Relay is electrically operated switches that open and close the circuits by receiving electrical signals from outside sources.
The “relays” embedded in electrical products work in a similar way; they receive an electrical signal and send the signal to other equipment by turning the switch on and off.


Types of Electrical Relay

Relay technology can be divided into two main categories: Movable contacts (mechanical relay) and no movable contacts (MOS FET relay, solid state relay).

Movable contacts
( Mechanical Relay )

relay movable type

This type of relay has contacts that are mechanically actuated to open/close by a magnetic force to switch signals, currents and voltages ON or OFF.

No movable contacts
( MOS FET relay, Solid State Relay )

relay no movable

Unlike mechanical relays, this type of relay has no moving contacts but instead employs semiconductor and electrical switching elements such as triac and MOS FET. By the operation of these electronic circuits, signals, currents and voltages are switched ON or OFF electronically.

Electrical relay Structure and Operating Principles

  1. Mechanical Relay
    Relay consists of a coil, which receives an electric signal and converts it to a mechanical action and contacts that open and close the electric circuit.
  2. MOS FET Relay
    MOS FET relay is a semiconductor relay that uses power MOS FETs in output elements.

Electrical relay Characteristics and Mechanism

Characteristics of Electrical Relay
Mechanical Relay

One of the major characteristics of a mechanical relay is the physical spacing between the coil and the contact component in order to achieve appropriate level of insulation (insulation distance) on both input and output.

One of the major characteristics of a MOS FET relay is that it utilizes semiconductor so the contacts do not mechanically open/close. As a result, benefits include reduction of footprint, quiet operation, longer operating life, and eliminating the need for additional maintenance.

Relay Parts Name

1 ) Relay contacts

An electromechanical relay has these two fixed contacts; “NO” normally open and “NC” normally closed. They share a common mobile contact called “COM” or common. Under normal conditions, a relay can be open or closed.
In the open position, the contacts are made only when energized and return to their normal position, that is, normally closed once de-energized.
You can think of these contacts as a metal conductor made of elastic material such as beryllium or phosphor bronze alloy.
They have a special welded contact strap. These contact belts are made of high conductivity material with resistance to damage due to electrical sparks.
A conductive belt of an electromechanical relay can be made of alloys of silver, copper silver, tungsten silver, nickel silver, platinum, and gold.

2 ) Electromagnet

The magnetic field is produced when current passes through a conductor.
When current passes through a coil wound on a soft iron core, a magnetic field is created perpendicular to the direction of the current. This magnetic field when flowing through this soft iron core makes it an electromagnet.
An electromagnet maintains its magnetic property until the current is turned off, so it can be turned off and again play an important role in the principle of operation of the relay.
Conventionally, relays usually have latency in the magnetic field. This means that they retain some of their magnetic property even when the current is removed.
This is due to the magnetic lag created due to hysteresis, a hysteresis is a magnetic phenomenon where the magnetic induction in a product lags behind its own magnetic field/strength.

3 ) Movable armature

An armature in the relay is that moving conductor that makes or breaks contact based on the magnetic flux of the iron core.
When energized, these armatures pull against spring tension to make or break contact based on the normally closed/normally open relay type.

4) Yoke

A yoke in an electromechanical relay is that piece of metal attached to the soft iron core that does the job of holding and attracting the armature.
They are very small metal pieces attached at the top of the central element. In many designs, the armature is hinged to the yoke connected with a lead wire.
It ensures the connectivity/continuity of the current between the contacts and the mobile armature. The other main function of the yoke is to provide a low reluctance path for magnetic flux to flow.
Ideally when we look at an electromagnetic relay; we find an energizing coil called primary circuit. A primary circuit has two key parts, one is movable and spring loaded.
This part is called the truss while the other part that remains fixed is called the yoke. When power is supplied to the energizing circuit; yoke pull the movable armor towards itself closing the air gap in between. Modern yokes are made of ferromagnetic material with composite material as an intermediate layer.

5) Spring

Not all relays have a spring attached, but those that do have one attached to the armature to facilitate its movement. This spring allows the armature to move freely within the generated magnetic field to make or break contact with the electrical connection.
These springs are usually made of flat sheet metal that is cut and then stretched into a spring shape; while some high output relays have a nickel silver spring

Working and function

An electromechanical relay works on the simple principle of electromagnetism. When a low voltage direct current is supplied to the energizing coil, electrical contacts are made by the effect of the magnetic field.

You can think of this phenomenon as a simple switch where the circuit is completed with the push of a button. A simple relay is a two-way switch that connects to a different circuit on one side, with three NC, COM, and NO contacts.


Relays with one common contact and two main contacts in a single pole arrangement are called single pole two way type. Similarly, a single pole with a single throw has a NO connection and two poles with a double throw have two NO and NC respectively.

Initially, when no current is supplied to the energizing coil; contact is made with NC and COM. If you connect a bulb with COM and NC at that time it will light up; Similarly, when the relay is energized, you can connect your bulb with COM and NO to make it glow.

While watching a relay upside down; you will find five points of contact. Three on one side while two on the opposite; the two contacts on the opposite side are for NO and NC while the others are Coil +, COM and Coil – respectively.

