What is a Solid State Relay?

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.

How do Solid State Relays work?

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 then 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.

Key features of the IMO SSR range

• We have a full comprehensive range that include Pluggable, PCB and Panel Mounted. • IMO’s SSR range are suited to cover a wide range of applications including Temperature control, Motor Control, PLC and Home applications. • IMO have Transient overvoltage protection options across various SSR types. • Compact design and competitively priced. • High quality