Simple SSR has zero-cross on/off switching

This Design Idea is a solid state relay (SSR) which uses a triac to switch AC loads at high current. There are plenty of simple SSR circuits available, but this may be the simplest circuit that achieves turn-on and turn-off only when the AC line voltage is near zero. Transistor Q2 acts as a clamp to disable the SCR gate drive voltage when the AC line is above about 15V. When the line voltage is below that v ...

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