When we supply current to these two points coil + and coil – magnetic flux is produced; and the position of the armor is altered. In the same way, when we disconnect the supply, the armature returns to its position and the NC contact closes.

How to Test a Relay?

Testing a relay involves checking whether it properly opens and closes in response to a control signal. Relays are electromechanical switches that use an electromagnet to open or close electrical contacts.
General guide on how to test a relay:

Note: Before performing any tests, make sure to disconnect the power source to ensure safety.

Tools and Materials:

Jumper wires
Power source (if needed)


Identify the Relay Pins:

Look at the relay and identify the different pins. There are typically:
Coil Pins (Control Side): These are the pins that connect to the coil.
Switched (Load) Pins: These are the pins that connect to the load (the device being controlled).

Check the Relay Ratings:

Ensure that the relay you’re testing is rated for the voltage and current levels of your application.

Check the Coil Resistance:

Measure the resistance across the coil pins using the multimeter.
Compare the measured resistance with the relay’s specifications. If the resistance is significantly higher or lower, the coil may be damaged.

Apply Control Voltage:

Connect the coil pins to a power source (usually a low-voltage source within the relay’s specified range).
If the relay is a normally open (NO) relay, you should hear a click as the relay closes its contacts. If it’s a normally closed (NC) relay, you should hear the click as it opens.
Alternatively, you can visually check the movement of the relay armature.

Check Contact Continuity:

Use the multimeter to check the continuity (resistance) between the switched (load) pins when the relay is energized.
If it’s a normally open (NO) relay, there should be continuity (low resistance) when the relay is energized.
If it’s a normally closed (NC) relay, there should be continuity when the relay is not energized.

Test for Open Contacts:

Check for continuity across the switched (load) pins when the relay is de-energized.
If it’s a normally open (NO) relay, there should be no continuity when the relay is not energized.
If it’s a normally closed (NC) relay, there should be continuity when the relay is not energized.

Visual Inspection:

Visually inspect the relay for any signs of physical damage or burnt components.

Repeat Tests:

Repeat the tests a few times to ensure consistency in the results.

Why relay failure?

The term “relay failure” can refer to issues or malfunctions related to electrical relays. Electrical relays are devices that control the flow of electric current in a circuit. They act as switches and are commonly used in various applications to control high-power devices with low-power signals.

Relay failure can occur for various reasons, including:

Electrical Overload:

Relays have a specified current-carrying capacity, and exceeding this limit can lead to overheating and failure. Electrical overload can occur due to a short circuit or an excessive load on the relay.

Mechanical Wear and Tear:

Over time, mechanical components within a relay can wear out. This wear and tear can result from repeated switching operations, leading to degraded performance and eventual failure.


Environmental factors, such as moisture and corrosive substances, can lead to corrosion of relay contacts. Corrosion increases electrical resistance and can interfere with the proper functioning of the relay.

Manufacturing Defects:

Defects in the manufacturing process or the use of substandard materials can contribute to relay failures. These defects may not be immediately apparent and could manifest over time.

Improper Installation:

Incorrect installation, such as incorrect wiring or inadequate cooling, can contribute to relay failure. Ensuring proper installation and adherence to specifications is crucial for reliable relay operation.


Like any electronic component, relays can experience aging effects over time. This can result in a decrease in performance and an increased likelihood of failure.

Environmental Conditions:

Extreme temperatures, humidity, and other environmental conditions can impact the reliability of relays. Some relays are designed to operate in specific temperature or humidity ranges, and exposure outside of these limits can lead to failure.

Vibration and Shock:

In applications where relays are subjected to vibrations or mechanical shocks, the internal components may become displaced or damaged, leading to failure.

To prevent relay failure, it’s important to choose relays that are appropriate for the specific application, follow proper installation procedures, and conduct regular maintenance checks.


A DPDT AC coil relay with “ice cube” packaging
Relays are used wherever it is necessary to control a high power or high voltage circuit with a low power circuit, especially when galvanic isolation is desirable.
Railway signaling
Motor device Control


What is the principle of relay?
It works on the principle of electromagnetism. The electromagnetic field that creates the temporary magnetic field is energised when the relay’s circuit detects the fault current. This magnetic field moves the relay armature to open or close connections.

What are the 5 applications of relay?
Relay Drive by Means of a Transistor, SCR.
Relay Drive from External Contacts.
LED Series and Parallel Connections.
Electronic Circuit Drive by Means of a Relay.
Power Source Circuit.
PC Board Design Considerations.

What are the 3 functions of relay?
Relay is an electrical control device, which has the interaction between the input circuit and the output circuit. It plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

Are relays AC or DC?
The working power of the AC relay is AC, and the working power of the DC relay is DC. The coil diameter of the AC relay is thicker and the number of turns is less, and the coil diameter of the DC relay has more turns than the thin wire diameter.

Why relay is used in PLC?
It is used to energize the starter, which, in turn, switches the motor voltage while the PLC controls the relay.

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By Aditya

Hi, I’m Aditya Sharma, a professional blogger from Gurgaon, India and I launched this blog called aadityacademy on July 2021. is a mechanical Project-oriented platform run by Aditya sharma and I got the motivation to start aadityacademy blog after seeing less technical education information available on google.

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