60631522-IEexptPDF

EXPERIMENT I ______ THE UNI-JUNCTION TRANSISTOR (UJT) I. OBJECTIVES: After completing and performing the laboratory experiment, you will able to: 1. 2. 3.

Demonstrate a practical go/no method of testing a UJT with an ohmmeter. Show a practical method for finding Vp and Vv and for determining the intrinsic standoff ratio () of a UJT. Relate the UJT variables of peak voltage, intrinsic standoff ratio, and interbase voltage, and calculate any of these, given the other two.

INTRODUCTION: The unijunction transistor is a breakover-type switching device. The PN junction of a UJT can be tested with ohmmeter similar to testing diodes and bipolar transistors. With the negative lead placed on the emitter and the positive lead placed at either, the junction is reverse biased and the resistance should high or infinite. When the positive lead is placed at either base, the junction is forward biased and the resistance should be low. There should be a resistance reading of several thousand ohms when the meter is placed across the base leads. The Vp or the firing point of UJT can be found by connecting a voltmeter to the emitter as shown figure 1.2. The voltage in the emitter is slowly increased by adjusting RE. When the UJT fires, the voltmeter reading will decrease rapidly to the minimum value of Vv. This procedure may be required several times to obtain as accurate a reading as possible. Before firing, IE should be zero, but after firing, IE should increase significantly. Resistor RS is used to limit the current through the emitter, in case RE is adjusted too close + VBB. Resistor R2 and the UJT form a resistive voltage divider, with VB2 being slightly less than VBB before firing. After firing, the resistance of the UJT decreases and the voltage drop across it, VB2 also decreases. 1

Since the voltage is proportional to resistance, the intrinsic standoff ratio can be found by the formula VP - VD  = -----------------, Where VD = 0.7 V VBB II. MATERIALS NEEDED: 1 1 1 1 1 1 1 1

Fixed + 12 – V power supply Standard or digital voltmeter Standard or digital ammeter 2N2646 UJT or equivalent 220 -  resistor at 0.5 W (R2) 1 - k resistor at 0.5W (RS) 10 - k potentiometer, linear (RE) Breadboard for constructing circuit

III. PROCEDURE: PART I : TESTING A UJT 1. Set the ohmmeter to the midrange scale. 2. Refer to figure 1.1 to connect the ohmmeter to the UJT properly for each lead and record the readings in the indicated ohmmeter table 1.1.

Figure 1.1 Testing a UJT With an ohmmeter

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PART II : FINDING THE VP, VV , AND  OF A UJT

Figure 1.2 Finding Vp’ Vv’ and  of a UJT



=

VP - VD VP – 0.7V ----------------- = ----------------VBB VBB

III. PROCEDURE: 1. Construct the circuit shown in figure 1.2. 2. Adjust RE until VE reads 0V. 3. Measure VB2 and record in the place in the data table 1.2 in labeled “Before firing”. 4. Measure IE and record next to VB2 in the same row. 5. Slowly adjust RE (with VE increasing) until the meter reading drops suddenly. Record the reading as close as possible to the VP before the UJT fires. Repeat this step a few times to obtain accuracy. 6. With the UJT fired, measure VV and record in its proper place in the data table (second row). 7. Measure VB2 and record next to VV in the data table. 8. Measure IE and record next to VB2 in the row “After firing”. 9. Using the value of VP from the data table, calculate and record the intrinsic standoff ratio () in table 1.2. 3

_________________________ ______ THE UNI-JUNCTION TRANSISTOR (UJT) Experiment 1 Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: PART I : TESTING A UJT Table 1.1: Ohmmeter Reading Emitter Positive Negative Positive Negative None None

Base 1 Negative Positive None None Positive Negative

Base 2 None None Negative Positive Negative Positive

Reading( H or L)

FILL-IN QUESTIONS: 1. A forward-biased PN junction should have

resistance.

2. A reverse-biased PN junction should have

resistance.

3. A forward-biased PN junction with a high ohmmeter reading indicates that UJT is . 4. A reverse-biased PN junction with a low ohmmeter reading indicates that the UJT is . 5. The resistance of a UJT from base to base reads the same regardless of the of the ohmmeter leads.

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PART II : FINDING THE VP’ VV’ AND  OF A UJT Table 1.2 Condition Before firing After Firing 

VE

=

VB2

IE

VP - VD VP – 0.7V ----------------- = ----------------- = VBB VBB

COMPUTATIONS:

GRAPH:

Draw the characteristic curve of a UJT for fixed value of VBB.

5

FILL-IN QUESTIONS: 1. The voltage on the emitter just before the UJT fires is called _______________________. 2. The voltage on the emitter after the UJT fires is called _______________________. 3. Before the UJT fires, IE is equal to

.

4. After the UJT fires, IE

.

5. When the UJT fires, VB2 = __________________.

ANALYSIS OF RESULTS:

6

QUESTIONS AND PROBLEMS: 1. Is a unijunction transistor a continuously variable device or a switching device? Explain.

2. Roughly speaking, what range of values does intrinsic standoff ratio fall into?

3. Explain why a UJT is not a thyristor.

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EXPERIMENT II CURRENT – VOLTAGE CHARACTERISTICS OF A UJT CIRCUIT I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. Demonstrate the reaction of voltages and currents when UJT fires. 2. Calculate current values from voltages and resistance values. 3. Interpret the current-voltage characteristic curve of a UJT.

INTRODUCTION: For the figure 2, voltage measurements are taken before and after the UJT fires. The current is calculated from the formulas shown in the data table 2. From this table, it can be determined what occurs to each voltage and current after the UJT fires. II.

MATERIALS NEEDED: 1 1 1 2 1 1 1

III.

