Relay | What is a relay, its function, types and how does it work

A relay is an electrically operated switch that opens and closes circuits by receiving electrical signals from external sources.

When the relay contact is open (NO), the relay isn’t energized with the open contact. However, if it is closed (NC), the relay isn’t energized given the closed contact. However, when energy (electricity or charge) is supplied, the states are prone to change.

Relays are normally used in the control panels, manufacturing, and building automation to control the power along with switching the smaller current values in a control circuit. However, the supply of amplifying effect can help control the large amperes and voltages because if low voltage is applied to the relay coil, a large voltage can be switched by the contacts.

What are the functions of a relay?

There are 3 main functions of a relay:

1. The relay permits a small amount of electrical current to control high-current loads

When voltage is supplied to the coil, a small current passes through it, resulting in a larger current passing through the contacts to control the electrical load.

2. The relay sends different types of electrical signals

AC loads can also be electrically controlled (Switching) from a DC power source.

3. The relay drives multiple outputs with only one input

One input signal to the coil can simultaneously control several independent circuits (switched).

1. Movable contacts - Mechanical Relay 

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.

2. No movable contacts - MOS FET relay, Solid State Relay 

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 operating these electronic circuits, signals, currents, and voltages are switched ON or OFF electronically. 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.

Typical examples of relays

The diagram below presents typical examples of relays, categorized into relays for control circuits and I/O applications. It highlights various types of relays, such as those designed for higher load capacity, greater reliability, specific environments, or terminal connections, along with their corresponding series names and features

Relay structure and operating principles

Frequently Asked Questions (FAQ)

1. What is the purpose of a relay?

The relay permits a small amount of electrical current to control high-current loads. When voltage is supplied to the coil, a small current passes through the coil, resulting in a larger amount of current passing through the contacts to control the electrical load. 

2. Why use a relay instead of a switch?

Switches can regulate the flow of electricity to some extent but are not as efficient as relays in safeguarding large electrical systems against overloads. Unlike relays, switches cannot detect excessive power flow in a circuit and thus cannot automatically sever the connection.

3. What are the main functions of a relay?

A relay is an electrical control device that facilitates interaction between the input and output circuits. It serves critical functions in the circuit, including automatic adjustment, safety protection, and circuit conversion.

Download Brochure 

Free Ebook