A Solid State Relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. They serve the same function as an electromechanical relay, but solid-state electronics contain no moving parts. SSRs consist of a sensor which responds to an appropriate input (control signal), 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 are characterized by a number of parameters including the required activating input voltage, current, output voltage and current, whether it is AC or DC, voltage drop or resistance affecting output current, thermal resistance, and thermal and electrical parameters for safe operating area. One of the main components of an SSR is an opto-isolator (also called an optocoupler) which contains one (or more) infra-red light-emitting diodes (LEDs) and a photo-sensitive device such as a phototransistor or a photo-SCR. When the LED is turned on, an output photo-sensitive device, such as a TRIAC, turns on and conducts current.
SSRs are used for switching on and off applications. They provide a voltage range of 3 VDC to 32 VDC, making them useful for most electronic circuits. An SSRs output type (AC, DC, or AC/DC) is determined by the type of switching device: a transistor (either bipolar or MOS), an SCR, or a TRIAC.
