A relay is an electronic component that uses electromagnetic principles or other physical effects to achieve the “automatic on/off” of circuits. Its core function is to control the on-off of large current/high voltage circuits with small current/signals, while also achieving electrical isolation between circuits to ensure the safety of the control end.

Its main functions can be classified into three categories:

1. Control and Amplification: It can convert weak control signals (such as milliampere-level currents output by single-chip microcomputers and sensors) into strong currents sufficient to drive high-power devices (such as motors and heaters), functioning as a “signal amplifier”. For instance, in smart homes, the tiny electrical signals sent by mobile phone apps can be controlled through relays to turn on and off the power of household air conditioners and lamps.

2. Electrical isolation: There is no direct electrical connection between the control circuit (low voltage, small current) and the controlled circuit (high voltage, large current). Control instructions are only transmitted through electromagnetic or optical signals to prevent high voltage from entering the control terminal and damaging the equipment or endangering personnel safety. This is commonly found in the control circuits of industrial machine tools and power equipment.

3. Logic and Protection: It can be combined to implement complex circuit logic, such as interlocking (preventing two motors from starting simultaneously) and delay control (delaying the connection of the load for a certain period of time after power-on). Some dedicated relays (such as overcurrent relays and overheating relays) can also monitor circuit abnormalities. When the current is too large or the temperature is too high, they will automatically cut off the circuit to protect electrical equipment from overload damage.