Fixed + 12-V power supply Standard or digital voltmeter 2N2646 UJT or equivalent 100- resistors at 0.5W (R1 and R2) 1-k resistor at 0.5W (RS) 10-k potentiometer, linear (RE) Breadboard for constructing circuit

PROCEDURE: 1. Construct the circuit shown in figure 2. 2. Adjust RE for VE reading 0V. 3. Measure VB1 and VB2 and record inn the data table 2 (row1). 8

4. Calculate IB1, IB2 and IE and record in the data table 2 (row1). 5. Adjust RE until the UJT fires (see Experiment 1 for reference to this indication). 6. Measure VV, VB1, and VB2 and record in the second row of the data table. 7. Calculate IB1, IB2, and IE and record in the second row of the data table. Compare the values of the data table for differences of voltages and currents when UJT fires and after UJT fires.

Figure 2 Current-voltage characteristics of a UJT circuit:

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CURRENT – VOLTAGE CHARACTERISTICS OF A UJT CIRCUIT Experiment No. 2 Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: Table 2 Condition VE

VB1

VB2

VB1 IB1 = -------------R1

At the firing point (t = t1) After firing VV

COMPUTATIONS:

10

+VBB - VB2 IB2 = --------------------------R2

IE = IB1 + IB2

ANALYSIS:

QUESTIONS AND PROBLEMS: 1. If the UJT has an intrinsic standoff ratio of η = 0.65 and an externally applied voltage of 18 V, calculate the peak voltage.

2. If the UJT has an RB1 = 5.7 kΩ and RB2 = 2.5 kΩ, what is the intrinsic standoff ratio?

3. How can a UJT be turned on?

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EXPERIMENT III UJT RELAXATION OSCILLATOR I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. Show how a UJT is used as a switch in a relaxation oscillator and to determine the approximate output frequency. 2. Explain the operation of UJT relaxation oscillator and UJT timers, and properly select the timing resistors and capacitors in these circuits. 3. Determine the factors that affect the frequency of oscillation.

INTRODUCTION: A UJT relaxation oscillator has three available outputs; a positive pulse train at B2 and exponential sawtooth waveform at E. Pulses at B1 and B2 are normally used to trigger other circuits. Caution is needed when using the output at E, since any circuit placed here is parallel to CE, which tends to load it down and changes the frequency of the oscillator. Components RE and CE primarily determine the frequency of the oscillator and can be approximated by the formula:

f

1  ----------------------RECE

However, the intrinsic resistance of the UJT and  also play a part in the charge and discharge time CE, and the actual frequency will usually be more or less. The power supply voltage has the least effect on the oscillator frequency.

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II.

MATERIALS NEEDED: 1 1 1 1 2 1 1 1 1 1 1 1

III.

Fixed + 12-V power supply Standard or digital voltmeter Oscilloscope (dual trace preferred) 2N2646 UJT or equivalent 100- resistors at 0.5 W (R1 and R2) 10-k resistor at 0.5 W (RE) 22-k resistor at 0.5 W (RE) 4.7- k resistor at 0.5 W (RE) 0.1-F capacitor at 15 WV dc (CE) 0.2-F capacitor at 15 WV dc (CE) 0.05-F capacitor at 15 WV dc (CE) Breadboard for constructing circuit

PROCEDURE: PART I : UJT RELAXATION OSCILLATOR 1. Construct the circuit shown in Figure 3.1. 2. Using the voltmeter, measure and record VE, VB, VB2, VBB, VEB1, andVEB2 in the blank spaces provided in Table 3.1. 3. Using the oscilloscope, examine the voltage waveforms at E, B1 and B2. 4. Draw the voltage waveforms in the graphs provided and indicate their peak-to-peak values. 5. Calculate the approximate frequency of the oscillator from the values of RE and CE and record in the proper place in the data table of figure 3.2. 6. Place the oscilloscope at E and measure the actual frequency. Remember that f = 1/T, where T is the time period of one cycle. Record this frequency in the proper place in the data table. 7. Change components RE and CE as indicated by the data table and repeat steps 5 and 6.

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Figure 3.1 UJT Relaxation Oscillator

PART II: UJT BASIC TROUBLESHOOTING APPLICATION Construct this circuit using your own values or those of a circuit from a previous section. Open or short the components as listed and record the voltages in the proper place in the table. The abbreviations will help indicate conditions with each problem. All voltages are references to ground. When the circuit is inoperative, there will be no voltage waveforms at E, VB1 and VBB.

Figure 3.2

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_________________________ UJT RELAXATION OSCILLATOR Experiment III Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET:

PART I : UJT RELAXATION OSCILLATOR Table 3.1: Voltmeter Reading VE VB1 VB2 VBB VEB1 VEB2

Table 3.2: Frequency Reading RE (k)

CE F)

f1/RECE hertz Calculated

10 22 4.7 10 10

Measured

0.1 0.1 0.1 0.2 0.05

15

COMPUTATIONS:

GRAPHS: VB1

VB2

16

VE

FILL-IN QUESTIONS: 1. A

pulse train is found at B2 of the UJT oscillator.

2. A

pulse train is found at B1 of a UJT oscillator.

3. The voltage waveform at E of a UJT oscillator is in the form of a . 4. If the value of RE or CE increases, the frequency of the oscillator . 5. If the value of RE or CE decreases, the frequency of the oscillator .

PART II : UJT BASIC TROUBLESHOOTING Table 3.3 Condition

VE

VB1

VB2

Normal RE open

R2 open

DEC

DEC

DEC

INC

DEC

DEC 17

Comments All voltages are proper Open RE at +VBB: capacitor can not charge Open R2 at +VBB: no voltage applied to B2

R1 open

VBB

CE open

DEC

CE shorted

DEC

E open

DEC

VBB

VBB

INC

DEC

DEC

INC

DEC

NOC

B2 open

INC

INC

VBB

B1 open

VBB

DEC

VBB

E-B1 shorted

DEC

NOC

NOC

E-B2 shorted

DEC

INC

DEC

Open R2 at GND: no current path through UJT Open CE at GND: emitter pulled up toward + VBB; UJT saturates. Emitter pulled toward GND; UJT cut off Open junction of RE and CE: no current path through emitter Open at bottom of R2 and measure same place: no current through UJT Open at bottom of R2 and measure same place: no current through UJT R1 parallel with CE

R2 in parallel with RE; UJT saturated

Table Abbreviations: VBB power supply voltage INC increase DEC decrease NOC No change GND ground

ANALYSIS OF RESULTS:

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QUESTIONS AND PROBLEMS: 1. Refer to the relaxation oscillator shown in figure 3.1. Assume that the UJT has the following characteristics: Vv = 1.2 V RB1 = 5.7 kΩ RB2 = 2.7 kΩ Ip = 5µA Iv = 2.75 mA (a) Find the peak voltage, Vp. (b) What is the approximate oscillator frequency? (c) Describe the waveform that appears across R1. What voltage appears across R1 during the time when the UJT is not conducting?

