Triac and Diac

Description About Triac And Diac

1. TRIAC & DIAC Guidance by:- Mr. Goverdhan Yadu Submitted by:- Rahul Sahu M.Sc.- Physics 2 nd Sem

2. Contents:- TRIAC Construction Equivalent Circuit and Working Mode of Operation Characteristic Curve Application DIAC Construction Mode of Operation Characteristic Curve Application

3. TRIAC TRIAC= TRI ode for A lternating C urrent. A TRIAC is a three terminal and five layer, semiconductor switching device which can control alternating current in a load. It is bidirectional device. Means it can conduct current in both directions. The TRIAC of maximum rating of 16KW is available.

4. C onstruction o f TRIAC:- A TRIAC is equivalent to separate SCRs connected in inverse parallel. Anode of each connected to the cathode of the other with gates commoned . Therefore a TRIAC acts like a bidirectional switch i.e. it can conduct current in either direction. It consist of three terminals namely, Main terminal1(MT1),Main terminal2(MT2) and gate terminal (G).

5. E quivalent c ircuit And Working o f TRIAC:- The transistor equivalent circuit of TRIAC will be composed of four transistor. The transistor Q1 and Q2 constitute the equivalent circuit of SCR1 and transistor Q3 and Q4 constitute the equivalent circuit of SCR2. Operation :- When MT2 is positive with respect to MT1 and appropriate gate current is allowed in the gate circuit,SCR1 is turned ON while SCR2 remains OFF. In term of transistor equivalent circuit Q1 and Q2 are forward biased while Q3 and Q4 are reverse biased. Therefore, Transistor Q1 and Q2 conduct current. Since Q1 and Q2 from a positive feedback, both transistor are quickly driven to saturation and a large current flow through the load R L . This is as if switch between MT2 and MT1 were closed.

6. D iagram :-

7. D iagram :- (2) When MT2 is negative with respect to MT1 and appropriate gate current is allowed in the gate circuit, SCR2 is turned ON and SCR1 is OFF. In term s of transistor equivalent circuit, Q3 and Q4 are forward biased while Q1 and Q2 are reverse biased. Therefore, transistor Q3 and Q4 will conduct. The current in load R L will quickly attain a large value. The circuit will behave as if a switch is closed between MT2 and MT1.

8. M ode o f o peration :- MT2 is positive with respect to MT1 and Gate polarity positive with respect to MT1.

9. M ode o f o peration :- MT2 is positive with respect to MT1 with a Gate polarity negative with respect to MT1.

10. M ode o f o peration :- MT2 is negative with respect to MT1 with a Gate polarity positive with respect to MT1.

11. M ode o f o peration :- MT2 is negative with respect to MT1 with a Gate polarity negative with respect to MT1.

12. C haracteristic c urve o f TRIAC:- The V-I characteristic for TRIAC in the I st and III rd quadrants are essentially identical to those of an SCR in the I st quadrant. The TRIAC can be operated with either positive or negative gate control voltage but in normal operation usually the gate voltage is positive in quadrant I st and negative in quadrant III rd . The supply voltage at which the TRIAC is turned ON depends upon the gate current.

13. A pplication o f TRIAC:- It can operate and switch both half cycles of an AC waveform. In DC applications, SCRs are required to be connected with a parallel diode to protect against reverse voltage. But the TRAIC may work without a diode, a safe breakdown is possible in either directions. It can be used as a high power lamp switch It can be used to Phase Control.

14. dIAC DIAC = DI ode for A lternating C urrent. A DIAC is a two terminal, three layer bidirectional device which can be switched from its OFF state to ON state for either polarity of applied voltage. There is no control terminal on this device. This device works when avalanche breakdown occurs.

15. C onstruction o f dIAC :- The DIAC can be constructed in either npn or pnp . Show the basic structure of DIAC in PNP form. The two lead s are connected to p-regions of silicon separated by an n-region. The structure of DIAC is very much similar to that of a transistor, but there are several important differences:- There are no terminal attached to the base layer. The three regions are nearly identical in size. The doping concentrations are identical to give device symmetrical properties.

16. M ode o f o peration :- Forward Bias T1 is positive with respect to T2.

17. M ode o f o peration :- Reverse Bias T1 is Negative with respect to T2.

18. C haracteristic o f dIAC :- When a positive or negative voltage is applied across the terminals of a DIAC, only a small leakage current I BO will flow through the device. As the applied voltage is increased, the leakage current will continue to flow until the voltage reaches the breakover voltage V BO . At this point, avalanche breakdown of the reverse biased junction occurs and the device exhibits negative resistance i.e. current through the device increases with decreasing values of applied voltage. The Voltage across the device then drops to ‘ breakback ’ voltage V F .

19. C haracteristic c urve o f dIAC :-

20. A pplication o f dIAC :- DIAC are used primarily for triggering of TRIAC. Light Dimmer Heat Control

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