Relays are used in our lives as a control device , the switch is common sense , in the case of closed or open conditions are met . Switching characteristics of the relay has been widely used in many control systems in particular discrete control system. From another perspective , the design of electronic circuits used for a certain purpose , and eventually more or less need to interact in a number of machinery and equipment , so the interface relay also played the role of electronic equipment and mechanical equipment.

 Relay is automatically switching elements with isolation function , widely used in remote control, telemetry , communications , automation , mechatronics and power electronic devices, is one of the most important control elements.

    Relays generally reflects some input variables ( such as current, voltage, power , impedance, frequency, temperature , pressure, speed , light, etc. ) of the sensing means ( input portion ) ; able to achieve the controlled circuit is "on" , " off " control actuator ( output portion ) ; between the input and output portions of the relay , and the isolation of the input coupling , and the functional processing of the output portion for driving the intermediate mechanism ( driving section ) .

    As a control element , summed up the relay there are several functions:

    1 ) expand the control range . For example , multi- contact relay control signal reaches a certain value , the contact group can be in different forms , but for access , breaking , multi- turn road circuit .

    2 ) to enlarge. For example , sensitive relays , relays , etc., with a very small amount of control , you can control a lot of power circuits.

    3 ) integrated signal. For example, when a plurality of control signals in the form specified input multi-winding relay , through more integrated , to achieve the desired control effect.

    4 ) automatic , remote monitoring . For example , relays and other electrical devices automatically together with the program control circuit can be composed of , in order to achieve automated operation .

The most common relay to the number of thermal relay , thermal relay normally used for AC 50Hz, 60Hz, rated voltage up to 660V, rated current up to 80A of the circuit for AC motor overload protection . It has a differential mechanism and temperature compensation links with specific AC contactor plug installed.

Time relay is also very commonly used as a relay , its role is to be a delay element , usually it can exchange 50Hz, 60Hz, voltage up to 380V, DC to 220V control circuit for the delay element , according to the scheduled time or breaking the circuit . It can be widely used in electric drive systems, automatic control systems and procedures from time to control a variety of roles in the automatic control system of the production process .

Commonly used in the control relay is typically used as relay control , signal transmission and isolation amplifier and other purposes. There are also current relay used to limit the current, voltage relays to control voltage , static voltage relay , phase sequence voltage relay , phase sequence voltage differential relays, frequency relays, power relays direction , differential relays, ground relays, motor protection relays, etc. and so on. It is precisely because of these different types of relays , we might be on different physical exercises control , complete a full control system.

In addition to the traditional relay , relay technology is also applied in other aspects , such as intelligent motor protection is based on the three-phase AC motor works , analyzes the main reasons leading to the development of motor damage , it is a unique design, reliable and versatile protection, when a fault occurs , cut off the power supply timely , easy to implement repair and maintenance of the motor, the product has a lack of protection, short-circuit , overload protection , suitable for all types of AC motors , switchgear, distribution security and limited power to control electrical boxes and other electrical equipment , various types of ancillary products is the preferred design and installation of electrical equipment . The technical installation dimensions, wiring, the same current adjustment with the same type of bimetal thermal relay. Directly instead of bimetal thermal relay replacement of advanced electronic products. And its true principles or relay technology.

Relay technology development to the present , and computer technology have combined to produce a programmable controller technology. Referred to as a programmable logic controller PLC. It is an advanced technical means used directly in the microcomputer automatically controlled. It has high reliability, strong anti-interference , fully functional , small , flexible scalable , software direct, simple , easy maintenance, appearance , etc. ; previous relay control elevators have hundreds of contacts to control operation of the elevator . There is a bad contact , can cause failure, maintenance is quite troublesome , but there are hundreds of PLC controllers internal solid state relays, dozens of timer / counter with power failure memory function , input and output using optical isolation , the control system only 10% of the fault relay control mode . Because of this , the state authorities have expressly provides new capacity from 1997 onwards may not use the elevator control elevator relay , switch PLC microcomputer control elevators.