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2. For the relaxation oscillator of figure 3.1, what would be the effect on the oscillator frequency of doubling CE? Of doubling R1?

3. Why there is a maximum and minimum limit on the size of the emitter resistor in a UJT oscillator circuit?

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4. Explain the purpose of inserting R2 in the base 2 lead of the relaxation oscillator.

5. What three factors determine the period of oscillation?

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EXPERIMENT IV SILICON-CONTROLLED RECTIFIER I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. 2. 3. 4.

Demonstrate a practical go/no go methods of testing an SCR with ohmmeter. Show the turn-on (fire) and turn-off (reset) methods for an SCR. Determine the electrical characteristics of a particular SCR. Define the various SCR parameters.

INTRODUCTION:

The PN junction from gate to cathode of an SCR can be tested with an ohmmeter similar to a regular diode. However, testing from anode to gate will not indicate if an SCR is working properly, because one of the PN junctions is always reverse biased. The SCR can be tested with an ohmmeter by placing the positive lead on the anode and the negative lead on the cathode with the gate left open. The meter should read high or infinite resistance. Placing a clip lead from the anode or positive lead of the ohmmeter to the gate triggers the SCR and the meter should indicate low resistance. When the clip lead is removed, the meter continues to indicate low resistance if the power source is sufficient to produce the required holding current. To conduct, the SCR must have its anode more positive than its cathode. When the gate voltage is made more positive than its cathode, the SCR turns on or fires and current flows from cathode to anode. When the gate voltage is again made equal to or more negative than the cathode, current continues to flow through the SCR. The SCR is turned off or reset by reducing the current through it below its holding current.

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II.

MATERIALS NEEDED: 1 1 1 1 1 1 2 1

Fixed + 12-V power supply Standard or digital voltmeter C106Y1 SCR or equivalent 100- resistor at 0.5 W (R1) 10-k resistor at 0.5 W (RG) 22-k resistor at 0.5 W(RA) DPST switches (S1 and S2) Breadboard for constructing circuit

PART I; TESTING AN SCR WITH AN OHMMETER 1. Set the ohmmeter to the midrange scale. 2. Connect the ohmmeter to the SCR as shown in figure 4.1 and record the meter reading in table 4.1.

Figure 4.1 Testing an SCR with an ohmmeter: (a) without clip lead; (b) with clip lead; (c) again without clip lead.

PART II: OPERATION OF AN SCR 1. 2. 3. 4.

Construct the circuit shown in figure 4.2. Set switches S1 and S2 as indicated and then apply power to the circuit. In the first row of the data table 4.2, record the values of VG and VA. Move S1 to position B and record the values of VG and VA in the second row of the data table. 5. Move S1 to position A and record the values of VG and VA in the fourth row of the data table. 23

6. Move S2 to position B and record the values of VG and VA in the fourth row of the data table. 7. Move S2 to position A and record the values of VG and VA in the fourth row of the data table.

Figure 4.2 Operation of an SCR.

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________________________ SILICON-CONTROLLED RECTIFIER Experiment 1V Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: PART I : TESTING AN SCR WITH AN OHMMETER Table 4.1: Ohmmeter Reading Gate (G) None Positive None

Anode (A) Positive Positive Negative

Cathode (K) Negative Negative Positive

Reading( H or L)

FILL-IN QUESTIONS: 1. An SCR will have

resistance before being triggered.

2. An SCR will have

resistance before after triggered.

3. The -tochecked like a normal diode.

resistance of an SCR can be

4. An SCR is being tested with an ohmmeter. When the clip lead on the gate is removed, the meter indicates high resistance. This does not prove that the SCR is defective, but that the power source of the meter is not sufficient to produce the necessary through the device.

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PART II: OPERATION OF AN SCR Table 4.2 S1 Condition A B A A A

S2 Condition A A A B A

VG

VA

Condition of SCR (on or off)

FILL-IN QUESTIONS: 1. Before firing, the voltage from anode to ground of the SCR is equal to . 2. When the gate is made more .

the SCR fires and IAK

3. Once the SCR fires, the gate control and the current to flow through the SCR. 4. When the SCR is conducting, the voltage from the anode to ground is equal to . 5. The SCR can be below its holding current.

by reducing the current through it

ANALYSIS OF RESULTS:

26

QUESTIONS AND PROBLEMS: 1. The letters SCR stand for silicon-controlled rectifier. Explain the use of the word rectifier in the name.

2. What two things must happen to cause an SCR to fire?

3. What is the difference between triggering gate current and holding current?

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4. How much voltage across the anode-cathode terminals of a mediumpower SCR after it has fired?

5. How can an SCR be made to conduct?

6. How can an SCR be turned off?

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EXPERIMENT V CURRENT CONTROL OF AN SCR I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. Demonstrate the effect that gate current has to turn on an SCR, and to determine the minimum holding current to keep the SCR conducting. 2. Define parameters associated with SCRs, such as gate trigger current, holding current, forward ON-state voltage and give the approximate range of values expected for these parameters.

INTRODUCTION: The experiment shows that sufficient gate current must flow in order to turn on the SCR and that the minimum holding current can be found with the addition of large-value potentiometer in anode circuit. II.

