise anticlockwise of the balanced balanced or nulled nulled position. The control transformer can therefore be considered as a null detector and is most often used in servo systems.

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engineering

CT Rotor Position

EMF Induced in CT Rotor

90° clockwise

90° anticlockwise

5° clockwise

5° anticlockwise

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Control Synchro Servo System

The diagram below shows a control synchro sy stem employed in its most common c ommon role as part of a servo system.

 As shown, the system is balanced, zero emf is induced in the CT rotor, there is no output to the servo motor and the motor and pointer are stationary. If the rotor of the CX is now moved clockwise, the resultant field across the stator of the CT will also move around around clockwise. The rotor of the CT CT is now no longer longer at 90 ° to the resultant field and therefore has an emf induced in it. The emf is applied to a discriminator amplifier to sense its phase relationship to the excitation supply, to obtain direction information, information, and then applied applied to the motor. The motor turns, driving the pointer and at the t he same time driving the rotor of the t he CT towards the balanced position (90 ° to the resultant field). When the rotor is at 90 ° to the resultant field, the induced emf falls to zero and the motor stops, the pointer having moved to indicate the new position. If the rotor of the CX had been moved anticlockwise, the error signal in the CT rotor would have been of opposite phase and the motor would have turned in the opposite direction to once again null or balanced the system.

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4.2.9 DIFFERENTIAL CONTROL SYNCHROS

These are in common common use. Their operation is the same as for Torque differential differential synchros, and they can also be wired to produce an electrical signal proportional to the sum or difference between two inputs.

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ELECTRONIC FUNDAMENTALS

SERVO SYSTEMS

Servomechanisms are a type of of automatic control system. The action of the output output in slavishly following the demands demands of the input gives the system its name. name. (Servus is the Latin name for slave). Human operators are incapable of providing the degree of precision necessary to operate complex machines requiring fast and accurate control. They are also also limited in the amount of power they can apply apply to a load. Servomechanisms provide the precise control and power that humans are unable to provide. Servomechanisms, or Servo’s possess the following properties: are error activated. • They are amplification. • They have power amplification. • They contain moving parts. • They are automatic in operation.

5.1

CATEGORIES OF SERVO SYSTEMS

Servomechanisms can be classified according to two main categories: • Open loop systems. • Closed loop systems. 5.1.1 OPEN LOOP

In an open loop system, system, the input demand generates demand generates an electrical equivalent of the demand position. This signal is amplified to the required power level and applied to a motor to position the load. The speed of response response and the final position position of the load depend on the following factors: •

Any variations in load conditions.

•

Frictional forces within the motor and its load, load, and and any mechanical interconnections.

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•

Variations in power supplies.

•

The value of the demand voltage.

•

Variations in amplifier gain.

 As the open loop system suffers from the variable factors shown above, the output is unlikely to follow the input precisely and cannot provide the close tolerance required. 5.1.2 CLOSED LOOP

If the errors in the output of a system are detected and f ed back to the input so that the necessary corrections can be made to eliminate the error, the system is said to be a closed loop system. below. system. A Closed loop system is shown below.

The essential features of a closed loop s ystem are: •

Information concerning the behaviour of the the load is fed fed back to the input. This is called feedback. feedback.

•

The position of the output (feedback) (feedback) is compared to that demanded by the input. Typically in a summing amplifier.

•

The production of an error signal proportional to the difference between between the demand and feedback signals.

•

Power amplification of the error signal to control control the the load.

•

Movement of the load in such a direction as to reduce reduce the error signal to zero, at which point the output is the same s ame as that demanded by the input.

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REMOTE POSITION CONTROL SERVOMECHANISMS

One form of closed loop servo system is a Remote Position Control (RPC) servo serv o system. This system is used to control the position of a remotely located device device in response to a change in the demanded input. The essential parts of an RPC servo are as follows: •

device for converting one form of Transducers - In general, a transducer is a device energy into another, for example, electrical to mechanical, heat to electrical or light to electrical. In servo systems these are generally used used to convert a mechanical input to an electrical signal for the servo.

