Relay is a kind of electronic control devices, through the relay can use low voltage controller to control high voltage electrical appliances, it has control loop (output)It can separate the high voltage controlled system from the low voltage controlled system. According to the class of the input signalThe relay can be divided into current relay, voltage relay, temperature relay, pressure relay and time relayIt can also be divided into electromagnetic relay, electronic relay, electric relay, inductive relay, optical relay, thermal relay, etc.