MATERIALS NEEDED: 1 1 1 1 1 1 1 1 2 1

Fixed + 12-V power supply Standard or digital voltmeter Standard or digital ammeter C106Y1 SCR or equivalent 100- resistor at 0.05 W (R1) 22-k resistor at 0.5W (RB) 100-k resistor at 0.5W (RA) 50-k potentiometer (RH) DPST switched (S1 and S2) Breadboard for constructing circuit

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III.

PROCEDURE: 1. Construct the circuit shown in figure 5.1. 2. Set both switches as indicated and then apply power to the circuit. 3. Calculate the gate current IRA, flowing through RA and record in table 5.1. 4. Measure VAK and record in the place indicated next to IRA. Is the SCR on or off? 5. Move S1 to position B. 6. Calculate the gate current IRB flowing through RB and record in the place indicated. 7. Measure VAK and record in the place indicated next to IRB. Is the SCR on or off? 8. Remove the power supply voltage from the circuit. 9. Modify the circuit by adding the ammeter and 50-k potentiometer (RH) in series with load resistor RL. 10.Set the wiper RH so that the resistance is completely “shorted out”. 11.Make sure the S1 and S2 are set as indicated and then apply power to the circuit. 12.Momentarily move S1 from position A to position B and back again. 13.Recording the reading of VAK and IA in table 5.2. 14.Slowly adjust RH so that the current IA begins to decrease. 15.Remember the reading of IA when VAK increases to +VAA. Record this value in table 5.2 indicated for the minimum holding current of the SCR. (Perform steps 10 through 15 a few for a more accurate reading).

Figure 5.1 Current control of an SCR. 30

___________________ CURRENT CONTROL OF AN SCR Experiment V Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: Table 5.1 S1

S2

A B

A A

Figure 5.1: Without potentiometer. IRA = VAA/RA IRB = VAA/RB VAK 0A 0A

Table 5.2 Figure 5.1: With potentiometer. Potentiometer setting VAK IA (%) 0 5 10 20 30 40 50 60 70 80 90 100

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Condition (ON or OFF)

FILL-IN QUESTIONS: 1. If the gate resistor is too large, not enough gate current will flow to ____________ the SCR. 2. When the gate resistor is _____________ the proper value of gate current will flow to trigger the SCR. 3. Sufficient _______________ is required to keep the SCR conducting.

4. If the load resistance in series with the anode is too large, not enough current flows from cathode to anode and the SCR will turn ________________ .

ANALYSIS OF RESULTS:

32

QUESTIONS AND PROBLEMS: 1. How much gate current is needed to trigger a medium-power SCR?

2. After an SCR has fired, what effect does the gate signal have on the SCR?

3. What effect does an increase in anode current have on anodecathode voltage?

4. Explain why an SCR is superior to a series rheostat for controlling and limiting current through a load.

5. Explain the difference between an SCS and an SCR.

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EXPERIMENT VI AC TRIGERRING OF AN SCR I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. Show how an ac current to a load can be controlled by an SCR, depending on what portion of the positive alternation of s sine wave the SCR turns on. 2. Define firing delay angle and conduction angle. 3. Show how conduction angle affect the average load current. 4. Control average current delivered to a load.

INTRODUCTION: The first part of this experiment uses only a variable resistance to vary the trigger time from 0o to 90o. An oscilloscope is used to view the voltage waveforms across the SCR and RL. A capacitor and diode are added to the original circuit to complete the second part of the experiment. The capacitor extends the trigger time to nearly 180o, and the diode produces a sharpener current pulse when it conducts, to provide more trigger control. II.

MATERIALS NEEDED: 1 1 1 1 1 1 1 1 1

12-Vrms transformer or ac source Oscilloscope (use only one channel) C106Y1 SCR or equivalent 1N4001 diode or equivalent 100- resistor at 0.5W(R1) 1-k resistor at 0.5 W(RA) 500- k potentiometer (RG) 0.2-F capacitor or 25 25WV dc (CG) Breadboard for constructing circuit 34

III.

PROCEDURE: 1. Construct the circuit shown in figure 6.1 2. Place the oscilloscope’s channel 1 across the SCR, VAK. 3. Vary RG back and forth and view the voltage waveform across the SCR. 4. Adjust RG so that the SCR triggers about halfway between 0o and 90o 5. Draw the voltage waveform across the SCR in the space provided, making sure to align it with the proper degrees for one cycle. (Indicate peak-to-peak voltage). 6. Place the oscilloscope’s channel 2 across RL. 7. Draw the voltage waveform across RL in the space provided, making sure it with proper degrees for one cycle. (Indicate peakto-peak voltage). 8. Set the firing delay angle and complete table 6.1. Before measuring the new setting of RG using ohmmeter, turn off the power supply first. 9. Modify the circuit as shown in Figure 6.1 by adding the capacitor and diode (Figure 6.2). 10.Place the oscilloscope across the SCR. 11.Vary the RG back and forth and view the voltage waveform across the SCR. 12.Adjust RG so that the SCR triggers past 90o, but not at 180o point. 13.Draw the voltage waveform across the SCR in the space provided, making sure to align it with the proper degrees for one cycle. (Indicate peak-to-peak voltage) 14.Place the oscilloscope’s channel 2 across RL. 15.Draw the voltage waveforms across RL in the space provided, making sure to align it with the proper degrees for one cycle. (Indicate peak-to-peak voltage). 16.Set the firing delay angle and complete table 6.2. Before measuring the new setting of RG using ohmmeter, turn off the power supply first.

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Figure 6.1: Trigger time ≈ 0° to 90°

Figure 6.2: Trigger time ≈ 0° to 180°

36

_________________________ AC TRIGGERING OF AN SCR Experiment V1 Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: Table 6.1: Trigger time ≈ 0° to 90° Firing Delay Angle 0 30 RG Setting (kΩ)

45

GRAPHS: Voltage waveform across the SCR

Voltage waveform across the load resistor.

37

60

75

90

Table 6.2: Trigger time ≈ 0° to 180° Firing Delay 10 30 45 60 75 90 100 120 140 150 160 170 180 Angle RG setting (kΩ) GRAPHS: Voltage waveform across the SCR

Voltage waveform across the load resistor.

