THE RELAY

THE RELAY

• A Relay is an electromagnetic switch.

• A Relay is activated when a current is applied to it.

Relays

Relays are a special type of switch which is turned on and off by an electromagnet.

Next is a diagram of a simple relay.

When a current flows through the coil an electro-magnetic field is set up.

The field attracts an iron armature, whose other end pushes the contacts together, completing the circuit.

When the current is switched off, the contacts open again, switching the circuit off.

When a current When a current flows through the flows through the coil an electro-coil an electro-magnetic field is magnetic field is set up.set up.

The field attracts an iron The field attracts an iron armature, whose other end armature, whose other end pushes the contacts together, pushes the contacts together, completing the circuit.completing the circuit.

When the When the current is current is switched off, switched off, the contacts the contacts open again, open again, switching the switching the circuit off.circuit off.

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A useful property of relays is that the circuit powering the coil is completely separated from the circuit switched on by the relay. For this reason relays are often used where a safe low-voltage circuit is being used to control a high-voltage circuit.

The symbol for a relay makes the separation of the two circuits clear by separating the coil symbol from the switch symbol

Coil symbol

Switch symbol

The relays used in schools generally a have voltage between 6V and 15V (ours 9 V). When using a diode with a relay, be sure that it is connected across the poles (terminals) of the relay to prevent damage to the transistor from back EMF generated when the relay coil switches off.