What is Electrical Relay ?

what is electrical relay??It is the most essential component for protection and switching of various  control circuits & other electrical components. It react to voltage or current  by performing open or close the contacts or circuits. 

Relay is an electrical switch that control a high voltage circuit ( On and off ) by  using a low voltage source .It is the basic component for automation of any electrical circuit




There are four type of terminal in a relay.

1-Control input or coil terminal

Control input are the two input terminals which control its switching mechanism. A low power source is connected to this terminal to activate and deactivate it. These source may be AC or DC depending on the types.

2-COM Or Common terminal

COM means common terminal

This is the output terminal where we connect one side of the load terminal. Depending upon the state this terminal is internally connected with either of the other two terminals

Parts of Relay

3-NO terminal

NO or Normally open terminal is the load terminal of a relay which remain open when it is not activated. The NO terminal become close  with COM terminal when the relay is activated.

4-NC Terminal

NC or Normally close terminal is also the load terminal which remain close when the relay is not activated .The NC terminal become close with COM terminal when the relay is activated

Poles and Throw

Pole is the switches inside a relay. The number of switches inside is called poles of the relay..The number of circuit which is being controlled per pole is called throw .

With the help of single throw we can control only one circuit. either ON or OFF but the double throw can control two circuits that is by alternating from one circuit to another by opening one circuit and closing another  during switches.( ON and OFF )


To understand the operation of relay assume  SPDT (single pole double throw) relay


When there is no power supply to the actuating coil , the relay is inactive or off & the position of its pole remains at NC terminal .Due to this electric current can flow between the COM terminal & NC terminal. Thus due to this allows the flow of current through the circuit connected to COM & NC terminal.

When the power supply is given to the actualing coil ,the pole of the relay shifts to the NO terminal.  Thus the NC terminal becomes open & the COM terminal becomes closed or electrically short to NO terminal. Subsequently, allowing the flow of current through the circuit connected with COM & NO terminal.

Type of Relay Base on Poles and throw

Type of relay base on pole and throw


SPST means single pole single throw

Single pole means it can control only one circuit and single throw means its poles has only one position through which it can conduct. The SPST relay two states I.e. either Open or Close circuit.



SPDT means single pole double throw

Single pole means it can control only one circuit and double throw means its poles has two position through which it can conduct.


The SPDT has two states & in each state, its one circuit remains closed while the other remains open & vice versa.


DPST means Double pole single throw

Double pole means its can control two completely isolated circuits. The single throw means that each pole has only one position in which in which it can conduct .


The DPST relay can switch two circuits simultaneously i.e. either providing a close or open circuit.


DPDT means Double pole double throw

Double pole means its can control two completely isolated circuits. The double throw means that each has two position in which it can conduct


The DPDT can be interpreted as two SPDT relays but their switching is simultaneous.

In this article we will briefly discuss about the relay ,its working and the type of relay use in circuit.

Different Types of Relays base on operation principle

Relay are of different types depending upon the operating principle and structural feature .


Electromagnetic  relays have electrical, mechanical and magnetic component and  also have operating coil and mechanical contacts. Therefore, when the coil gets activated with the help of supply, these mechanical contacts gets opened or closed. To activate these coil we AC or DC supply .

relay electromagnetic

DC and  AC Relays

Both AC and DC relays work on the same principle  which is base on electromagnetic induction, but the construction is different and also depends on the application for which these relays are constructed. DC relays are employed with a freewheeling diode to de-energize the coil, and the AC relays uses laminated cores to prevent eddy current losses.

The very interesting aspect of an AC is that for every half cycle, the direction of the current supply changes; therefore, for every cycle the coil loses its magnetism because  zero current due to every half cycle makes the relay continuously make and break the circuit. So, to prevent this – additionally one shaded coil or another electronic circuit is placed in the AC relay to provide magnetism in the zero current position.


Attraction type electromagnetic Relay

These type of relays work with both AC and DC supply and attract a metal bar or a metal piece when power is supplied to the coil. This can be a plunger being drawn towards the solenoid or an armature which is attracted towards the poles of an electromagnet. These relays do not have any time delays for its operation  so these are used for instantaneous operation without time delay .

2.Induction Type Relays

These are used as protective relays in AC systems alone .In this relay the actuating force to move the contacts is developed by a moving conductor that may be a cup or disc type through the interaction of electromagnetic fluxes which is develop due to fault currents.

This type of relay are of several types like watt hour ,shaded pole and induction cup structures and are mostly used as directional relays in power-system protection and also for high-speed switching operation applications.

Magnetic Latching Relay

This type we use permanent magnet or parts with a high remittance to remain the armature at the same point as the coil is electrified when the coil power source is taken away.

3. Solid State Relays

This type we uses solid state components to perform the switching operation without moving any parts. Since the control energy required is much lower compared with the output power to be controlled by this types that results the power gain higher when compared to the electromagnetic types. These are of different types: reed relay coupled SSR, photo-coupled SSR, transformer coupled SSR,and so on.

In photo coupled SSR where the control signal is applied by LED and it is detected by a photo-sensitive semiconductor device. The output form this photo detector is used to trigger the gate of TRIAC or SCR that switches the load.

solid state relay

3. Hybrid Relay

These have both electromagnetic and electronic components. Usually, the input part contains the electronic circuitry that performs rectification and the other control functions, and the output part include electromagnetic relay.

4.Thermal Relay

Thermal relays are depend upon the effects of heat which means its operate when the ambient temperature rise from the limit, directs the contacts to switch from one position to other. These are used mainly in motor protection and consist of bimetallic elements  like a temperature sensor as well as control elements. Example of this type are thermal overload relay.

5. Reed Relay

Reed Relays which consist of a pair of magnetic strips (also called it as  reed) that is packed within a glass tube. This magnetic strip acts as both an armature and a contact blade.  The magnetic field which is  applied to the coil is wrapped around this tube that makes these magnetic strip or reed  move so that switching operation is performed.

Depending upon dimensions,these type of relays are differentiated as micro miniature, subminiature and miniature relays. Depending upon the construction, these are classified as hermetic, sealed and open type relays. Depending upon the load operating range, these are of micro, low, intermediate and high power types.

reed relay

It is also available with different pin configurations like 3 pin, 4 pin and 5 pin .Switching contact can be SPST(single pole single throw), SPDT(single pole double throw .), DPST (double pole single throw.)and DPDT (double pole double throw. ) types. Some relay are available  as normally open (NO) type and the other are normally closed (NC) types.

Magnetic Contactor and Relay

Magnetic contactor is a switching device like a relay but it has the higher current capacity whereas relay has low current capacity. Magnetic contactor is the robust version of relay.
Magnetic contactor can handle high power but Relay handles small power for its weak contacts, A magnetic contactor operate by itself it always needed an external signal or power like relay. On the other hand circuit breaker is not made for frequent operation but its operate by itself and send some signal to the external circuit

relay and contactor

Relay can operated by below 1 ampere but magnetic contactor can used in over 1KA power circuits. Contactor made for frequent switching operation and its may be switched on & off 1000 times in a day. Hence it is mechanically robust and strong to withstand such switching and it is capable of handling high power.

Relay can operated by below 1 ampere but magnetic contactor can used in over 1KA power circuits. Contactor made for frequent switching operation and its may be switched on & off 1000 times in a day. Hence it is mechanically robust and strong to withstand such switching and it is capable of handling high power.

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