FILL-IN QUESTIONS: 1. Using only a potentiometer, the ac trigger tine of an SCR can be varied from about to degrees.

38

2. Using a potentiometer and capacitor, the ac trigger time of an SCR can be varied from about to degrees. 3. When the SCR conducts, the voltage across RL is about equal to the . 4. When the SCR conducts, the voltage across its A-K terminals is about V. 5. The voltage across RL when the SCR conducts is the result of times .

ANALYSIS OF RESULTS:

39

QUESTIONS AND PROBLEMS: 1. Which condition would cause the larger load current, a firirng delay angle of 35° or a firing delay angle of 60°?

2. If the conduction angle of an SCR is 90° and it is desired to double the average load current, calculate the new conduction angle for a 60 Hz ac supply.

3. For figure 6.1, assume the supply is 12 Vrms, IGT = 10 mA, and RA = 1 kΩ. The firing angle is desired to be 90°. To what value should R2 be adjusted?

40

4. In figure 6.1, if the resistance of the load is 100Ω and the supply is 12 Vrms, how much power burned in the SCR when the firing delay angle is 0°? When the SCR is turned ON the voltage across is 1.0 V.

5. In figure 6.1, the supply is 115 Vrms, 60 Hz. The SCR has a triggering gate current of 35mA; RA = 1 kΩ; what value of RG will cause a firing delay angle of 90°?

41

EXPERIMENT VII TESTING A DIAC I. OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. Demonstrate how a DIAC can be tested with a power supply voltage. 2. Explain the characteristic curve. 3. Discuss the equivalent circuit and bias conditions. INTRODUCTION: A DIAC is a type of thyristor that can conduct current in both directions. It can be tested with a power supply voltage that is at least a few volts greater that its VBO. The DIAC is connected in series with a smallvalue limiting resistor, which in turn is connected to a potentiometer or a variable power supply voltage. The voltage across the circuit is then increases until VBO is reached. The power supply is then set to zero volts. The DIAC is reversed in the circuit, and the same procedure is performed to test the reverse direction.

II.

MATERIALS NEEDED: 1 1 1 1 1 1

III.

100-v POWER SUPPLY Standard or digital voltmeter RS 276-1050 DIAC or equivalent with VB=40V 470-k resistor at 0.5W (RS) 10-k potentiometer (RA) Breadboard for constructing circuit

PROCEDURE: 1. Construct the circuit shown in figure 7.1. 2. Place the wiper of RA to ground. 3. Place the voltmeter across the DIAC. 42

4. Record the condition of the DIAC in the first line in the data table 7. 5. Adjust RA so that the voltage indicated by the meter begins to increase. 6. Continue to increase the voltage until there is a slight decrease in the voltage reading. 7. Record the condition of the DIAC on the second line of the data table. 8. Place the wiper of RA back to ground. 9. Reverse the DIAC in the circuit. 10.Record the condition of the DIAC on the third line of the data table. 11.Repeat steps 5 and 6. 12.Record the condition of the DIAC on the fourth line on the data table.

Figure 7.1 Testing a DIAC

43

_____________________________ TESTING A DIAC Experiment VII Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: Table 7.1 Circuit condition

VT

Before Firing After Firing Before Firing After Firing

Condition of DIAC (ON,OFF)

0 to VBO Reverse DIAC in circuit 0 to VBO

FILL-IN QUESTIONS: 1. The DIAC turns on when its in either direction.

is

reached

2. When the DIAC turns in either direction, the voltage across its terminals slightly, indicating a resistance action. 3. The voltage across the DIAC decreased when it turns on, but the current through it . 4. The DIAC similar to two or back to back series.

face to face

44

ANALYSIS OF RESULTS:

QUESTIONS AND PROBLEMS: 1. Compare the diac to the Shockley diode in terms of basic operation.

2. Sketch the current waveform for a series ac circuit having the following components: V = 35 Vrms, load resistor RL = 1 kΩ. The diac has a breakover potential of 20 V, IH = 1.2 mA.

3. Sketch and explain the characteristic curve of a diac. What other names are used for a diac?

45

EXPERIMENT VIII TRIAC I.

OBJECTIVE : After completing and performing the laboratory experiment, you will able to: 1. 2. 3. 4. 5.

Define and discuss the important electrical parameters of triac. Discuss the equivalent circuit and bias conditions. Show how to test a TRIAC for conduction in both directions. Demonstrate how the TRIAC conducts in both directions, and how it can be triggered with positive and negative. Explain the characteristic curve.

INTRODUCTION: A TRIAC is like a diac with a gate terminal. It can conduct current in either direction when it is triggered on, depending on the polarity of the voltage across the triac. A triac can be tested with an ohmmeter similar to testing an SCR or PUT. The positive lead of the ohmmeter is placed on T2 and the negative lead is placed on T1. The meter should read infinite resistance. A clip lead is placed from the positive lead to gate, which should trigger on the TRIAC. The meter should now continue to indicate low resistance if the lower source is sufficient to produce the required holding current. The meter leads are reversed on the main terminals of the TRIAC and a clip lead is placed from the negative lead to the gate to test for conduction in the reverse direction. This is a go/no go test. There are four modes of triggering a TRIAC. 1. 2. 3. 4.

Positive terminal voltage with positive trigger voltage. Positive terminal voltage with negative trigger voltage. Negative terminal voltage with positive trigger voltage. Negative terminal voltage with negative trigger voltage.

46

II.

MATERIALS NEEDED 1 1 1 1 1 1 1 1 1

III.

15-V dual power supply Standard or digital voltmeter 2N5754 TRIAC or equivalent 100- resistor at 0.5 W (RL) 1-k resistors at 0.5 W (RA and RB) 10-k resistor at 0.5 W (RG) TPST switch (S1) (a single wire may be used) DPST switch (S2) Breadboard for constructing circuit

PROCEDURE:

PART I: TESTING A TRIAC WITH AN OHMETER 1. Set the ohmmeter to the low-range scale. 2. Connect the ohmmeter to the TRIAC as shown in figure 8.1a and record the meter reading in table 8.1 3. Connect the clip lead as shown in figure 8.1b and record the reading. 4. Remove the clip lead as shown in figure 8.1c and record the reading. 5. Connect the ohmmeter to the TRAIC as shown in figure 8.1d and record the meter reading. 6. Connect the clip lead as shown in figure 8.1e and record the reading. 7. Remover the clip lead as shown in figure 8.1f and record the reading.

