What is Relay? - Electrical C & V


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06 March 2020

What is Relay?


Relay is an electronic switch that is opened (ON) or closed (OFF) by a second electrical circuit that is isolated from the main circuit.


Relay is a switch operating on flame voltage, which can turn the supply on or off, common switches are installed in our homes which we can turn on or off by hand. But to use the relay, we have to supply the relay, only then it does its work. Wherever switching work is done such as UPS, stabilizer etc., relay is definitely used. Today semiconductor devices are used for switching, but today relays are used in many circuits.

Relay is a faster switching device than normal switching, inside it there are two main components, the first one is Coil and the other is the contactor and the second one is given below the list of components.


1.Armature: It works by connecting Common Terminal to NC Contact and NO Contact.

2.Spring: It connects the armature to the NC contact when the relay is closed. Or simply say that the spring connects the armature to the NC contact.

3.Contact: This is the terminal of the relay where we can connect the equipment or circuit to whomever we have to switch. such as NC Contact, NO Contact.

4.Coil: (Electromagnet) in which electric current is given which is the common terminal move.

Working of Relay?

 DC voltage is required for the relay to function. The Common Terminal is directly connected to the NC Contact when the relay is OFF until the relay coil is given DC supply. Then the Common Terminal and the NC Contact are connected to each other, but as soon as we supply DC to the Coil of the relay then the Coil will energize and pull the armature towards us so that the Common Terminal will now move from the NC Contact to the NO Contact. Will go and supply will come to Normally Open Terminal. But as soon as you stop the supply of the relay, the spring back in it will pull the armature towards you and will be connected to the Common Terminal NC Contact again.


On the basis of working principle

There are two types of relays based on the principle of work:

1. Electromagnetic relays.
2. Semiconductor relays.


                                                                  Electromagnetic Relay


                                                                 SSR (Solid State Relay)

Out of which electromagnetic relays are very large and they are used in inverter, stabilizer etc. The same semiconductor based relays are very small and are used in small circuits.

By number of poles

We can only take a connection in the photo from the NO and NC contacts. But if we need more than one terminal somewhere or more than one switching is needed, for this we have to use two or more poles relay instead of one pole. In which we can turn on or off more than one circuit or device at a time.


1. Single Pole Single Throw Normally-Open
2. Single Pole Single Throw Normally-Close
3. Single Pole Double Throw
4. Double Pole Single Throw
5. Double Pole Double Throw

Suppose there are two Water Pump in your house and you want that as soon as your water tank is filled with first Water Pump and second Water Pump starts automatically then for this you will need Single Pole Double Throw Relay.

you will get 1 NC and 1 NO Terminal, then you have to connect the first Water Pump to the first NC Terminal and second Water Pump to the NO Terminal. So as soon as the first water tank is filled, your relay will be activated, then the NC contact will automatically become NO and the NO contact will become NC and the supply will start in the second water pump. So in this way you can use Single Pole Double Throw Relay.

Depending on the volt or, depending on the size, in the market, you will find relays running on different bolts which will run from 5 V DC, 12 V DC, or 24 V DC, and 110V AC, or 220 V AC.
 There are many types of relays. Which are based on their shape and texture.

1. Mercury relay
2. Overload Protection Relay
3. Latching Relay
4. Vacuum Relays
5. SSR (Solid State Relay)
6. Force-Guided Contacts Relay
7. Multi-Voltage Relays
8. Time delay relay

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