ECE121 Experiment No. 5 - UJT Waveform Generator

UJT Waveform Generator

Laboratory Exercise No. 5

Performance Objectives

1. Demonstrate the operation and determine the frequency of a UJT relaxation oscillator.
2. Determine the effect of a change in timing components on the frequency of oscillation in a UJT relaxation oscillator.
3. Demonstrate the operation of a UJT square wave generator.

Equipment and Materials

Power Source 0-12Vdc, 20mA

Electronic VOM

Oscilloscope

C1 0.22ųF

C2 25ųF electrolytic

C3 0.022ųF

CR1 Silicon Diode IN4004

Q1 Unijunction Transistor 2N2646

R1 10KΩ, 1W

R2 1KΩ, 1W

R3 47Ω, 1 W

R4 100KΩ, 1W

R5 1MΩ, 1W

R6 22KΩ, 1W

R7 47KΩ, 1W

Breadboard

Exercise Procedure

Objective A. Demonstrate the operation and determine the frequency of a UJT relaxation oscillator.

1.

a) Connect the relaxation oscillator shown in Fig. 6-3.

Fig. 6-3.

b) Adjust V_BB to 12Vdc.

c) Monitor the waveform across C1 using the oscilloscope. What type is displayed and what is the amplitude?

The waveform displayed is a saw tooth waveform. The amplitude is 4V.

d) Measure and record the period of the waveform displayed on the oscilloscope. Measure between two successive peaks on the waveform.

t = __2.2 _ msec

e) Calculate the frequency of oscillation.

f = 1/t = _1/(2.2 msec) = 454.545 Hz_

f) Monitor the waveform across base 1 resistor R3. What type of waveform is displayed and what is the amplitude?

A pulse waveform is displayed. The amplitude is 5.6V.

g) Monitor the waveform from base 2 to ground. Describe the waveform.

There is a pulse waveform and a square wave from b1.

h) Calculate the time period from the relaxation oscillator using the values of R1 and C1.

t = R1C1 = _(10K)(0.22ųF) = 2.2 msec_

i) Now calculate the frequency using the RC time constant.

f = 1/t = 1/R1C1 = _1/(2.2 msec) = 454.545 Hz_

j) Compare the frequency using the RC time constant with the frequency determined using the measured time period. Are the two values in agreement?

Yes, the two values are equal and in agreement.

k) Decrease V_BB to 11Vdc.

l) Measure the time period of the saw tooth across C1.

t = _2.15 msec_

m) Reduce V_BB to 10Vdc.

n) Measure the time period saw tooth again.

t = _2.05 msec_

o) Does decreasing V_BB show any noticeable change in the period of the saw tooth?

No, there is no noticeable change.

Objective B. Determine the effect of a change in timing components on the frequency of oscillation in a UJT relaxation oscillator.

2.

a) Replace 10KΩ timing resistor in your circuit with 100KΩ resistor R4.

b) Adjust V_BB to 12Vdc.

c) Measure the period of the saw tooth across C1 using the oscilloscope.

t = _0.035 msec_

d) Calculate the new frequency.

F=1/t = _28.57 Hz_

e) Reduce V_BB to zero.

f) Replace 100KΩ resistor R4 with 1MΩ resistor R5 in the timing circuit of your UJT relaxation oscillator.

g) Adjust V_BB to 12Vdc.

h) Now measure the period of the saw tooth across C1 again.

t = _0.23 sec_

i) Calculate the new frequency.

f = 1/t = _4.35 Hz_

j) Reduce V_BB to zero.

3.

a) Remove 1MΩ resistor R5 from the timing circuit in your UJT relaxation oscillator and replace it with 10KΩ resistor R1.

b) Remove 0.22ų F timing capacitor C1 and connect 25ųF electrolytic capacitor C2 in its place. The positive end of C2 connects to the emitter and the negative end connects to the ground.

c) Adjust V_BB to 12Vdc.

d) Measure the time period of the saw tooth waveform across C2.

t = _0.035 sec_­

e) Calculate the frequency of oscillation.

f = 1/t = _28.57 Hz_

f) Compare the frequency of (e) with the value determined in procedure 1(e). Underline the correct answers. Increasing the value of capacitance in the timing circuit of a UJT relaxation oscillator (increases, decreases) the RC time constant and (increases, decreases) the frequency of oscillation.

g) Reduce V_BB to zero.

Objective C. Demonstrate the operation of a UJT square wave generator.

4.

a) Examine the circuit shown in Fig. 6-4. Capacitor C3 charges through diode CR1 and resistor R6.

Fig. 6-4

b) Connect the circuit as shown in Fig. 6-4.

c) Adjust V_BB to 10Vdc.

d) Monitor the waveforms at base 2, emitter and across C3 using the oscilloscope and compare them with the ones in Fig. 6-5. Are they as shown?

Yes they are the same.

e) Measure the period of the square wave at base 2.

t = _16 msec_

f) Reduce V_BB to zero.

