Functions of Relay The relay is a kind of electric device that can connect or disconnect the control circuit according to the change of some input signal in order to realize automatic control and protect the electric driving system. The  input signals here mentioned can be electrical quantity of voltage or current etc. (they can connect or disconnect the electric circuit when they reach to a certain value of current and voltage), or they can be the non-electrical  quantity of rotating speed, time, temperature and pressure( the monitored equipment can connect or disconnect the electric circuit when they reach to a certain rotating speed, a period of time or temperature value or a certain gas/liquid pressure). 

Generally the relay is not directly used for controlling the main circuit of strong current with stronger signal, but for controlling the main circuit via the contactor or other electric device, therefore compared with the contactor, the breaking capacity of the relay contactor is very small, generally the arc extinguishing device is not required. The relay is featured by simple structure, small volume and light weight, but the requirement on the action accuracy of the relay is higher. A simple electromagnetic relay without shell: when the flowing current reaches a certain value, the coil magnets will attract the contact to connect or disconnect the electric circuit. 

There are many kinds of relays, the working principle and structure of which  are different, but generally speaking, the relay is composed of three parts of bearing mechanism , intermediate mechanism and actuators. The relays can be divided into the control relay and the protective relay according to the difference of the usage. 

According to the principle of the action, they can be divided into: Electromagnetic relay, induction type relay, thermal relay, mechanical relay, electric relay and electronic relay. 

According to reaction parameters, they can be divided into Current relay, voltage relay, time relay, speed relay, pressure relays etc. 

According to the action time, they can be divided into: instantaneous relay and delay relay.