Solid state relay is a contactless electronic switch with relay characteristics developed by the combination of separated electronic components, integrated circuits (or chips) and hybrid microcircuit technology. It is a four terminal active device. Two of them are low current input control terminal, and the other two ends are high current output controlled end, namely load end. Photoelectric isolation is adopted in the middle, which is used as electrical isolation between input and output. When the input is added with DC or pulse signal, the output can change from off state to on state, and it will be blocked when there is no signal, so as to control the large load. But because of this isolation, the load side of the relay is actually powered by switch circuit, so to start the relay, voltage and load must be provided at the load side. Solid state relay has the characteristics of long life, high reliability, fast switching speed, small electromagnetic interference, no noise and no spark, etc. it can be widely used in aerospace, navigation, home appliances, machine tools, communication, chemical industry, coal mine and other industrial automation fields.