Working of Relay

Relays are electrically operated switches. They have an electromagnet which controls the switching operation. Relays find uses in stabilizers, mechanical bots for switching action.

Now let us come to the working of the relays.

Relays as you can see has 5 terminals – Three on one and two on other. Two of the three terminals (those at the end) are connected to the electromagnet (coil’s terminals). The third one at the centre is the common (COM) and the two terminals on the other end are Normally Close (NC) terminal and Normally Open (NO) terminal. The metal part over the electromagnet is the armature .It’s one end is connected to the COM and the protruding end of it is the contact which switches between NO and NC terminal.

Normally Closed is the terminal to which the contact is resting when the electromagnet is not energized. NO remains open at that moment and hence its name.

When small DC voltage (12V in this relay) is applied across the electromagnet, the electromagnet get magnetised and the contact moves from NC to NO. As the source is withdrawn from the electromagnet, the contact returns back to NO position either by action of Spring or metal of suitable elasticity . The switching action takes around 30ms. Thus it allows us to switch between two states be it ON/OFF or left/right movement of motor or any other relevant condition. The appliances are connected at the NO, NC and the COM terminal.



These can be AC appliances or DC appliances depends upon the voltage and current rating of the relay.

Now the very important thing is how shall we determine which one is NO and which one is NC? It is very easy if you have a multimeter. Just put it to buzzer mode and check for conductivity with one end at the COM terminal and other on either of the 2 terminals without applying voltage to the electromagnet. Whichever terminal gives the buzzing sound is the NC. Even if you don’t have a multimeter, we can determine the terminals using a bulb and a cell and small wires. Just connect one end of the bulb to the COM and the other to any one end of the cell. Using small wire connect the other end of the cell to either of the terminals. The terminal for which the light glows is NC terminal.