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.