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.