Conclusion

After performing the experiment, the group has learned that:

• A UJT relaxation oscillator operates through the following. C1 charges by resistor R1. UJT will turn ON when its Vp value is reached through voltage charging across C1. Resistance between emitter and base 1 will decrease, which allows C1 to discharge to the emitter-B1 junction to the ground. As C1 discharges, its voltage decreases, which causes the UJT to switch OFF. Its frequency depends on the UJT’s Vp rating, V_BB, and RC time constant.

ECE121 Experiment No. 6 - UJT-SCR Time Delay Circuit

UJT-SCR Time Delay Circuit

Laboratory Exercise No. 6

Performance Objectives

A. Demonstrate the operation of a UJT-SCR time delay circuit.

B. Calculate and measure the time period of a UJT-SCR time delay circuit.

Equipment and Materials

ü Power Source 0-12 Vdc, 500mA

ü Electronic VOM

ü Oscilloscope

ü Bread board

ü C1 10μF, electrolytic

ü CR1 Silicon Diode, IN4004

ü DS1, DS2 Miniature lamps

ü Q1 UJT, 2N2646

ü Q2 SCR C106B1

ü R1 270Ω, 1W

ü R2 4.7KΩ, 1W

ü R3 500KΩ, ½ W Potentiometer

ü R4 100Ω, 1W

ü R5 470Ω, 1W

ü S1 SPST

ü S2 PBNC

Objective A. Demonstrate the operation of a UJT-SCR time delay circuit.

1. a) Connect the circuit shown in Fig. 7-1. Leave S1 open.

b) Adjust R3 to about mid-position.

c) Adjust the power supply to 12Vdc.

d) Close S1 and wait about 5 seconds. Do S1 and DS2 light? Yes.

Figure 7-1

e) Do DS1 and DS2 remain lit? Yes.

f) Depress S2 momentarily and then release it. Do DS1 and DS2 go out? Yes.

g) Do DS1 and DS2 light again after about 5 seconds? Yes.

h) Adjust R3 to increase the timing circuit resistance.

i) Depress and release S2. Does increasing the resistance of R3 make the time delay period longer? Explain.

Yes. Varying the resistance determines the time the capacitor charges to Vpeak thus increasing the resistance value makes it longer for the capacitor to charge for the UJT to short which triggers the SCR to turn on, as seen in the lamps when they light on.

j) Would decreasing the resistance of R3 shorten the time delay period? Explain.

Yes. Decreasing the value of R3 shortens the time delay since it just needs a short time for the capacitor to charge to its Vpeak value. After a quick charge, it then discharges and supplies voltage to the UJT component.

k) Adjust R3 to decrease the timing delay circuit resistance.

l) Depress and release S2. Is the time delay shorter than before? Yes.

Objective B. Calculate and measure the time period of a UJT-SCR time delay circuit.

2. a) Readjust R3 to mid-position.

b) Connect the oscilloscope vertical input leads between the emitter of Q1 and circuit common. Set the oscilloscope controls for dc mode operation, a slow sweep speed and a convenient vertical deflection.

c) Depress and release S2. What does the gradually increasing oscilloscope waveform represent?

The increasing waveform represents the charge of the capacitor going to its Vpeak value.

d) What terminates the oscilloscope sweep?

The time the capacitor reaches its Vpeak and the start of its discharge time.

e) Depress and release S2 and record the maximum voltage across C1.

EC3 = 10 Volts

f) What does the amplitude of this voltage represent with regard to the UJT?

This is the maximum voltage value it reaches during the capacitor’s charging time before it supplies voltage to the UJT and triggers the SCR to switch on so that the lamps will also be turned on.

g) Adjust R3 for minimum trigger circuit resistance.

h) Assume R3 has no resistance when set to minimum. Calculate the time period of the time delay circuit using the values of R2 and C1.

T = RC = (4.7K)(10μF)

T = 47 msec

i) Depress and release S2 and measure the changing time constant of C1 using the oscilloscope.

T = 44 msec

j) Compare your calculated and measured time periods. Do they agree? Explain.

Yes. The values for the calculated and measured time periods agree since, R3 set to its minimum, calculating the RC time constant is the time when the capacitor C1 charges to its Vpeak – also ideally the same when it is actually measured.

k) Open S1 and reduce the power supply voltage to zero.

Conclusion

• In a UJT-SCR time delay circuit in this experiment, which consists of a UJT triggering the SCR component so that delay in time is achieved, increasing the resistance also increases the time delay output of the circuit. This is so because it takes a longer time for the capacitor to charge. When it reaches Vpeak, it discharges and supplies voltage to UJT to short then triggers the SCR to turn on. While decreasing the resistor value also shortens the time delay period of the circuit. Therefore, varying the resistance for a timing delay circuit using UJT and SCR, determines the time delay period.

• The calculated and measured time periods in a UJT-SCR time delay circuit across the capacitor both agree since they have the same RC time constant values which is defined by T=RC. Also, the time period increasing amplitude in the oscilloscope represents the voltage peak value of the capacitor during which the time it charges. After it saturates, the voltage across the capacitor discharges thereby producing a drop in the waveform as seen in the oscilloscope.