•

power of the input signal signal to a level Amplifier - The amplifier increases the power suitable to drive the device device being positioned. Large mechanical work outputs outputs are therefore possible for very small work inputs.

•

Motors - Motors are used to move the device being controlled. They are usually coupled to a gearbox and produce either a linear or rotary motion.

5.2.1 POSITIONAL FEEDBACK

 As already mentioned. For a closed loop system to function correctly is requires the use of Position Feedback (PFB). In a perfect system incorporating positional feedback, the output shaft would would exactly follow the input shaft position. The diagram below shows a simple system using positional feedback.

 Assuming the angular position of the output shaft corresponds exactly with the angular position of the input shaft, the demand and feedback voltages from the potentiometers will be equal. equal. These voltages could be any value, we will assume they are both 5 volts.

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In the summing amplifier, the feedback signal is subtracted from the demand signal resulting in no error signal. With no error signal applied applied to the amplifier, the motor will be stationary and the system is said to be at rest. This may be expressed mathematically as: Error = Demand - Feedback = 0 If the input shaft is now rotated clockwise through some angle, changing the demand voltage from 5 volts to 6 volts, the difference between the input shaft voltage, now 6 volts, and the output shaft, still 5 volts, will be: Error = Demand - Feedback Error = 6V - 5V = 1V The motor now runs in the direction determined by t he polarity of the error voltage.  As the motor runs to reposition the load, the wiper on the potentiometer potentiometer is moved so as to increase the feedback voltage. voltage. When the feedback feedback voltage is again equal equal to the demand voltage (6 volts), the error signal will be zero, the motor will stop and the output shaft would be realigned with the input shaft. If the input shaft had been rotated anti-clockwis e through the same angle, instead of clockwise, the demand voltage would have decreased from 5 volts to 4 volts, and the error signal would have been –1 volt. This would have caused the motor to drive in the opposite direction, decreasing the feedback voltage. Error = Demand - Feedback Error = 4V - 5V = -1V The example uses a d.c. system sy stem but the same principles apply in the t he case of the equivalent a.c. circuit, except that t hat the direction of rotation of the motor is determined by the phase relationship between the output and a reference phase.

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TYPES OF INPUTS

There are three possible types of inputs to t o a servo, these are:

5.3.1 STEP INPUT

This type of input can be achieved by switching off the servo power, moving the input shaft and then re-applying power. The response of a servo system to this type of input reveals a great deal deal of information about about the servo system. It is therefore used as a test signal. 5.3.2 RAMP INPUT

This type of input is created when the input shaft is suddenly rotated at a constant angular velocity. The units would be radians / second. The diagram shows an example of a ramp Input. Servo systems are subjected to this type of of input during normal operation. 5.3.3 ACCELERATING ACCELERATING INPUT

 An accelerating function is created when the input shaft is rotated with a constant acceleration. The units would be radians / second 2. The diagram shows an example of an accelerating input. Systems are subject to this type of of input during normal operation.

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SYSTEM RESPONSE

How well a servomechanism responds to a particular change in input signal, in terms of transient response and overshoot, is a measure of its overall performance.  Any servomechanism will take a finite time to start to move and also to settle down at a new position. 'Settling Time' is defined as the time taken to approach approach a final steady state within specific limits. The diagram below shows the response of a RPC system to a step input.

Unless special precautions are taken a servomechanism will oscillate. The diagram above, when the output response reaches the required value at point 'x', the load has acquired considerable momentum momentum and consequently consequently overshoots. The error now increases in the opposite sense and a reverse-torque is applied which brings the load to rest at point 'y', 'y ', and then accelerates it back in the opposite direction where it again overshoots the desired desired position, at point Z. This process can continue continue indefinitely if the frictional losses in the system are negligible, and system would oscillate continuously. This is called 'hunting'. 'hunting'. To avoid oscillation and subsequent hunting, some form of damping is required.

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5.5 DAMPING Different amounts of damping produces different response curves. •

and transient oscillations are observed at Underdamped - When overshoots and the output of a system the servomechanism s ervomechanism is said to be underdamped.

