Solid State Relay (SSR) is a solid-state replacement of electro-mechanical relay, and is commonly used for general purpose switching of signals and low power AC/DC loads. It consists of a light-emitting diode (LED), a photo-sensitive FET driver and high-voltage MOSFETs. The relay turns on (contact closes) when the LED is on, and turns off (contact opens) when the LED is off. The SSR provides both switching and galvanic isolation functions.
SSR is highly effective in applications where there is a need to separate high voltage circuits from low voltage or low power circuits. By using SSR, circuits can exchange signals, and at the same time be galvanically isolated. The SSR allows for a safe interface between high voltage and low voltage circuits by breaking the ground loop to eliminate cross talk and interference.
SSR has been extensively used in a variety of application, such as industrial control, motor control, data acquisition, signal multiplexer, alarm switch, lamp control, and telecommunications equipment. In particular, for telephone applications it is always necessary to isolate telephone equipment from incoming telephone lines. Isolation is important to protect electronic equipment from harmful voltages or current caused by lightning. SSR, with high input-output transient rejection specifications, can provide excellent isolation and surge protection. This article introduces two common phenomenons of the transient voltage effect on SSR as well as the various methods of transient rejection measurements. In addition, an over-voltage protection device is recommended to further shield the SSR from hazardous surge voltage.