What is a relay, how it works, Types, and its applications?

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Relay have many applications in the electronic and electrical industries. They serve a great function in automatic switching on or off circuits and many other applications such as testing and measurement of equipment, automation, and even protection of electrical circuits. The variety of its applications makes people start to ask, what is a relay and what can it use to achieve.

What is a relay?

A relay is an electromechanical switch, which uses a small power signal to control a large power circuit. It consists of a coil, which when energies by a small DC signal causes a moveable lever to make or break up electrical contact. It is a switch that uses a DC signal to energize an electromagnetic which then, causes the switch to turn OFF or ON depending on how the circuit was configured.

There are two different types of relays, electromechanical and solid-state relays. Electromechanical relays are the most popular used relays in our workplace or project works. Let’s talk about the parts of this relay and how it works.

Every electromechanical relay consists of the following parts.


  • Electromagnetic
  • moveable contacts
  • switching points
  • spring


Electromagnet is formed by wounding an insulated copper coil on a metallic core and passing a DC on the coil to make the metallic core magnetic. Just like a solenoid. when DC is passed through two ends of yeh coil it cause the core to be magnetic and demagnetizes when the DC is removed. The two ends of the coil for energizing the coil are connected to the relay pins.

Mechanical moveable parts

The mechanical movable contact is the part that is causing the switching mechanism. This is the part that connects to the COM (common) pin of the relay. It moves from NC to NO pin when the relay is energized. And move back to NC when the DC source is removed from the solenoid coil of the relay.

Switching point

The switching point is the channels the relay can switch to during operation. For instance, a 5-pin relay has two switching points. The circuit will be programmed in such a way that when it switches to point A, point B will be OFF, and the point A circuit start to run. And when the relay is switched to point B, then point B will start operation shorting down point A and vise vasa.


The main function of the spring in this type of relay is to return the moveable contact from NO(Normally Open) to NC normally close. when DC sources are removed from the solenoid, the spring pulls it back to NC and makes the circuit connects to normal Close.

How a relay works.

How relay work

A relay works with the principle of electromagnet induction. When a DC volt is applied to two wires of the solenoid coil of the relay, it induces a magnetic field around the core. And this magnetic field on the metallic core then causes the mechanical moveable part of the relay to move from NC (normally close) point to the NO (normally open) position. And current can then flow from COM to NO. When the DC source is removed or the switch turned off, there will be no magnetic field on the solenoid and the spring then pulls the moveable part back to NC. And current can then pass from COM to NC.

Each time a relay is energized, the contacts change from their initial position to another. From NC to NO or from NO to NC. Some relays have several switching poles that they can switch many poles at a time.

Relay contact types.

As we said earlier that a relay is a switch, is good we know the type of contact it uses to switch from one position to another. So the terminology of poles and throw used in the switch is also used here on relays.

A relay can be classified according to the number of the circuit it can switch ON or OFF at a time.

Numbers of poles and throw of a relay.

Many different types of relays are classified by the number of poles and throw they have. The number of poles and throw a relay has, is what determines the number of switches it can control at a time.

A pole in a relay is referred to switch while a throw is the number of the circuit it can switch to.

Classification of relay by their number of pole and throw.

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

Single pole single throw (SPST).

An SPST relay switches only one contact during operation. It can either turn a single circuit ON or OF just like a normal single pole single switch used in household installations. This type of relay can be used in triggering the ON or OFF of our alarm system.

Single pole double throw (SPDT).

This type of relay has one input and two outputs. It connects the input with one of the outputs at a time.

This type of relay is used in switching the operation of one circuit to another. It is mostly used as a change-over switch.

Double pole single throw (DPST).

The relay has a double pole with a single throw, it breaks two lines each time the relay is open and closes the two lines when the relay is closed.

When this type of relay is used for switch control, it breaks both the phase and neutral line of the switch when it is open and connects both lines when close.

Double pole double throw.

This type of relay is what is used in electric motor forward and revised controls. It has a double pole and double throw and can switch both poles when get energized.

Types of relays.

There are many classifications of relays depending on their structure functions and applications. But here is the list of common types of electrical relays in use.

  • Electromagnetic
  • Latching
  • Electronic
  • Non latching
  • Reed
  • Thermal
  • Small signal
  • Differential
  • Multi-Dimensional
  • moving coil
  • Rotary
  • Sequence
  • Polarized
  • Time delay
  • Automotive
  • Frequency
  • Thermal

Applications of relays.

Relays are used for the protection of our power systems equipment against damage that might be caused by over current/voltage. Relays help to isolate electrical equipment each time there is a high current or voltage across the line.

They are used to control a high-voltage circuit using a small signal to operate the relay. This application is found on audio Amplifiers and electronic devices.

They are still used to control high-current circuits using a low-current signal. This can be seen in motor starters. Where the relay trip a motor when there is a high current that might damage the electrical motor.

These are where we can find the approbation of relays

  • Lighting control system
  • Electric motor drive control system
  • Traffic control lights
  • Protection system of a power system lines
  • Telecommunications
  • Industrial process controller
  • Computer interface
  • Automotive

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