•

Critically damped - When the system responds to an error in such a way that the output moves to the required position at the fastest possible rate without producing overshoots it is said to be critically damped. This is a theoretical position and provides the division divis ion between underdamping and overdamping.

•

but a time lag is introduced Overdamped - When no overshoots are produced, but into the system, the servomechanism is said to be overdamped.

In practice, servo systems sys tems are designed to be slightly underdamped in order to reduce response delays. This This is shown by the dotted line in the diagram. This degree of damping is often called 'ideal damping'. damping'. Under ideally damped conditions, the system reaches the r equired position more quickly than when critically damped, but it overswings the demanded position and has to move back onto it. This means the system takes slightly longer longer to reach the steady state.

5.5.1 FRICTIONAL FORCES WHICH PRODUCE DAMPING

There are frictional forces inherent in a servo system, which provide damping. These are coulomb coulomb friction and viscous friction. Another friction present when the servo is at rest is stiction. 5.5.1.1

Stiction

Stiction is present in the system when it is at rest. This initial friction must be overcome in order for the system to move. Once moving, stiction falls to zero. The name stiction comes from Static Friction. Issue 1 - 02 October 2002

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Coulomb Friction

Coulomb Friction is a constant force independent of speed and is demonstrated by the rubbing friction between two plates. The diagram below shows a s ystem both undamped and damped with Coulomb Coulomb friction. The number of overshoots overshoots in the system using Coulomb friction is proportional to the size of the initial error, the larger the error the greater the number of overshoots. It will be noted that the response r esponse curve of the system using Coulomb frict ion brings the system to a steady state but that a positional error is present. present. For this reason coulomb friction is not used in i n practical systems, and although always present in the form of inherent friction, good system design keeps it to a minimum.

5.5.1.3

Viscous Friction

Viscous Friction is proportional to velocity and provides satisfactory damping for servo systems. When the velocity of the system is zero, viscous friction is zero, and therefore it will not cause a position error. If the system moves more rapidly, the viscous friction will increase, as necessary to provide the additional damping required.

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The response of a system to a ramp input is shown below. below. The oscillations of the system are damped out, but but a constant error is produced. This error is called Velocity Lag is proportional to the the amount of viscous 'Velocity Lag'. Lag'. The amount of Velocity damping.

5.5.1.4

Efficiency of Output Damping

Both Coulomb and Viscous damping have the great disadvantage of being applied to the output of systems. This requires large amounts amounts of energy to control high power outputs. This inevitably generates generates heat, which entails entails the provision of complex cooling systems. It is more efficient to apply damping damping to the input input of the system, where power levels are much lower. 5.5.2 VELOCITY FEEDBACK DAMPING DAMPING

 A simple and commonly used method of providing damping damping at the input is to use Negative Velocity Feedback (NVFB).  A system using NVFB damping is shown in the diagram below. below. The feedback is applied to the the input and therefore must be electrical. Velocity feedback provides damping similar to viscous friction, but because it is applied to the input, little power is required.

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In servomechanisms, velocity feedback is achieved by using a tacho-generator (TG) driven by the output shaft of of the system as shown above. A Tacho-generator is a small electrical generator, which is either A.C or D.C. operated. operated. The great advantage of this type of feedback is i s that amount of voltage fed back to the system, and therefore the amount of damping, can be controlled by using a si mple potentiometer. 5.5.2.1

Velocity Feedback Curves

The diagram below shows the result of applying Velocity feedback in the circuit shown in above. As RPC servos are concerned with the position of the load, the velocity lag will only be present when the load is moving and will therefore only cause a slight increase in the response time.

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VELOCITY CONTROL SERVOMECHANISMS

In some applications it is the rotational speed of a shaft and not its position that must be controlled. A Rate Servo is shown shown in the diagram on the following page. page. The input demand signal is used to control the angular velocity of the output shaft and not its position. To make the speed of the driving motor exactly exactly proportional to the input demand a servomechanism is essential. essential. If a servomechanism were not used the speed of the output motor would vary with changes in the supply voltage or any changes of the friction in the motor or its load. Note that there is no position feedback. Movement of the speed control potentiometer produces a voltage proportional to the demanded speed. speed. The tacho-generator provides provides a voltage proportional proportional to the angular velocity of the output output shaft. If there is a difference between between these two signals an error voltage will be be fed to the amplifier. amplifier. The output of the amplifier will accelerate or decelerate the motor until the output of the tacho-generator produces a voltage exactly equal to the input i nput demand voltage and the motor will run at the demanded speed.

