What is Relay? Types, Applications, and Working of Relay

We all use Relay in various Applications such as Automobiles, Home automation, DIY projects, Industrial Applications, Testing and Measurement equipment, and many more. But most of us don’t know the technical concepts of Relay.

what is relay

In this tutorial, we will learn What is Relay?, the Construction and Working of Relay, Different Types of Relays, Applications of Relays, and many other concepts about Relays. So let’s start.

What is Relay?

A relay is an electrically operated switch that uses an electromagnet to open or close any electrical circuit. In other words, We can say that A Relay is an electromechanical switch that has the ability to turn ON or OFF any electrical circuit. It consists of a flexible moving mechanical part that can be controlled electrically through an electromagnet.

While we use normal switches to open and close any electrical circuit manually, Relay is also a Switch that connects and disconnects circuits. But instead of manual operation, the Relay uses an electrical signal to control the circuit’s ON-OFF operation. Relays are used to controle a high-powered circuit using a low-power signal.

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Depending on the applications, Relays can perform various operations such as Relays can be used as Electrical switches, or as an amplifier to convert smaller currents into larger values. They can also be used to control low-power signal circuits or when multiple circuits need to be controlled by a single signal.

The simplest type of relay is single pole single throw (SPST) relay, which has only one switch and one possible connection. Whereas a double pole double throw (DPDT) relay has two pole and two throws for each pole. These are used for phase or polarity reversal in motor direction controle circuit.

Why Relay is important?

Electrical Relays are essential and crucial because they help to control the high current loads by using a small amount of current. Relay applies voltage to the coil that causes low current flow through it. It allows a larger current to pass through the contacts and control the electrical load.

Relays are essential for Applications where a low-power control signal needs to command high-power circuits.

Function of Relay

The main function of the relay is to control the high-voltage circuit (230V AC) with the help of a low-voltage DC signal. They control one electrical circuit by opening and closing contacts in another circuit.

It detects undesirable conditions in the assigned area of the electrical circuit and gives command to the circuit breaker to disconnect the affected area. In this way, Relay protects the electrical system from damage. So It is also considered as protective equipment.

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Construction of Relay

The Relay operates both electrically and mechanically. Also mainly, there are two kinds of Relay available: Electromechanical Relay and Solid State Relay. In this tutorial, we will focus on Electromechanical relays because of their frequent use.

It consists of an electromagnet and a set of contacts that performs switching operation. The construction of relays is mainly grouped into four different parts named as: Mechanically movable Contacts, Electromagnets and Electromechanical Design, Springs and Bearings, and Terminals and Housing. So let’s discuss the Construction and Parts of Relay in brief.

Parts of Relay

Mechanically movable Contacts:

Contacts are the most important part of Relay that are mechanically movable and affect the reliability. The good contacts should have minimum contact resistance.

Generally, two more contacts are available, also called Switching Points to connect high Amp-Load. Another contact called Common Contact is present to connect Switching Points. These Contacts are named as Normally Open (NO), Normally Close (NC), and Common (COM) Contacts.

The selection of contact material depends upon several factors like nature of the current to be interrupted, the magnitude of the current to be interrupted, Frequency and voltage of operation.

Electromagnets and Electromechanical Design:

Electromagnets are constructed by wounding copper coils on a soft iron core. The two ends of the coil are connected to two pins of the relay and the coil current is restricted to 5A at 220V.

On the other hand, Electromechanical design includes the design of a Magnetic circuit and the mechanical attachment of the Core, Yoke, and Armature. The reluctance of the magnetic path is kept minimum to make the circuit more efficient.

Spring and Bearings:

Spring is connected to one end of the armature and pushes the armature back into place if no current passes through the coil.

The assembly of an armature with the magnet and the base is made with the help of spring. The spring is insulated from the armature by a molded block to provide dimensional stability. The fixed contacts are usually welded at the terminal link.

The bearing may be a single ball, multi-ball, pivot-ball, and jewel bearing. The single ball bearing is used for high sensitivity and low friction. Whereas, multi-ball bearings provide low friction and greater resistance to shock.

Terminals and Housing:

Generally, there are four types of terminals in Relay.

Control input or Coil Terminal: There are two Control input terminals in the Relay that control its switching mechanism. A low-power source is connected to these terminals to activate and deactivate the relay. The source can be AC or DC depending on the type of Relay.

