Solid State Relay

A solid-state relay (SSR) is an electronic switching device that switches states when an external voltage is applied along its n-type and p-type junctions. SSR has a small control signal that controls a larger load current or voltage. It consists of a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device which switches power to the load circuitry, 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.
Solid-state relays are composed of semiconductor materials, including thyristors and transistors. Solid-state relays have current ratings that extend from a few microamps for low-power packages up to around a hundred amps for high-power packages. Solid-state relays have extremely fast switching speeds usually ranging between 1 to 100 nanoseconds. Solid-state relays are not easily affected by contact wear. Solid-state relays have several shortcomings: a high susceptibility to damage (a relatively high vulnerability to overloads in comparison to electromechanical relays); limited switching arrangements (SPST switching); a need for finer tuning due to high "on" resistances.