phototriac

Phototriac, also called an optically triggered triac, is a type of optoisolator that uses a light-sensitive output stage in the form of a triac. It provides electrical isolation between control electronics and AC power circuits and can be used to trigger a larger triac or to switch low-power AC loads.

Internally, it consists of an LED input and a bidirectional phototriac output. When the LED is driven, photons activate the output device, causing it to conduct in either direction. The output conducts only while the load current remains above its minimum holding current and the input is energized.

Because the output is a triac-like device, phototriacs are not suitable for high-current switching by themselves; they are typically used to trigger a main triac or SCR in AC circuits. Some versions include a zero-cross switching feature to reduce EMI; non-zero-cross devices can trigger at any point in the cycle but may induce more EMI.

Key specifications include the trigger current required on the LED side, the input-output isolation voltage, the output triac's voltage rating and maximum current, the off-state leakage, and the dv/dt rating. These devices are commonly packaged in DIP-6 or SMT optocoupler packages.

Typical applications are control of mains AC loads, such as lamp dimming, fan or motor control, and solid-state relays that isolate control circuitry from power electronics.

See also: optocoupler, triac, optotriac, solid-state relay.