Solid State Relays

Solid-state relays (SSR) are electronic switching devices that switch on or off when a current is applied to their energizing circuit. (Please click here for an explanation of relay basics.) Instead of mechanical parts solid state relays have an optical sensor functioning as the pole which responds to an appropriate signal or input, and a solid-state electronic switching device functioning as the throw which switches power to the load circuitry (also known as the controlling or contact circuit). 

Solid state relays turn on and turn off quicker and require less voltage to turn on and off because they have no moving, physical parts. Without moving parts, solid-state relays can also be smaller and have a longer life cycle. With solid state relays there is residual electrical resistance and / or current leakage whether switches are open and closed due to their construction, but the small voltage drops this creates usually aren’t a problem.