Abstract: A solid state relay in which the control circuit for controlling a bidirectional conduction device (triac) includes a silicon controlled rectifier (SCR) and a photo transistor that is responsive to a light coupled control signal. Both the SCR and the photo transistor are coupled in parallel between the d.c. terminals of a full wave rectifier. The base of the photo transistor and the gate of the SCR are coupled to the collector of a transistor that is connected in an common emitter configuration between the d.c. terminals of the rectifier. The last-named transistor has its base connected to the collector of the photo transistor. The last-named transistor is a zero voltage crossover detector and conducts only when the photo transistor is nonconducting. The photo transistor and SCR may conduct only when the crossover detector transistor is nonconducting.