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. 7 - Triac and Diac

Triac and Diac

Laboratory Exercise No. 7

Performance Objectives

A. Demonstrate bi-directional conduction of a gated TRIAC.

B. Demonstrate the four triggering modes of a TRIAC.

C. Demonstrate bi-directional operation of DIAC.

Equipment and Materials

ü Power Source 6.3 Vac, 500mA

ü Power Source 6 Vdc, 500mA

ü Power Source

ü Electronic VOM

ü Oscilloscope

ü Bread board

ü C1 0.1μF, electrolytic

ü DS1 Miniature lamp

ü Q1 DIAC, ST2

ü R1 47Ω, 1W

ü R2 10KΩ, 1W

ü S1 PBNO

ü S2 SPST

Objective A. Demonstrate the bi-directional conduction of a gated TRIAC.

1. a) Connect the circuit shown in Fig. 8-3.

Figure 8-3

b) Adjust the main terminal voltage to 6.3 Vac.

c) Adjust the gate supply voltage to 6 Vdc.

d) Depress and hold S1. Does S1 light? Yes.

e) Release S1. Does DS1 go out? Yes.

f) Connect the oscilloscope across DS1 to monitor the load voltage.

g) Depress and hold S1. Is the voltage waveform a complete sine-wave? Yes.

Release S1.

Objective B. Demonstrate the four triggering modes of a TRIAC.

2. a) Change your circuit that shown in Fig. 8-4. Be sure S2 is open initially.

Figure 8-4

b) Adjust the main terminal and the gate voltage sources to +6Vdc.

c) Close S2 and depress S1 momentarily. Does DS1 light? Yes.

d) Look at your circuit and then record in Table 8-1 the triggering mode of Q1 in this configuration.

Procedure	TRIAC TRIGGERING MODE
	Quadrant	MT2 Polarity	Gate Voltage Polarity	Gate Current Polarity
2	I	+	+	+
3	I	+	–	–
4	III	–	–	–
5	III	–	+	+

Table 8-1

e) Open S2. Does DS1 go out? Yes.

3. a) Reduce the gate voltage supply to zero.

b) Reverse the polarity of the gate voltage supply by reversing the power supply leads connected to your circuit.

c) Adjust the gate voltage to -6Vdc.

d) Close S2 and depress S1 momentarily. DS1 should light.

e) Record in Table 8-1 the data for this triggering mode of Q1.

f) Open S2. DS1 should go out.

4. a) Reduce the main terminal supply voltage to zero.

b) Reverse the polarity of the main terminal voltage by reversing the power supply leads connected to your circuit.

c) Adjust the main terminal voltage to -6Vdc.

d) Close S2 and depress S1 momentarily. DS1 should light.

e) Record the data for this triggering mode of Q1 in Table 8-1.

f) Open S2. DS1 extinguishes.

5. a) Reduce the gate voltage supply to zero.

b) Reverse the polarity of the gate voltage supply again.

c) Adjust the gate voltage to 6Vdc.

d) Close S2 and depress S1 momentarily. DS1 should light.

e) Record the characteristics of this Q1 triggering mode in Table 8-1.

f) Open S2 and reduce all voltage sources to zero.

g) Look at the recorded data of Table 8-1. Would you say that TRIAC has four separate and distinct triggering modes? Yes.

Objective C. Demonstrate bi-directional operation of a DIAC.

6. a) Examine the circuit shown in Fig. 8-5. Connect the circuit as shown.

Figure 8-5

b) Adjust the power supply voltage to 30Vdc.

c) Momentarily ground the oscilloscope vertical input and position the trace at the center of the CRT for a zero volt reference. Measure the dc plus peak waveform across CR1. This is the break over voltage of CR1.

Sawtooth amplitude = 30 volts

d) Reduce the power supply voltage to zero.

e) Reverse the polarity of the power supply connected to your circuit.

f) Adjust the power supply to -40Vdc.

g) Is the polarity of the sawtooth inverted? Yes.

h) Measure the peak waveform as in step (c) above.

i) Is the break over voltage approximately the same value in both directions? Yes.

j) Reduce the power supply voltage to zero.

Conclusion

• When ac voltage is supplied across the terminals of the bilateral gated TRIAC, it is shorted, wherein a complete sine-wave input is supplied across the load to the TRIAC. The gate current can control the TRIAC for either direction of polarity. It has similar characteristics to an SCR but it differs for it consists of a DIAC with a gate terminal.

• Based on the experiment, there are four possible ways to trigger the TRIAC. It can be triggered by reaching its breakover voltage (+ or -). Positive or negative potential across the terminals can be triggered by reaching +/- breakover voltage. Once the gate V_BO is reached, the TRIAC can only be turned off when the potential across its terminals reaches 0V.

• For a DIAC, there is a breakover voltage present in either direction. It produces a sawtooth waveform that when polarity is reversed, an inverted sawtooth is displayed on the oscilloscope.