triac

A triac is a solid-state electronic switch that can conduct current in both directions through a single device. Unlike a standard thyristor, which blocks current in the reverse direction, a triac contains two thyristor structures in parallel, oriented to pass current regardless of polarity. Once triggered by a small gate signal, it remains conducting until the current naturally falls below the holding threshold, typically at a mains voltage zero-crossing.

The name comes from "triode for alternating current". The three terminals are the anode (main terminal 1), cathode (main terminal 2), and gate. In practice, both main terminals are interchangeable because the device conducts bidirectionally. The gate retains a single polarity convention for triggering purposes, though either gate polarity can fire the device depending on the main current direction.

Triacs are the standard component in ac power control circuits. They appear in light dimmers, fan speed controllers, heater regulators, and motor soft-starters. By triggering the gate at different points in each mains cycle, you control the fraction of the cycle that current flows, thereby controlling average power delivered to the load. A triac conducts the entire sine wave if triggered near the start; it blocks most of it if triggered near the zero-crossing.

Practical constraints and failure modes

Triacs have ratings for continuous current (typically 4 to 40 amps in general-purpose industrial grades) and peak gate current. The rate of change of voltage (dv/dt) is critical: if the voltage across the device changes too rapidly without gate triggering, the device may fire unintentionally due to junction capacitance effects. Industrial triacs are therefore often paired with snubber networks (resistor and capacitor in series) across their terminals to limit dv/dt to safe levels, especially in inductive circuits.

Triacs suffer from latching at high temperatures and can be destroyed by excessive gate current, reverse gate voltage exceeding rated limits, or surge currents beyond their peak non-repetitive current rating. Many fail shorted (conducting at all times), which is dangerous because it locks the load permanently on. Capacitive loads, common in power factor correction circuits, can degrade triac lifetime significantly because of high dv/dt stresses during turn-off.

The device is largely superseded by insulated gate bipolar transistors (IGBTs) and gate turn-off thyristors (GTOs) in high-power applications because these allow active commutation and better control. However, triacs remain entrenched in low-cost ac switching below about 50 amps because they require minimal support circuitry and no forced commutation.