Relay (English Name: Relay) is a kind of electric control device, when the input (excitation volume) changes to the requirements, the electrical output circuit to make the controlled quantity of an electric appliance the occurrence of a predetermined step change. It has a control system (also known as the input circuit) and the controlled system (also known as the output circuit) the interaction between the. The control circuit is usually applied to automation, it is actually a kind of "automatic switch to control large current operation with small current". It plays in the automatic adjustment circuit, safety protection, the role of conversion circuit.