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A solid state relay is an electronic switching device that switches on or off when an external voltage is applied across its control terminals. They serve the same function as an electromechanical relay, but as solid-state electronics contain no moving parts and have a longer operational lifetime.
SSRs consist of a sensor which responds to an appropriate input , an electronic 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. They may be designed to switch either AC or DC loads.
Packaged SSRs use power semiconductor devices such as thyristors and transistors, to switch currents up to around a hundred amperes.
SSRs have fast switching speeds compared with electromechanical relays, and have no physical contacts to wear out.