47

Figure 8.1 Testing a TRIAC with an ohmmeter: (a) Without clip lead; (b) with lead; (c) again without clip lead; (d) without clip lead; (e) with clip lead; (f) again without clip lead

PART II: TRIGGERING MODES OF A TRIAC 1. 2. 3. 4. 5.

Construct the circuit shown in figure 8.2. Open and close S2 to make sure that the TRIAC is off. Measure and record VG and VT2 in the first line of the data table 8.2. Indicate on the same line of the data table if the TRIAC is on or off. Move S1 to position B. Measure and record the data on the second line as done in steps 3 and 4. 6. Move S1 to position A and again measure and record the data on the third line of the data table. 7. Move S2 to position B and then back to position A. Measure and record on the fourth line of the data table. 8. Move S1 to position C. Measure and record data on the fifth line of the data table.

48

Figure 8.2

Figure 8.3

49

9. Move S1 to position A. Measure and record the data on the sixth line of the data table. 10.Move S2 to position B and then back to position A. Measure and record the data on the seventh line of the data table. 11.Reverse the power supply voltages as shown in figure 8.3 to test the TRIAC for conduction in other direction. 12.Repeat the steps 1 through 10, but record the data in table 8.3.

50

_________________________ TRIAC Experiment VIII Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: PART I : TESTING A TRIAC WITH AN OHMMETER Table 8.1: Ohmmeter Reading Figure 8.1 Circuit (a) Circuit (b) Circuit (c) Circuit (d) Circuit (e) Circuit (f)

Reading ( H or L)

FILL-IN QUESTIONS: 1. A TRIAC will have before being triggered.

resistance in either direction

2. A TRIAC will have after being triggered.

resistance in either direction

3. A TRIAC is being tested with an ohmmeter. When the clip lead is removed, the meter indicates high resistance. This does not prove that the TRIAC is defective, but that the power source of the meter is not sufficient to produce the necessary through the device. 4. If the ohmmeter shows low resistance before the TRIAC is triggered, this indicates that the TRIAC is . 51

5. If the ohmmeter shows infinite resistance after the TRIAC is triggered, this indicates that TRIAC is . PART II: TRIGGERING MODES OF A TRIAC Table 8.2 S1 condition A B A A C A A

S2 condition A A A A-B-A A A A-B-A

VG

VT2

Condition of TRIAC (on or off)

Table 8.3 S1 condition A B A A C A A

S2 condition A A A A-B-A A A A-B-A

VG

VT2

Condition of TRIAC (on or off)

FILL-IN QUESTIONS: 1. Before the TRIAC is triggered on, the voltage across its main terminals is equal to the voltage. 2. When the TRIAC is triggered on, the voltage across its main terminals is about V. 3. Once the TRIAC is conducting, the gate control and the current to flow through the TRIAC. 52

4. The TRIAC can be turned off by reducing the current through it below its . 5. The TRIAC can have or terminal voltage and be triggered on by voltage applied to the gate.

or

ANALYSIS OF RESULTS:

QUESTIONS AND PROBLEMS: 1 What is the difference between a triac and an SCR in terms of basic operation?

53

2. How does a triac differ from diac?

3. Define VGT. What range of values does it have for a medium-sized triacs?

4. Are triacs inherently temperature stable? Explain.

54

EXPERIMENT IX AC TRIGGERING OF A TRIAC I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. 2.

Demonstrate how ac current to a load can be controlled by a TRIAC. Explain the operation of a triac in controlling both alternations of an ac supply driving a resistive load.

INTRODUCTION: The first part of this experiment uses only a variable resistor to vary the trigger time from 0o to 90o and 180o to 270o. An oscilloscope is used to view the voltage waveforms across the TRIAC and RL. In the second part of this experiment, a capacitor is added to the original circuit to extend the trigger tome for nearly the entire cycle. The zener diodes to simulate a DIAC, which makes the trigger time more symmetrical. II.

MATERIALS NEEDED: 1 1 1 2 2 1 1 1

24-V rms transformer or ac source Oscilloscope (use only one channel) 2N5754 TRIAC or equivalent 1N5231 zener diodes or equivalent (Z1, Z2) 100- resistors at 0.5W (RL and RA) 10-k potentiometer (RL) 5.0-F capacitor at 50 WV dc (CG) Breadboard for constructing circuit

55

III.

PROCEDURE: 1. Construct the circuit shown in figure 9.1. 2. Place the oscilloscope across the TRIAC. 3. Vary RG back and forth and view the voltage waveform across the TRIAC. 4. Adjust RG so that the TRIAC triggers about a halfway between 0o and 90o and 180o and 270o. 5. Draw the voltage waveform across the TRIAC in the space provided, making sure to align it with the proper degrees for one cycle (indicate peak-to-peak voltage). 6. Place the oscilloscope across RL. 7. Draw the voltage waveform across RL in the space provided, making sure to align it with proper degrees for one cycle. (Indicate peak-to-peak voltage)

Figure 9.1

8. Modify the circuit as shown in figure 9.2 by adding the capacitor. 9. Place the oscilloscope across the TRIAC. 10.Vary RG back and forth and view the voltage waveform across the TRIAC. 11.Adjust RG so that the TRIAC triggers past 90o and 270o. 12.Draw the voltage waveform across the TRIAC in the space provided, making sure to align it with the proper degrees for one cycle (indicate peak-to-peak voltage). 13.Place the oscilloscope across RL. 14.Draw the voltage waveform across the RL in the space provided, making sure to align it with the proper degrees for one cycle (Indicate peak-to-peak voltage). 56

Figure 9.2

57

________________________ AC TRIGGERING OF A TRIAC Experiment IX Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: GRAPH: Voltage waveform across the TRIAC using figure 9.1

Voltage waveform across the load resistor using figure 9.1

58

Voltage waveform across the TRIAC using figure 9.2

Voltage waveform across the load resistor using figure 9.2

FILL-IN QUESTIONS: 1. Using only a potentiometer, the ac trigger time of a TRIAC can be varied about to degrees and to degrees. 2. Using a potentiometer and a capacitor, the ac trigger time of a TRIAC can be varied over nearly one cycle.

