What Is A Relay

Relay is one of the most used components in industrial automation and control. In this article, I will explain what a relay is, its type, and how to use it correctly.

What is a relay?

A Relay is an electronically controlled switch.  It consists of a set of input terminals for single or multiple control signals and a set of operating contact terminals.

Let’s say you want to turn on a fan when the level of carbon monoxide reaches a certain level.

Instead of having a carbon monoxide detector and someone to start the fan when the level reaches a certain level, most fixed gas detectors come with a relay that turns on or off the fan when the carbon monoxide level reaches a certain level.

Relays basics

A relay consists of two things: a coil and contacts.

A magnetic coil

This part will produce the magnetic field; its main function is to activate the switching action.


These are connections for power to feed through to the device to be activated. Usually, there are three types of contacts: Common, Normally Open, and Normally Closed.


In most relays, this will be labeled as C; the primary function of this contact is to provide a power connection.

Normally Open


In most relays, this is labeled as NO; the main function is to provide power to an external device.

Electricity can not pass from one contact to another, and this is an open circuit.

Normally Closed

This is usually labeled NC. The main function of this contact is to cut power to an external device.

The electricity is allowed to pass from one contact to another.


How does the relay work?

When a current passes through a solenoid, a magnetic field is produced; this can then attract ferrous metal components in its vicinity.

This attraction is used to operate a switch with the relay. Relays can thus be used to control a larger current or voltage and, additionally, to isolate the power used to initiate the switching action from that of the controlled power.

For a relay connected to the output of a controller, when the output switches on, the solenoid magnetic field is produced, and this pulls on the contacts and closes a switch or switches.

The result is that much larger currents can be switched on. Thus, the relay might switch the current to a motor.

A relay’s solenoid might operate more than one set of contacts, and the term pole might be used for each set of contacts.

Contacts can also be obtained as either normally open (NO) or normally closed (NC) in the absence of any input.

Thus, when selecting relays for a particular application, consideration has to be given to the number of poles required, the initial contact conditions, and the rated voltage and current.

Application of relays

Relays are used for various applications such as home automation, cars and bikes (automobiles), industrial applications, DIY Projects, test and measurement equipment, and many more.

Typical application areas of the relays include

  • Lighting control systems
  • Telecommunication
  • Industrial process controllers
  • Traffic control
  • Motor drives control
  • Protection systems of electrical power system
  • Computer interfaces
  • Automotive
  • Home appliances

Types of relays

There are many ways to classify relays. Relays can be classified depending on the poles and throws.

Single Pole Single Throw Relay (SPST)

Single Pole Single Throw Relay (SPST)

This is the simplest type of relay. It consists of only one pole and throw, making the path easier to close or open.

The best example of this is a Push Button. It is used for applications that require the ON or OFF state.

Single Pole Double Throw Relay (SPDT)


This is the most common type of relay. It consists of only one pole but has two throws. An example of this is a Slide switch. It is also called a changeover relay.

Double Pole Single Throw Relay (DPST)

It consists of two poles and a throw. It can connect two terminals of a single circuit at a time. Toggle Switch works on this property.

Double Pole Double Throw Relay (DPDT)


It consists of two poles, but each pole has two throws. These are used for phase or polarity reversal in motor direction control.

Advantages and Disadvantages of Relay

There are some Advantages and Disadvantages of Relay given below :

Advantages of Relay

  • Relay allows us to control a remote device.
  • At High temperature, it works well.
  • Relay can activate large machines of great power and be active with low current.
  • The relay can switch to direct current (DC) or alternate current (AC).

Disadvantages of Relay

  • When relay use is continuous, the contacts are damaged.
  • At the time of activation and deactivation, it generates a lot of noise.
  • It has a high switching time.

What Is A Relay

FAQ: What is the relay?

What is the latching relay?

A latching relay is one whose contacts remain open or closed even after the power has been removed from the solenoid. 

What is Protection Relay?

A protection relay is a crucial electrical system component that safeguards infrastructure, employees, and equipment from electric problems and malfunctions.

Why use a relay instead of a switch?

Relays are much more versatile than switches as they handle complex systems. From homes and electronics to various industries, relays can come in handy to assist in handling larger circuits.

Selecting between relay and switch is not difficult as their utilities are specified for different purposes.

What is the difference between a relay and a switch?

A relay is an electrically operated switch that harnesses the power of an electromagnet to either open or close a circuit, while a switch is a mechanical device that opens or closes a circuit physically. Switches can be activated manually, but relays are typically controlled by electrical signals.

What is the purpose of a relay?

The primary purpose of a relay is to protect the electrical system from voltages or currents that are too high, allowing the safe operation of any equipment it connects to.

They’re commonly found in various applications, from commercial and industrial uses to home and consumer products.

Key takeaways

As per the discussion of the Working Principle and types of Relay, we understand the principle of electromagnetic attraction when it gets energized and operates to the given signal.




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