A solid-state relay (SSR) is an ON-OFF control device in which the load current is conducted by one or
more semiconductors – e.g., a power transistor, an SCR, or a TRIAC. It is an electronic switch, which, unlike an electromechanical relay, contains no moving parts.
The following schematic shows the Hybrid Solid State Relay SSR circuit diagram. It composed of a control signal, reed relay, TRIAC, optional preamplifier, trigger circuit, AC power and load. The control signal is applied (directly, or through a preamplifier) to the coil of a reed relay. The closure of the reed switch then activates appropriate circuitry that triggers the thyristor switch. Clearly, the input-output isolation achieved is that of the reed relay, which is usually excellent.