Short note: Working principle of TRIAC

A TRIAC (Triode AC switch) is a three-terminal, bidirectional thyristor that can conduct in both directions. Its terminals are MT1, MT2 and gate G. It is equivalent to two SCRs connected in inverse parallel with a common gate.

        MT2
         |
    +----+----+
    |         |
   SCR1 ->   <- SCR2    (inverse parallel)
    |         |
    +----+----+
         |       G
        MT1 -----+

Working principle:

• When MT2 is positive with respect to MT1, a gate pulse (positive or negative) turns ON the left PNPN path and current flows MT2 → MT1.
• When MT1 is positive with respect to MT2, a gate pulse turns ON the other path and current flows MT1 → MT2.
• Once ON it latches until the current falls below the holding current, which happens naturally at each zero crossing of AC.
• It can be triggered in four modes (MT2 +/−, gate +/−); sensitivity is highest in quadrants I+ and III−.

V-I characteristic is identical in the first and third quadrants: blocking until breakover (reduced by gate current), then a low on-state voltage.

Applications: light dimmers, fan speed regulators, heater control, AC motor speed control, AC static switches. It is usually triggered by a DIAC.

Limitations: lower voltage/current and dv/dt ratings than SCRs, and suitable mainly for 50/60 Hz.