TRIAC is equivalent to two SCRs connected in inverse parallel with the gates connected together. As a result, the TRIAC functions as a Bidirectional switch to pass the current in both directions once the gate is triggered. TRIAC is a three terminal device with a Main terminal1 ( MT1), Main terminal 2( MT2) and a Gate. The MT1 and MT2 terminals are used to connect the Phase and Neutral lines while the Gate is used to feed the triggering pulse. The Gate can be triggered either by a positive voltage or negative voltage. When the MT2 terminal gets a positive voltage with respect to the MT1 terminal and the Gate gets a positive trigger, then the left SCR of the TRIAC triggers and circuit completes. But if the polarity of the voltage at the MT2 and MT1 terminals is reversed and a negative pulse is applied to the Gate, then the right SCR of Triac conducts. When the Gate current is removed, the TRIAC switches off. So a minimum holding current Ih must be maintained at the gate to keep the TRIAC conducting.
Triggering a TRIAC
Usually 4 modes of triggering is possible in TRIAC:
A positive voltage at MT2 and a positive pulse at the gate
A positive voltage at MT2 and a negative pulse at the gate
A negative voltage at MT2 and positive pulse at the gate
A negative voltage at MT2 and a negative pulse at the gate
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