A relay is an electrically operated switch that isolates one electrical circuit from another. In its simplest form, a relay consists of a coil used as an electromagnet to open and close switch contacts. Since the two circuits are isolated, a lower voltage circuit can be used to trip a relay, which will control a separate circuit that requires a higher voltage or amperage. A 12-volt relay requires 12 volts direct current (DC) to energise the coil. Relays can be found in early telephone exchange equipment, in industrial control circuits, as a starter solenoid in automobiles, on water pumps, in high-power audio amplifiers, and as protection devices.
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A Relay Is an Electromagnetic Switch
Power is applied to the coil of a relay in a specified voltage to “energise” it. Shown as Points A and B on the schematic diagram, when 12 volts DC are applied across the terminals, associated switch contacts change their state.
Switch Contacts Can Be Normally Open or Closed
Switch contacts on a relay can be in one of two states, normally open (NO) or normally closed (NC). When the coil is at rest and not energised (no current flowing through it), the switch contacts are given the designation NO or NC. In an open circuit, no current flows, such as a wall light switch in your home in the down position when the light is off. In a closed circuit, metal switch contacts touch each other to complete a circuit, and current flows, similar to flipping up a wall light switch to the “On” position.
In the accompanying schematic diagram, Points C and D connect to the switch. When a voltage is applied across the coil at Points A and B, an electromagnetic field is created, which attracts a lever in the switch, causing it to make or break contact in the circuit at Points C and D (depending if the design is NO or NC). The switch contacts remain in this state until the voltage to the coil is removed.
Relays Come in Different Switch Configurations
Relays come in different switch configurations. The switches may have more than one pole, or switch contact. The diagram shows a single-pole single-throw configuration, referred to as SPST. This is similar to a wall light switch in your home. With a single throw of the switch, the circuit is closed.
Other common configurations include double-pole single-throw and double-pole double-throw. Relays are especially valuable in circuits where a small voltage can control the On/Off state of a separate circuit that uses high voltage or high amperage.
For example, a furnace in a home's basement may require 220 volts to power it. A switch can be located upstairs, but does not require that a 220-volt line be run to it. A simple low voltage wire can connect the switch to a relay located downstairs on the furnace. The relay can be activated with typically a low voltage 12-volt source. The switch upstairs can turn on the relay downstairs. The relay's switch contacts will turn on or off the higher voltage 220-volt circuit.