What is an Electrical Relay? And How Does it Work?

Have you ever wondered how a tiny switch can control a powerful machine?

Imagine you want to switch-on a powerful machine that takes a lot of current. For that you would need a Switch which can handle such large amount of current. Therefore, that switch is going to be a big, cumbersome switch. Also it would be more risky to manually operate a switch which makes or breaks the path for large currents.

An electrical relay is an electrically operated switch (an electromechanical device) that allows a low power signal to control a higher power circuit. Think of it as a bridge between two circuits: one that can be controlled easily (the control circuit) and another that needs more power (the load circuit).

Instead of using a big, cumbersome switch, we can use a small and easy-to-operate switch. The small switch controls the relay, and the relay controls the big machine. In simpler terms, a small electrical signal acts like a trigger, allowing you to control a much larger circuit with a relay. This is the basic purpose of an electrical relay.

How does an electrical relay work?

The below electrical relay diagram helps you understand the basic working principle of an electrical relay.

A relay consists of an electromagnet (coil), an armature (a movable lever), a spring, and one or more sets of contacts. Here’s how it works step-by-step:

1. Control Circuit: When a small voltage is applied to the control circuit, current flows through the coil of the relay.
2. Electromagnetic Field Created: The current through the coil generates a magnetic field according to the Faraday’s law of electromagnetic induction.
3. Armature Movement: The magnetic field attracts the armature, pulling it towards the coil. Relay is activated.
4. Contacts Switch: The movement of the armature either makes or breaks the connection in the load circuit, thereby turning the high power device on or off.
• Normally Open (NO): These contacts are initially open and only connect when the relay is activated. Power circuit connected between NO and Common terminal is switched-on when relay is activated, and it is switched-off when relay is deactivated.
• Normally Closed (NC): These contacts are initially closed and disconnect when the relay is activated. Power circuit connected between NC and Common terminal is switched-off when relay is activated, and switched-on when relay is deactivated.
5. Return to Original Position: When the control signal is removed, the spring returns the armature to its original position, and the contacts revert to their default state.

[Also Read: What Is Power Electronics And How Does It Affect Your Life?]

Types of electrical relays

Relays come in various shapes and sizes, each suited for specific applications. Here are some common types:

• Electromechanical Relays (EMR): These are the traditional relays with moving parts, as described above.
• General Purpose Relays: Used in a wide range of applications, from simple home appliances to industrial machinery.
• Machine Control Relays: Designed for industrial applications where high reliability and robustness are required.
• Reed Relays: These use a magnetic reed switch enclosed in a glass tube. They are small and fast, making them ideal for high-speed switching applications.
• Solid State Relays (SSR): These relays have no moving parts and use semiconductor devices to perform the switching. They are faster and more reliable than EMRs but can be more expensive.
• Time Delay Relays: These relays delay the activation or deactivation of the load circuit for a specific period. They are used in applications where timing is critical.
• Latching Relays: These relays maintain their position even when the control power is removed. They are used in applications where the state needs to be preserved, like in memory storage systems.
• Contact Configuration Types:
• SPST (Single Pole Single Throw): Simplest type with one input (pole) and one output (throw). It can only connect or disconnect a single circuit.
• SPDT (Single Pole Double Throw): Has one input (pole) and two outputs (throws), allowing the input to be connected to one of the two outputs.
• DPST (Double Pole Single Throw): Has two inputs (poles) and one output (throw) for each input. It can control two circuits simultaneously with a single switch.
• DPDT (Double Pole Double Throw): Similar to SPDT but with two inputs (poles) and four outputs (throws). Each input can connect to two of the four outputs, allowing it to switch between two circuits.

Electrical relay symbols

 Source: Wikipedia

Where are electrical relays used?

Relays are important in many applications across various industries due to their ability to control high power circuits with low power signals. Some common applications of electric relays are:

• Automotive Industry: Used in car electrical systems to control headlights, starter motors, and other critical components.
• Home Appliances: Found in washing machines, air conditioners, and microwave ovens to manage different functions.
• Telecommunications: Relays were instrumental in early communication systems and are still used in some applications today.
• Industrial Automation: Used in programmable logic controllers (PLCs) to automate machinery and processes.
• Power Systems: Employed in protective relaying to safeguard electrical power systems from faults and overloads.
• Medical Equipment: Critical in medical devices where precise control and reliability are paramount.