An electromagnetic relay is an electromechanical switch made up of an electromag- net and a set of contacts.
Current flowing through it in one circuit can switch on and off a current in a second circuit.
The circuit symbol for a relay is
An electromagnetic relay operates in the following manner • Closing a switch in the primary circuit turns on the electromagnet.The primary circuit contains a low voltage power supply and an electromagnet.
• This attracts the armature, which rotates on a pivot, and this forces the contacts together in the secondary circuit.
• This completes the secondary circuit, allowing current to flow through it. Electromagnetic relay DEMONSTRATION 24.1
Demonstration of an Electromagnetic Relay Apparatus Relay, switch, two power supplies, leads, ammeter and resistor
Procedure Set up apparatus as illustrated in Fig 24.2 with the switch open.
Note the current in the secondary circuit (0 A). Close the switch. Again, note the current in the secondary circuit.
Coil Contacts A Coil terminals Coil Fig 24.1: Electromagnetic relay Pivot Armature Soft iron core
24 Contact terminals Contacts
Fig 24.2
Observation Current flows in the secondary circuit after the switch in the primary circuit is closed.
Conclusion The relay triggered the completion of the secondary circuit, allowing current to flow through it.
