This document discusses TRIACs and DIACs. TRIACs are bidirectional semiconductor switches that can control AC in a load. They consist of two SCRs connected in inverse parallel with a common gate. DIACs are also bidirectional semiconductor devices that can be switched from off to on with either polarity of applied voltage. They have no control terminal. Both devices exhibit avalanche breakdown and negative resistance characteristics. TRIACs are used for phase control and lamp switching. DIACs are primarily used to trigger TRIACs in applications like light dimmers and heat controls.

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

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

3. TRIAC
 TRIAC= TRIode for Alternating Current.
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. Construction Of 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. Equivalent Circuit And Working Of
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:-
(1) 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 RL. This is as if switch
between MT2 and MT1 were closed.

6. Diagram :-

7. Diagram :-
(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 RL will quickly attain a large
value. The circuit will behave as if a switch is closed between MT2 and
MT1.

8. Mode Of Operation:-
1) MT2 is positive with respect to MT1 and Gate polarity
positive with respect to MT1.

9. Mode Of Operation:-
2) MT2 is positive with respect to MT1 with a Gate polarity
negative with respect to MT1.

10. Mode Of Operation:-
3) MT2 is negative with respect to MT1 with a Gate polarity
positive with respect to MT1.

11. Mode Of Operation:-
4) MT2 is negative with respect to MT1 with a Gate polarity
negative with respect to MT1.

12. Characteristic Curve Of TRIAC:-
 The V-I characteristic for TRIAC in the Ist and IIIrd quadrants are
essentially identical to those of an SCR in the Ist 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 Ist and negative in quadrant IIIrd.
The supply voltage at which the TRIAC is turned ON depends upon the
gate current.

13. Application Of 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 = DIode for Alternating Current.
 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. Construction Of 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:-
i. There are no terminal attached to the base layer.
ii. The three regions are nearly identical in size.
iii. The doping concentrations are identical to give device symmetrical
properties.

16. Mode Of Operation:-
Forward Bias
1) T1 is positive with respect to T2.

17. Mode Of Operation:-
Reverse Bias
2) T1 is Negative with respect to T2.

18. Characteristic Of DIAC:-
 When a positive or negative voltage is applied across the
terminals of a DIAC, only a small leakage current IBO 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 VBO.
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 VF .

19. Characteristic Curve Of DIAC:-

20. Application Of DIAC:-
 DIAC are used primarily for triggering of TRIAC.
 Light Dimmer
 Heat Control

21. THANK YOU☺