When a small external voltage is applied across its control terminals, a solidstate relay (aSSR) is an electronic switching device that switches on or off. SSRs consist of a sensor which responds to an appropriate input, a solid-stateelectronic switching device which switches power to the load circuitry, and a coupling mechanism to enable the control signal to activate this switch without mechanical parts. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.
