Q.Mention four modes of operation of TRIAC.

The TRIAC (Triode for AC) is a member of the thyristor family, but unlike an SCR — which conducts only from anode to cathode — a TRIAC conducts in both directions, making it a bidirectional controlled switch. This is why it is widely used for AC power control in lamp dimmers, heater controls and fan-motor speed controllers.

Its three terminals are Main Terminal 1 (MT1), Main Terminal 2 (MT2) and the gate (G). In its circuit symbol, two triangles are drawn back-to-back (anti-parallel) between MT1 and MT2, with a gate lead near MT1 — a picture of two thyristors in inverse parallel.

Because either main terminal can be positive or negative and the gate can be driven either way, there are four triggering (electrode-potential) modes: MT2 positive with G positive, MT2 positive with G negative, MT2 negative with G negative, and MT2 negative with G positive (all referenced to MT1). …