What is Electrical Relay? Definition, Symbol, Types, Working and Uses


Relays are one of the most popular electrical devices which are mainly used for switching purposes. But these devices do not work like normal switches. They are operated electrically according to the principle of electromagnetic induction. These devices are used in heavy-duty machines, such as in air conditioners, television, and used in many more remote control devices.

In this tutorial, I am going to unlock the details of the electrical relay. Here you will learn much more about electrical relays that how they work and how they are used in circuits. Below is a table of contents of what you can learn from here.

• What is an electrical relay?
• Working of an electromechanical relay
• Types of electrical relay
• Symbols of electrical relays
• Applications of electrical relay

What is Electrical Relay?

Relay is an electrical device that controls (turned on and off) a high voltage circuit using a low voltage source where the low voltage circuit and the high voltage circuit are completely isolated from each other.

As we already know that relays are such electrical devices that are commonly used for switching purposes. It is used to open and close the electrical circuits by receiving electrical signals from outside sources. Although relays work like ordinary switches but they are slightly different from normal switches. The normal switches need physically turned on and off where the relays can be turned on and off by applying a small electric current.

Working of an Electromechanical Relay

Basically, relays are consist of a set of input terminals for single or multiple control signals, and a set of output terminals. For this single or multiple signal controller, the relay uses an electric coil wound in an iron core. When a relatively small current is applied to the electric coil, the coil becomes electrically energized and creates a magnetic field in it.

This energized magnetic coil is then used to operate a switch that can control much larger currents. In this way, an electrical relay can use to switch a very small current to a larger current and enable both circuits to be electrically separated from each other.

So if we talk about the internal parts of an electromechanical relay, it has the following parts

• Electromagnetic coil
• Armature
• Core
• Movable contacts
• Spring return arrangement

What is Electrical Relay?

Types of Electrical Relay

Electric relays come in different types using slightly different technologies. The technologies in which relays are made can be divided into two main groups one is movable contacts relay and another is no movable contacts relay.

The movable contact relays such as mechanical relay may be considered in some respects to use old technology. Where no movable contact relays such as solid-state relays, MOS FET relays might be thought to be a more effective means of switching electrical current.

So, there are different types of relays and they are classified into different categories according to their properties. Each of these types of relays is used for a specific application and these are

1. Based On Operation Principles

EMR (Electromechanical Relay)
SSR (Solid State Relay)

2. Based on Poles & Throw

• SPST Relay (Single Pole Single Throw)
• SPDT Relay (Single Pole Double Throw)
• DPST Relay (Double Pole Single Throw)
• DPDT Relay (Double Pole Double Throw)

3. Forms Of Relay

• 'Form A' Relay
• 'Form B' Relay
• 'Form C' Relay
• 'Form D' Relay

4. Hybrid Relay
5. Reed Relay
6. Electrothermal Relay (Thermal Relay)
7. Polarized & Non-polarized Relay

Symbol of Electrical Relay

In the diagram below, the symbols of different types of electrical relays are given.

What is Electrical Relay

Application of Electrical Relay

Electrical relays have a very wide range of applications. Starting from various appliances used in the home to the telecommunication systems electrical relays play a very important role. Below are some common uses of this electrical relay.

• Electrical relays are used for isolating low voltage DC circuits from high voltage AC circuits.

• They are used in motor control circuits for switching, as well as protecting the motor.

• They are used in many power distribution centers for detecting various faults.

• They have single or multiple control signals, which are used to control multiple circuits.

• They are used in many industries for automation systems.

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