Common Terminal (COM): COM refers to the common Terminal of relay. This is the output terminal of the relay where the load is connected. This terminal is internally connected with either NO or NC depending on the state of the relay.

Normally Open (NO): NO is also a load terminal that remains open when the relay is not active. NO terminal closed with the COM terminal when the relay gets activated.

Normally Close (NC): NC is another load terminal that remains closed with the COM terminal when there is no control input. When the relay gets activated, the NC terminal disconnects from COM and remains open until the relay is deactivated.

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Poles and Throw: The number of Switches inside the relay is called the Poles of Relay. Whereas, The number of circuits being controlled per pole is known as the Throw of Relay.

A single throw relay can control only one circuit i.e. either ON or OFF. While a double throw relay can control two circuits i.e. alternating from one circuit to another by opening one circuit and closing the other during the switching operation.

Working of Relay

Now let’s understand how Electromechanical Relay works.

Relay works on the principle of Electromagnetic Attraction. The below image shows the working of the Relay. A copper coil wound on an iron core acts as an Electromagnet.

When the circuit of the relay senses the fault current, it energizes the Electromagnet circuit and produces a temporary magnetic field. This magnetic field moves the armature for opening and closing the contacts.

working of relay
Working of Relay

In an Electromechanical Relay, A small circuit has the ability to switch ON and OFF a large circuit through its contacts by using Electromagnets. Depending on the uses, Different contact configurations are available namely Normally Open (NO) and Normally Close (NC).

In the Normally Open (NO) Relay, contacts are open when there is no current passing through it. When an Electromagnetic circuit gets energized, it creates a magnetic field that attracts the armature and closes the contacts.

In the Normally Close (NC) Relay, contacts are closed when there is no current passing through it. Unlikely to Normally Open (NO) Relay, When an Electromagnetic circuit gets energized, it creates a magnetic field that attracts the armature and opens the contacts.

Types of Relay

There are various types of Relay and they are classified into different categories according to their properties and uses.

Based on Poles and Throw:

According to their Poles and Throw, Relays are classified as:

  1. Single pole single throw (SPST)
  2. Single pole double throw (SPDT)
  3. Double pole single throw (DPST)
  4. Double pole double throw (DPDT)

Based on Operation Principles:

According to their operation principles and uses there are various types of Relay available.

  • Electromechanical Relay
  • Solid StateRelay
  • Hybrid Relay
  • Reed Relay
  • Electro-Thermal Relay
  • Directional and Non-directional Relay
  • Differential Relay
  • Time delay Relay
  • Impedance Relay
  • Admittance Relay
  • Mho Relay
  • Frequency Relay
  • Buchholz Relay
  • Over-current Relay

We have published an article in which we discussed in detail all the different types of Relay. So please go through my article titled “Different Types to Relay to learn more in deep.

Applications of Relay

The Application of Relays is limitless. As a relay is a protective device, It is used to protect the electrical system from fault and to minimize the damage to the equipment connected in the system due to over-voltage and over-current.

Relay can detect the fault that occurred in electrical power systems and isolate it with the help of a circuit breaker. So the relay is used for the purpose of protection of the equipment connected with the electrical system.

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These are used to control the high voltage circuit with the low voltage signal in the application of audio amplifiers and some types of modems.

These are also used to control high-current circuits with low-current signals in applications like starter solenoids in automobiles.

Relays are not only used in large electrical circuits but also in computer circuits to perform arithmetic and mathematical operations.

Some typical application areas of relays are:

  • Lighting control system
  • Telecommunication system
  • Traffic control system
  • Electrical power protection system
  • Automotive home appliances system
  • Electrical drive control
  • Industrial process control system
  • DIY projects, and many more.

How to test Relay?

Testing of Relay: Since the relay is an electromechanical device. So the relay can wear out eventually and stop working. Then in that situation, we need to test a Relay to get to know whether it is working or not.

There are a few techniques to test the relay if it is working or not. So these techniques are:

  • Testing of the relay with a multimeter
  • Build a simple circuit to test the relay
  • Use a DC power supply to check relay is properly working or not.
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Hey, I'm Satish Gupta an Engineer by profession and blogger by passion. I am writer and founder of this blog, Here I publish contents related to Electrical and Electronics Engineering..

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