SSR is a kind of electronic switch with isolating function and without mechanical contacts,made up of semiconductor and passive element,make use of photoelectric coupler and transformer to lelctric isolate or electric couple the control circuit and load circuit,there isn't any movable parts inside.The solid state relay not only has switching function that is the same as electromagnetic relay,but also has advantages such as compatible with logic circuit,firm mounted switch,low electromagnet interference or radio frequency interference,long service life and high reliablity.