Triac,or TRIlode for alternating current, is an electronic device equivalent to two silicon-controlled rectifiers joined in inverse parallel (but with polarity reversed) with their gates connected together. The resulting bidirectional electronic switch can conduct current in either direction when triggered.

Like SCRs, triacs are also three terminal devices. Their MTl and MT2 terminals (main terminals I and 2) pass current in either direction, while the third terminal G (gate) sends trigger pulse to the device.

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Triacs can be triggered by either a positive voltage or negative voltage applied to their gate electrode. When the voltage on the MT2 terminal is positive with respect to MT2 and a positive voltage is applied to the gate, the left SCR in the triac conducts. If the voltage is reversed and a negative voltage is applied to the gate, the SCR on the right conducts. Minimum holding current 'Ih' must be maintained to keep the triac conducting. AC or DC pulses can trigger the triac. Four modes of triggering are possible:

1. Positive voltage to MT2 and posi¬tive pulse to gate

2. Positive voltage to MT2 and negative pulse to gate

3. Negative voltage to MT2 and positive voltage to gate

4. Negative voltage to MT2 and negative voltage to gate

Working of the triac is simple. Normally, the triac remains an open switch with the lamp turned off. When switch Sl is 'on: R1 provides gate current to the triac, so the triac self-latches shortly after the start of each half cycle and the lamp is provided with full power.