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MPSA92 MUS4988 1N4003 2N6071A MPSA42 2N5444

When control signal calls for power, optoisolator energizes circuit that provides load turn-on at zero-voltage time of AC waveform. If phototransistor Q3 of optoisolator is illuminated after S1 drops to 1-V conduction voltage of MUS4988, triacs will not be turned on. Circuit thus provides relay-enabling voltage window, lower limit of which is point at which all components involved in turning on triacs are forward-biased. Upper limit is nominal 8-V breakover voltage of unilateral switch S1. S2 performs similar function on negative voltage alternations. Load-controlling triac is rated 40 A.-T. Mazur, Solid-State Relays Offer New Solutions to Many Old Problems, EDN Magazine, Nov. 20, 1973, p 26-32.

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