5.6.1 RESIDUAL ERROR

Because of inherent frictional and damping losses, some torque is always required to turn the motor and load at a constant speed and therefore a difference between the input demand and the actual speed will always be present. By using high gain in the amplifier this difference can be kept very low. 5.6.2 VELOCITY LAG

 A Rate Servo using velocity feedback is just as prone to velocity lag as a RPC Servo, but as it is only the speed and not the actual position of the output that is measured, it may be ignored. Issue 1 - 02 October 2002

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A.C. SERVOMECHANISM COMPONENTS

The components associated with dc servo systems are simple in operation and require no further explanation, however, some of the components in ac servo systems require additional consideration. 5.7.1 E & I BAR TRANSDUCER

The E & I bar transducer is so called because of the shape of its component parts. The diagram below shows the construction and operation of an angular displacement E & I bar transducer. A winding on the centre limb of the E bar carries an A.C. excitation supply. Secondary coils are connected in series opposition. With the I bar in the centre position equal flux will flow in the outer limbs of the E bar, the voltages induced in the two secondary coils will be equal and opposite and will therefore cancel out and there will be no output output signal. If the I bar is displaced from the central position, more flux will flow in the limb of the E bar with the smaller air gap and less flux will flow in the limb with the larger air gap. The induced voltages in the two windings will no longer cancel out and an output v oltage will be produced.

The phase of the output voltage is determined by the direction of movement of the I bar. The magnitude magnitude is determined determined by how far the bar moves. In a servo system the amount of movement will be kept small due to the follow-up action.

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The E & I Bar may also be used to convert linear movement to an electrical signal.  A practical application of this is shown in the diagram below. The I bar is moved linearly by an evacuated capsule. capsule. Note that the diagram diagram contains a complete servo system.

5.7.2 A.C. TACHOGENERATORS TACHOGENERATORS

Tachogenerators provide the velocity feedback for servo systems. A tachogenerator normally utilises the drag cup principle and will always produce a voltage with the same frequency as the supply voltage.

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The diagram below below shows the electrical components of of the tachogenerator. With the drag cup stationary no voltage is induced in the secondary winding as it is placed at right angles to the primary winding and the output is zero.

 As the output shaft drives the rotor, the current in the input coil produces a field that induces a current in that part part of the cup passing through through the primary axis. As the cup rotates, rotating eddy currents are induced and this will in turn induce a voltage across the output winding. The amplitude of the voltage will be proportional proportional to the speed of rotation of the drag cup and the phase will be dependent on the direction of rotation.

Ideally, the output of the tachogenerator would be zero when stationary but in practical systems a small voltage is present. Issue 1 - 02 October 2002

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PRACTICAL SERVO SYSTEMS

The following are examples of typical servo systems, many more systems will be encountered and examined during the course. 5.8.1 DIRECT SERVO CURRENT SYSTEM

The potentials at the 2 potentiometer wipers are proportional to the input and output shaft positions θI and θo. Any difference in the relative positions results in a difference of potential between the wipers which is the error signal applied to the amplifier. The polarity polarity of the voltage indicated the direction of the error. The signal is amplified and produces a flux in the split field motor.

The motor armature carries current continuously, thus the presence of the field produces a motor torque which drives the load l oad in a direction corresponding to the polarity of the error signal, towards towards alignment. When alignment is reached reached the error signal falls to zero, t he motor field disappears and the motor stops.

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5.8.2 ALTERNATING CURRENT CURRENT SERVO SYSTEM

The input shaft fixes the position of the control transmitter (CX) rotor and hence the position of the stator field of the control transformer. The output shaft fixes the position of the rotor of the t he control transformer (CT). When the rotor of the CT is at 90° to the rotor of the CX, no emf's are induced in the rotor of the CT and the system is stationary (nulled).

