The bi-directional thyristor or TRIAC conducts current in both directions. It can be considered as two SCRs connected in antiparallel with a common gate connection.
The TRIAC structure, symbol and VI characteristics are shown in figure. In I+ mode, the terminal MT2 is positive and the device is switched on by a positive gate current pulse. In III– mode, the terminal MT1 is positive and the device is turned on by a negative gate pulse.
For ac power control, a triac is more economical than an inverse-parallel thyristor combination, but it has a few drawbacks.
  1. The gate current sensitivity is somewhat lower and turn-off time is slower than for a thyristor.
  2. Besides, the reapplied dv/dt capability is poor and, therefore, it is difficult to operate with an inductive load.
bi-directional thyristor or TRIAC conducts current in both directions
Triacs are commonly used in incandescent lamp dimming and heating control, where the load is resistive. Figure shows the popular triac light dimmer circuit. The gate of the triac is supplied from an R-C circuit through a diac.
A diac shown in the figure is a symmetric voltage-blocking device.Triac-symmetric voltage-blocking device
The variable resistance R1 controls the dimming level of the lamp, which is connected in series. When the capacitor voltage VC in either polarity exceeds the threshold voltage ±VS of the diac, a pulse of current triggers the triac at a firing angle (α), giving phase-controlled, full-wave current to the lamp.
When the triac is on, the R-C circuit is shorted and the line voltage is impressed across the lamp. At the end of every half-cycle, the triac turns off and the next half-cycle begins. An R-C snubber as shown is important to a triac circuit to reduce reapplied dv/dt.
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