How does a relay switch work? A relay switch is a simple device with an electromagnetic coil that pulls a contact to create or cut the circuit when the coil is activated.
Relays come in a wide variety of shapes and sizes and can have a single switch function or multiple sets of contacts. The most common relays are double throw and single-pole switches and are equipped with two contacts identified as normally open (NO) and normally closed (NC).
The NC or NO designation indicates the position of the contacts when the coil is not receiving power.
Currently, there are two types of connections to a relay. The first set is the switch wiring, which includes a standard wire and two switch wires, one normally open and one normally closed. The coil is not powered, and the normally closed contact completes the circuit.
The second set of wires to a relay are the coil wires. When the coil is powered, it pulls down the switch, cutting the circuit through the normally closed contact, and completes the circuit through the normally open contact.
Uses of a relay
The relays are used for a wide variety of purposes. They allow a small amount of electrical current (through the coil) to control the power of a wide variety of loads. Larger relays, called contactors, are used to control power to pumps and motors and other industrial loads. Smaller relays are used in electronic circuits and a variety of control functions.
You deal with relays every day without realizing it. Cars have relays that control common lights and flashing lights. A special type of relay controls the car’s ignition, usually referred to as a solenoid. Garage door openers use relays to control door movement and direction. While driving on the street, traffic lights are controlled by relays.
At home, your refrigerator uses a relay to control the compressor motor and fans. Your washing machine, dryer, oven, and dishwasher all use relays to control their operation.
Some relays even control themselves. A relay can be used to keep a circuit energized once the start button is pressed. Until the power is lost or the stop button is pressed, the circuit will remain energized. The light in this circuit is connected in parallel to the relay coil. The start button provides power to the circuit only for a moment.
Once activated, the relay coil keeps the switch in the normally open position, completing the circuit even after the start button is no longer pressed. The relay coil and light will remain activated until the stop button is pressed to cut the circuit. Once the circuit is stopped, no power is transmitted to the coil, so the relay returns to the normally open position, and the circuit remains unpowered.