ECE121 Experiment No. 4 - Unijunction Transistor

UNIJUNCTION TRANSISTOR

Laboratory Exercise No. 4

Performance Objectives

A. Measure the interbase resistance and determine the emitter-base 1 PN junction diode characteristics of a unijunction transistor.

B. Determine the intrinsic standoff ratio of a unijunction transistor.

C. Measure the peak emitter firing voltage of a unijunction transistor.

Equipment and Materials

ü Power Source 0-10 Vdc, 20mA

ü Electronic VOM

ü Bread board

ü C1 0.22μF

ü C2 0.1μF

ü CR1 Silicon Diode, IN4004

ü Q1 Unijunction Transistor, 2N2646

ü R1 10KΩ, 1W

ü R2 100KΩ, 1W

ü R3 1KΩ, 1W

ü R4 10KΩ, 1W

Objective A. Measure the interbase resistance and determine the emitter-base 1 PN junction diode characteristics of a unijunction transistor.

1. a) Look at the UJT and identify the base 1, base 2, and emitter leads. Set the Electronic VOM to the ohmmeter function on the R x 1K range.

b) Measure and record the resistance between B1 and B2 with the emitter open (R_BBO).

R_BBO = 5.45 KΩ

c) Reverse the ohmmeter leads and measure R_BBO again.

R_BBO = 5.38 KΩ

d) Do your measurements of R_BBO indicate that the UJT has a PN junction between B1 and B2? No.

e) Measure and record the forward emitter to base 1 resistance. Connect the ohms probe of the ohmmeter to the emitter and the ground lead to base 1.

R_{EB1 (forward)} = ___KΩ

f) Reverse the ohmmeter leads and measure the reverse resistance between emitter and base 1.

R_{EB1 (reverse)} = ∞

g) Compare your R_EB1 measurements. Do these values indicate the UJT has a PN junction between emitter and base 1? Yes.

Objective B. Determine the intrinsic standoff ratio of a unijunction transistor.

2. a) Examine the circuit shown in Fig. 5-2. Capacitor C2 changes through R1 until it reaches the firing voltage of the UJT. When Q1 fires and Q2 discharges through the emitter-base 1 circuit of the UJT, the charge on C2 then drops below the value required to hold Q1 in conduction and Q1 cuts off again.

Figure 5-2

b) Connect the circuit shown in Fig. 5-2.

c) Adjust V_BB to 10 Vdc.

d) Measure and record the peak voltage across C1.

E_C1 = 0.679 Volts

e) Calculate the intrinsic standoff ratio using the V_BB value of (c) and E_C1 of (d).

Ŋ = Vp/ V_BB = E_C1/V_BB

Ŋ = 0.679/10

­ Ŋ = 67.9 mV

f) Reduce V_BB to zero.

Objective C. Measure the peak emitter firing voltage of a unijunction transistor.

3. a) Now change your circuit to that shown in Fig. 5-3. Adjust R4 for minimum resistance initially.

Figure 5-3

b) Adjust V_BB to 10 Vdc.

c) Slowly increase the resistance of R4 while monitoring the voltage from emitter to ground on the Electronic VOM. What happens to the voltmeter reading?

As the resistance is increased, the voltage across UJT also increases until it reaches the maximum voltmeter reading. It attains the highest voltage then drops to a small value.

d) What does the peak reading on the voltmeter represent?

The voltage across the voltmeter represent the voltage to trigger your UJT to “on”.

e) What causes the sudden decrease in emitter voltage?

The decrease in emitter voltage is a result that the UJT was triggered by a positive voltage. When the UJT is on, the resistance across the base decreases.

f) Reduce R4 to minimum resistance again.

g) Repeat (c) and (f) as many times as necessary until you obtain an accurate measure of Vp.

Vp = 7.06 volts

h) Compare the value of Vp with the value E_C1 of in 2(d). Do the values agree? Explain any discrepancy.

No, they do not agree. They have small difference and the resistance with respect to ground differs at least 1KΩ.

i) What effect would a larger value of _ Vp?

The effect of a larger Vp will increase the voltage in the emitter.

j) What effect would a larger value of V_BB have on Vp?

If V_BB would have a large value, Vp will also increase because diode voltage and h are fixed values. Thus, varying V_BB will affect Vp. (Vp = V_D + h V_BB)

k) Reduct the power supply voltage to zero.

Conclusion

• Since there is an approximately equal resistance between the bases of the unijunction transistor, it can be concluded that there is they have no polarity, thus has no PN junction. While the emitter-base resistance has polarity so there is PN junction between them – positive on the emitter and negative on either base. R_BB = R_B1 + R_B2 when I_E= 0.
• Based on the experiment, the intrinsic standoff ratio of a unijunction transistor is defined by Ŋ = Vp/ V_BB = E_C1/V_BB where Vp has a small voltage value because of a small resistance in base 1.

• By increasing the resistance with respect to emitter also increases the voltage entering the UJT therefore triggering it to turn ‘on’. When V_E = Vp, conduction is established and emitter potential V_E will drop with increase in I_E. A larger Vp will increase the voltage in the emitter.