59

3. When the TRIAC conducts, the voltage across RL is at out equal to the . 4. When the TRIAC conducts, the voltage across its main terminals is about V. 5. When the TRIAC is not conducting, the voltage across its main terminals is equal to the .

ANALYSIS OF RESULTS:

60

QUESTIONS AND PROBLEMS: 1. Does the firing delay angle of a triac during the positive half cycle necessarily equal the firing delay angle during the negative half cycle?

2. What happens to firing delay angle as temperature increases, assuming everything else is constant?

3. Explain why the load power can be reduced by turning the potentiometer backward once the triac has just barely started firing.

61

EXPERIMENT X PROGRAMMABLE UNIJUNCTION TRANSISTOR

I.

OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. 2. 3. 4.

State how to set the PUT trigger voltage. Demonstrate a practical go/no/go method of testing a PUT with an ohmmeter. Demonstrate the turn on (firing) and turn off (reset) methods for a PUT. Compare the PUT structure to that of the SCR.

INTRODUCTION: The PN junction from anode to gate of a PUT can be tested with an ohmmeter similar to a regular diode. However, testing for cathode to gate will not indicate a working PUT because one of the PUT junctions with always be reversed biased. The PUT can be tested with an ohmmeter by placing the positive lead on the cathode with the gate will trigger the PUT, and the meter should indicate low resistance if the power source is sufficient to produce the necessary holding current. Ro conduct, the PUT must have its anode more positive that its cathode. When the gate voltage is made more negative than its anode, the PUT turns on or fires and current flows from cathode to anode. When the gate voltage is again made equal to or more positive than the anode, current continues to flow through the PUT. The PUT is turned off or reset by reducing the current through it below its holding current for a specific power supply voltage.

62

II.

MATERIAL NEEDED: 1 1 1 1 1 1 1 1

Fixed + 6-V power supply standard or digital voltmeter standard or digital ammeter 2N6027 PUT or equivalent 1-k resistor at 0.5W (RL) 10-k resistor at 0.5W (RG) SPDT switched (S1 and S2) Breadboard for constructing circuit

Figure 10.1 testing a PUT with an ohmmeter: (a) without clip lead; (b) with clip lead; (c) again without clip lead

PART I: TESTING A PUT WITH OHMMETER 1. Set the ohmmeter to the midrange scale. 2. Connect the ohmmeter to the PUT as shown in figure 10.1a and record the meter reading in table 10.1. 3. Connect the clip lead as shown in figure 10.1b and record the meter reading. 4. Remove the clip lead as shown in figure 10.1c and record the reading.

63

PART II: OPERATION OF A PUT 1. 2. 3. 4.

Construct the circuit shown in figure 10.2. Set switches S1 and S2 as indicated and then apply power to the circuit. In the first row of the data table 10.2, record the values of VA and IAK. Move S1 to position B and record the values of VA and IAK in the second row of the data table. 5. Move S1 to position A and record the values of VA and IAK in the third row of the data table. 6. Move S2 to position B and record the values of VA and IAK in the fourth row of the data table. 7. Move S2 to position A and record the values of VA and IAK in the fifth row of the data table.

Figure 10.2

64

_________________________ PROGRAMMABLE UNIJUNCTION TRANSISTOR Experiment X Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: PART I : TESTING A PUT WITH AN OHMMETER Table 10.1: Ohmmeter Reading Figure 10.1 Circuit (a) Circuit (b) Circuit (c)

Reading ( H or L)

FILL-IN QUESTION: 1. A PUT will have

resistance before being triggered.

2. A PUT will have

resistance after being triggered.

3. A PUT is being tested with an ohmmeter. When the clip lead on the gate is removed, the meter indicates high resistance. This does not prove that the PUT is defective, but that the of the meter is not sufficient to produce necessary through the device.

65

PART II: OPERATION OF A PUT Table 10.2 Condition

S1

S2

Before firing

A

A

Firing

B

A

After Firing

A

A

Reset

A

B

After reset

A

A

VA

IAK

FILL-IN QUESTIONS: 1. Before firing, the voltage from the anode of the PUT to ground is equal to . 2. When the gate is made more negative, the PUT IAK . 3. Once the PUT is fired, the

and

loses control and continues to flow through the PUT.

4. When the PUT is conducting, VA is equal to 5. The PUT can be reset by reducing the current through it below its .

66

V.

ANALYSIS OF RESULTS:

QUESTIONS AND PROBLEMS: 1. Explain the difference between a UJT and a PUT.

2. Compare PUT structure to that of the SCR.

3. What does the term programmable mean as used in PUT?

67

EXPERIMENT XI DETERMINING PUT OPERATIONAL FACTORS

I.

OBJECTIVES: After completing and performing the laboratory experiment, you will able to: 1.

II.

Show how to calculate and measure VG and VP, and how to measure IAK and IH.

MATERIALS NEEDED: 1 1 1 1 1 1 1 1 1 1 1 1

III.

Fixed +9-V power supply Standard or digital voltmeter Standard or digital ammeter 2N6027 PUT or equivalent 10-k potentiometer (RA) 6.8-k resistor at 0.5W 47-k resistor at 0.5W 68-k resistor at 0.5W 5.6-k resistor at 0.5W 33-k resistor at 0.5W 56-k resistor at 0.5W Breadboard for constructing circuit

PROCEDURE:

1. Construct the circuit shown in figure 11.1 using the values of R1 and R2 from the first row of the data table 11.1. 2. Adjust RA until VA reads 0V. 3. Calculate VG and record on the data table. 4. Measure VG and record on the data table. 5. Calculate VP and record on the data table.

