A solid state relay (SSR) is an electronic switching device in which a small control signal controls a larger load current or voltage. It comprises a voltage or current sensor which responds to an appropriate input (control signal), a solid-state electronic switching device of some kind which switches power to the load circuitry either on or off, and some 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.
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The control signal must be coupled to the controlled circuit in a way which isolates the two circuits electrically.
Many SSRs use optical coupling. The control voltage energizes an LED which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back thyristor, silicon controlled rectifier, or MOSFET to switch the load. The optical coupling allows the control circuit to be electrically isolated from the load.
An SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array, works well for DC loads.
There is an inherent substrate diode in all MOSFETs that conducts in the reverse direction. This means that a single MOSFET cannot block current in both directions. For AC (bi-directional) operation two MOSFETs are arranged back to back with their source pins tied together. Their drain pins are connected to either side of the output. The substrate diodes are alternately reverse biased in order to block current when the relay is off. When the relay is on, the common source is always riding on the instantaneous signal level and both gates are biased positive relative to the source by the photo-diode.
It is common to provide access to the common source so that multiple MOSFETs can be wired in parallel if switching a DC load. There is also commonly some circuitry to discharge the gate when the LED is turned off, speeding the relay's turn-off.
The relay is characterised by a number of parameters including the required activating input voltage, current, and whether it is AC or DC; the output voltage and current and whether it is AC or DC, voltage drop or resistance affecting output current, thermal resistance, and thermal and electrical parameters for safe operating area (e.g., derating according to thermal resistance when repeatedly switching large currents).
Most of the relative advantages of solid state and electromechanical relays are common to all solid-state as against electromechanical devices.