What is a Relay?

A relay can be defined as a switch. Switches are generally used to close or open the circuit manually .Relay is also a switch that connects or disconnects two circuits. But instead of manual operation a relay is applied with electrical signal, which in turn connects or disconnects another circuit.

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Relays can be of different types like electromechanical, solid state. Electromechanical relays are frequently used. Let us see the internal parts of this relay before knowing about it working. Although many different types of relay were present, their working is same.

Every electromechanical relay consists of an consists of an

  1. Electromagnet
  2. Mechanically movable contact
  3. Switching points and
  4. Spring

Electromagnet is constructed by wounding a copper coil on a metal core. The two ends of the coil are connected to two pins of the relay as shown. These two are used as DC supply pins.


Generally two more contacts will be present, called as switching points to connect high ampere load. Another contact called common contact is present in order to connect the switching points.

These contacts are named as normally open (NO),normally closed(NC) and common(COM) contacts.

Relay can be operated using either AC or DC.

In case of AC relays, for every current zero position, the relay coil gets demagnetized and hence there would be a chance of continues breaking of the circuit.

So, AC relays are constructed with special mechanism such that continues magnetism is provided in order to avoid above problem. Such mechanisms include electronic circuit arrangement or shaded coil mechanism.


  • Relay works on the principle of electromagnetic induction.
  • When the electromagnet is applied with some current it induces a magnetic field around it.
  • Above image shows working of the relay .A switch is used to apply DC current to the load.
  • In the relay Copper coil and the iron core acts as electromagnet.
  • When the coil is applied with DC current it starts attracting the contact as shown. This is called energizing of relay.
  • When the supply is removed it retrieves back to the original position. This is called De energizing of relay.

There are also such relays, whose contacts are initially closed and opened when there is supply i.e. exactly to opposite to the above shown relay.

Solid state relays will have sensing element to sense the input voltage and switches the output using opto-coupling.

Different Types of Relay:

Other than the Electromagnetic relay there are many other types of relays which work on different principles. Its classification is as follows

Types of Relay Based on the principle of operation

  • Electrothermal relay:

When two different materials are joined together it forms into a bimetallic strip. When this strip is energized it tends to bend, this property is used in such a way that the bending nature makes a connection with the contacts.

  • Electromechanical relay:

With the help of few mechanical parts and based on the property of an electromagnet a connection is made with the contacts.

  • Solid State relay:

Instead of using mechanical parts as in electrothermal and electromechanical relays, it uses semiconductor devices. So, the switching speed of the device can be made easier and faster. The main advantages of this relay are its more life span and faster switching operation compared to other relays.

  • Hybrid relay:

It is the combination of both electromechanical and solid state relays.

Types of Relay Based on the polarity:

  • Polarized relay:

These are similar to the electromechanical relays but there exists both permanent magnet and electromagnet in it, the movement of the armature depends on the polarity of the input signal applied to the coil. Used in telegraphy applications.

  • Non-polarized relay:

The coil in these relays doesn’t have any polarities and its operation remains unchanged even if the polarity of the input signal is altered.

Poles and Throws

Relays can switch one or more circuits. Each switch in relay is referred as pole. Number of circuits a relay connects is indicated by throws.

Depending on the poles and throws, relays are classified into

  • Single pole single throw
  • Single pole double throw
  • Double pole single throw
  • Double pole double throw

Single Pole Single Throw (SPST)

Single pole single throw switch SPST

A single pole single throw relay can control one circuit and can be connected to one output. It is used for the applications which require only ON or OFF state.

Single Pole Double Throw (SPDT)

Single pole double throw switch spdt

A single pole double throw relay connects one input circuit to one of the two outputs. This relay is also called as changeover relay.

Though the SPDT has two output positions, it may consist of more than two throws depends on the configuration and requirement of the application.

Double pole single throw (DPST)

double pole single throw switch dpst

A double pole single throw relay has two poles and single throw and it can be used to connect two terminals of a single circuit at a time. For example, this relay is used for connecting both phase and neutral terminals to the load at a time.

Double pole double throw (DPDT)

double pole double throw switch dpdt

A DPDT (double pole double throw) relay has two poles and two throws for each pole. In motor direction control, these are used for phase or polarity reversal.

The switching action between contacts for all these relays is performed when the coil get energized as shown in figure below.

Relays can be classified into different types depending on their functionality, structure, application etc. Learn about different types of relays .

Applications of Relay:

The applications of the relay are limitless, its main function is to control the high voltage circuit (230V circuit AC) with the low voltage power supply (a DC voltage).

  • Relays are not only used in the large electrical circuits but also used in the computer circuits in order to perform the arithmetic and mathematical operations in it.
  • Used to control the electric motor switches. To turn ON an electric motor we need 230V AC supply but in few cases/applications, there may be a situation to switch ON the motor with a DC supply voltage. In those cases, a relay can be used.
  • Automatic stabilizers are one of its applications where a relay is used. When the supply voltage is other than the rated voltage, set of relays sense the voltage variations and controls the load circuit with the help of circuit breakers.
  • Used for the circuit selection if there exists more than one circuit in a system.
  • Used in Televisions. An old picture tube television’s internal circuitry works with the DC voltage but the picture tube needs a very high AC voltage, in order to turn on the picture tube with a DC supply we can use a relay.
  • Used in the traffic signal controllers, temperature controllers.