optotriac

An optotriac, also called a triac optocoupler, is an optocoupler that uses a light-activated triac as its output stage. It provides galvanic isolation between a low-voltage control circuit and a high-voltage AC load while delivering a gate signal to a main triac or thyristor.

Construction and operation: The input side contains an infrared LED driven by the control logic. When illuminated, a photo-sensitive triac on the output side conducts and couples a triggering current to the gate of a larger triac. The optotriac itself is not designed to carry the load current; it only provides a controlled gate signal. Some devices include zero-crossing circuitry to trigger near the AC waveform zero crossing, reducing EMI, while others are random-phase and suitable for phase control.

Variants and common devices: Common families include non-zero-cross MOC3020/3021/3022/3023 and zero-cross MOC3060–MOC3063. The devices are designed for high input-output isolation, typically several kilovolts RMS, and have low output leakage when off. The LED input requires a forward current in the tens of milliamperes to guarantee triggering, while the output triac gate current (I_GT) is on the order of a few milliamperes to tens of milliamperes. They are packaged in DIP or surface-mount formats and are intended to trigger main triacs in AC circuits rather than switch loads directly.

Applications and usage: Optotriacs are used to drive mains-load triacs in light dimmers, motor controls, power regulators, and solid-state relays. They enable control logic, such as microcontrollers, to operate high-voltage AC equipment with galvanic isolation and simplified gate interfacing. Designers often add a gate resistor and a snubber network around the main triac to ensure reliable triggering and dv/dt immunity.