An electromagnetic relay is a switch operated by an electromagnet. It allows a small current in one circuit to control a much larger current in a separate circuit.
How a relay works
- A small current flows through the coil of an electromagnet in the low-voltage control circuit.
- The electromagnet becomes magnetised and attracts a soft iron armature (a pivoted lever).
- The armature moves and closes (or opens) the contacts in the high-power circuit.
- When the small current is switched off, the electromagnet loses its magnetism and a spring returns the armature to its original position.
Why relays are useful
A relay isolates the control circuit from the high-power circuit. This means:
- A person or a sensitive electronic component (such as a microchip or sensor) can safely control a circuit carrying hundreds of amps.
- The low-voltage circuit and the high-voltage circuit have no direct electrical connection, reducing the risk of damage or electric shock.
Applications
- A thermostat in a heating system uses a relay: a temperature sensor sends a small current to switch a relay that controls the mains-powered heater.
- Car starter motors draw very large currents. The ignition switch operates a relay rather than carrying that current directly.
Soft iron core
The armature and core of the relay are made from soft iron because it magnetises and demagnetises quickly. A steel core would retain its magnetism and the relay would not release when the current was switched off.
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