TRIAC - Basics, Operation, Applications, Structure, Testing

TRIAC = TRIode for Alternating Current
The TRIAC is a five layer, three terminal Power semiconductor device which has a pair of phase controlled SCRs connected in inverse parallel manner on the same chip. It is a bidirectional device, means it can conduct current in both the directions.

Symbol & Structure:


  • Remember that Thyristor looks like two PNP transistor connected in a back to back manner.
  • But it is clear from the structure that physically the triac does not comprise two thyristors connected in parallel.
  • It only functions as two inverse parallel connected thyristors on alternating current. 
  • The triac is not designed for work on DC and unlike a pair of inverse parallel connected thyristors, does not operate very stably on DC

Triac Equivalent Circuit:

The equivalent circuit of Triac is given below:

       This three terminal device can be triggered into conduction in both positive and negative half cycles of supply voltage by applying gate trigger pulses.

In mode-I, the terminal T2 is positive and the device is switched on by positive gate current pulse.
In mode-II, the terminal T1 is positive and it is switched on  by negative gate current pulse.
A triac is more economical than a pair of SCRs in anti parallel and its control is simpler.

Disadvantages of Triac:

  • Well designed RC snubber is required for protection.
  • The reapplied dv/dt rating is lower, so it is difficult to use with inductive load.
  • The gate current sensitivity is poorer.
  • The turn off time is longer due to the minority carrier storage effect.

Applications of Triac:

  • Light dimming
  • Heating control
  • Appliance type motor drives
  • Solid state relays with typically 50/60Hz supply frequency

Testing the Triac:

  • To test the this device, connect the ohmmeter leads to MT2 and MT1.
  • The ohmmeter should indicate no continuity.
  • If the gate lead is touched to MT2, the device should turn on and the ohmmeter should indicate continuity through the triac.
  • When the gate lead is released from MT2, the device may continue to conduct or it may turn off depending on whether the ohmmeter supplies enough current to keep the device above its holding current level.
  • The above mentioned procedure tests one half of the triac.
  • To test other half of the device, reverse the connection of the ohmmeter leads.
  • The ohmmeter should indicate no continuity. If the gate is touched again to MT2, the ohmmeter should indicate continuity through the device. 

Other Power Devices to Read:
Gate Turnoff Thyristor (GTO)
Power Mosfet
UJT - UniJunction Transistor

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1 Response

  1. yogita says:

    give description of modes

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