68

6. By adjusting RA, slowly increase VA until the meter indicates a sudden decrease. The point just before the meter reading decreases is VP. Slowly perform this step several times obtain an accurate measurement for VP. 7. Record VP in the data table. 8. With the PUT fired, (at the VP point), measure IAK and record in the data table. 9. With the PUT fired, slowly adjust RA so that IAK decreases. The meter reading will suddenly decrease to zero. The point just before this sudden decrease is the PUT’s holding current. Slowly perform this step several times to obtain an accurate measurement for IAK. 10.Record IH in the data table. 11.With the PUT fired, note the meter readings of VA and VG. The PUT is acting like a switch. 12.Repeat steps 2 through 11 for the other values of R1 and R2 given in the data table.

Figure 11.1 Determining PUT operational factors.

69

_________________________ DETERMINING PUT OPERATIONAL FACTORS Experiment XI Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: Table 11.1 R1

R2

6.8

5.6

47

33

68

56

R1 VG = ---------- (+VCC) R1 + R 2

VG

VP = VG + VD (0.7 V)

VP (VA)

FILL-IN QUESTION: 1. Vp is approximately

V greater than VG.

2. When the PUT fires, VA

.

3. When the PUT fires, VG

.

4. When the PUT fires, IAK

.

5. When the values of R1 and R2 increase, IH

70

.

IAK (mA)

IH (mA)

ANALYSIS OF RESULTS:

71

EXPERIMENT XII PUT RELAXATION OSCILLATOR

I.

OBJECTIVES: After completing and performing the laboratory experiment, you will able to: 1.

Demonstration how a PUT is used as a switch in a relaxation oscillator, and to determine the output frequency.

INTRODUCTION: The PUT oscillator is similar to the UJT oscillator, except that the trigger pulses at the gate and cathode appear to be sharper. Resistor R1 and R2 should be selected to produce a VG that when added to VP will equal 63.2% of +VCC (the value of VP). Resistor RA should be large enough to limit current below the holding current when the PUT is fired, or the PUT will never turn off. Therefore, no oscillations will be present. Components RA and CA determine the frequency of the oscillator and can be found in the formula: 1 f = --------------------RACA II.

MATERIALS NEEDED: 1 1 1 1 1 1 1 1 1 1

Fixed + 12 V power supply Standard or digital voltmeter Oscilloscope (dual trace preferred) 2N6027 PUT or equivalent 100- resistor at 0.5W (RK) 6.8-k resistor at 0.5W (R2) 47-k resistor at 0.5W (R1) 100-k resistor at 0.5W 56-kΩ resistor at 0.5W 0.1 µF capacitor 72

1 1 1 1 III.

220-k resistor at 0.5W 0.01-F capacitor 0.02-F capacitor Breadboard for constructing circuit

PROCEDURE:

Figure 12.1 PUT Relaxation oscillator

PART I: PUT RELAXATION OSCILLATOR 1. Construct the circuit shown in figure 12.1. 2. Using the voltmeter, measure and record the operating voltages VA, VG, and VK in the blank spaces provided in table 12.1. 3. Using the oscilloscope, examine the voltage waveforms at G, A and K. 4. Draw these voltage waveforms in the spaces provided and indicate their peak-to-peak values. 5. Calculate the frequency of the oscillator from the values of RA and CA and record in the record in the first row of the data table 12.2. 6. Place the oscilloscope at point A and measure the actual frequency. Remember that f=1/T, where T is the time period of one cycle. Record that frequency at the proper place of the data table. 7. Change components RA and CA as indicated by the data table and repeat steps 5 and 6.

73

PART II: PUT BASIC TROUBLESHOOTING APPLICATION Construct this circuit using your own value or those of a circuit from a previous section. Open or short the components as listed and record the voltages in the proper place in table 12.3. The abbreviations will help indicate the voltage conditions associated with each problem. All voltages are reference to ground. When the circuit is inoperative, there will be no voltage waveforms at VG, VK, and VA.

74

_________________________ PUT RELAXATION OSCILLATOR Experiment XII Group No._______ DOP:_______________ Section:___________ Time: _____________ Day ________Room______ Name:_______________________________ Signature_______________ Instructor:___________________________ DATA SHEET: PART I : PUT RELAXATION OSCILLATOR Table 12.1: Voltmeter Reading VA VG VK Table 12.2 RA (k)

CA (F)

100 100 100 47 220

0.01 0.1 0.02 0.01 0.01

f=1/ RA CA (HZ) Calculated

Measured

COMPUTATIONS:

75

GRAPH: Voltage waveform across the anode terminal, VA

Voltage waveform across the gate terminal, VG

Voltage waveform across the cathode terminal, VK

76

PART II: PUT BASIC TROUBLESHOOTING APPLICATION Table 12.3 Condition

VA

VG

VK

Normal RA open DEC

INC

NOC

CA open DEC

INC

NOC

DEC

INC

NOC

CA shorted RK open INC

R1 open R2 open

INC

INC

INC

+VCC

NOC

DEC

DEC

NOC

Comments All voltage proper Open RA at +VCC: no voltage applied to RC Open CA at GND: circuit may oscillate from stay capacitance; look for higher frequency Anode pulled to GND; PUT off Open RK at GND: no discharge path for CA; anode/ gate forward biased. Open R1 at GND; no reference voltage VG, PUT

Open R2 at +VCC : PUT saturates

Table abbreviations +VCC power supply voltage INC Increase DEC Decrease NOC No change GND Ground

FILL-IN QUESTIONS: 1. A oscillator.

pulse train is found at the gate of a PUT

77

2. A oscillator.

pulse train is found at the cathode of a PUT

3. The voltage waveform at the anode of a PUT oscillator is in the form of a . 4. If the value of RA or CA increases, the frequency of the oscillator . 5. If the value of RA or CA decreases, the frequency of the oscillator .

ANALYSIS OF RESULTS:

78