With a misalignment in the system an emf is induced in the rotor of the control transformer, this is the error signal. The error signal is amplified and passed to the motor, which with both phases excited drives the load in one direction or other according to the phase of the rotor emf. When alignment is reached reached there is no output from the CT, no input to the amplifier and the motor stops.

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6

OTHER TRANSDUCERS

6.1

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER

Linear variable differential transforms (LVDT's) are used to produce an electrical signal proportional to a linear movement. LVDT's consists of a moveable iron core that is mounted inside three three windings wound on a coil former. The centre winding is the excitation winding and and is connected to an a.c. reference reference voltage. The two outer windings are connected in series opposition and provide the output.

With the core centralised and a.c. applied to the excitation coil, an emf is induced in each of the output windings. windings. The emf's induced in each each winding are the same size, but phase displaced by 180º, and therefore cancel out, producing no output. When the iron core is moved, the emf induced in one output winding increases, and in the other it decreases. The two voltages no longer cancel, and an output is produced. If the core is moved the same same amount in the opposite direction, an emf of the same size, but of opposite phase will be produced.

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ELECTRONIC FUNDAMENTALS

ROTARY VARIABLE TRANSFORMER

Rotary Variable Differential transformers and E and I bar transducers work on the same principle as the LVDT. Each device being used to produce an electrical signal from a mechanical movement. The RVDT produces an electrical signal proportional to a rotational movement, and the E and I bar can be used to produce a signal from both a linear and rotary movement.

In the example studied, the excitation was applied to the single centre winding, this is not always the case, the excitation can be applied to the two outer windings. Each winding will induce induce an emf of opposite opposite polarity in the centre winding. winding. If both emf's are the same size they will cancel, if not a resultant output voltage will be produced. The phase of the output signal depends depends on which reference coil induced induced the larger emf into the single output winding, which in turn depends on the position of the core. 6.3

INDUCTIVE TYPE TRANSDUCERS

Inductive type transducers use the principles associated with inductance that were discussed earlier in the course. Inductance is generally used in one of two ways in aircraft transducers. Firstly a changing magnetic field is used as the transducer output and secondly by using changes in flux density to control a.c. current flow. 6.3.1 INDUCED EMF TYPE

This type of transducer comprises a coil and a permanent magnet and requires a steel target for its operation. When the target is displaced displaced from the transducer the field of the permanent magnet surrounds the coil, but does not induce an emf in it because the lines of flux are not moving relative to the conductor. Issue 1 - 02 October 2002

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

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engineering

ELECTRONIC FUNDAMENTALS

When the steel target is placed in close proximity to the transducer the magnetic flux density increases due to the reduction in reluctance, again, although the flux density increased, no emf is induced because there is no relative movement between the conductor and coil.

If the target is moved continually back and forward past the transducer, the flux density continually increases and and decreases. This changing flux induces an emf in the transducer. The frequency of the induced induced emf depends depends on the speed of of movement; the faster the target is moved, the higher the frequency. This form of transducer is used to measure rotational speed of items such as engine shafts. 6.3.2 A.C. CURRENT CONTROL

When a.c. current flows in an inductor, i nductor, the continually changing flux produces a continually changing back back emf that opposes opposes the current flow. flow. This opposition to current flow is called inductive reactance, and is dependent on the value of inductance of the coil. The inductance of the coil can be changed by c hanging the coil material, or by placing a piece of steel adjacent adjacent to it. Placing a piece of steel near the coil increases its inductance, which in turn increases the inductive reactance of the coil. Increasing the inductive reactance reduces the a.c. current flow in the coil, which can be detected and used to provide a signal to indicate when the steel is in close proximity to the coil or sensor. This form of transducer is used in proximity sensing systems such as those used to sense the position of the undercarriage.

Issue 1 - 02 October 2002

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JAR 66 CATEGORY B1 JAR 66 CATEGORY B1 MODULE 4 MODULE 4 ELECTRONIC FUNDAMENTALS

uk

engineering

ELECTRONIC FUNDAMENTALS

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Issue 1 - 02 October 2002

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