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How Does a Relay Work

Relays are used in many applications, such as test equipment, automotive, industrial, and home automation. This article will examine what a relay is, how it works, and some of its uses.

What is a Relay?

A relay is an electromagnetic switch that controls an electromagnet to join or disconnect two circuits using an electrical signal. Relays function without any need for manual intervention, unlike manual switches.

By employing an electrical signal to drive an electromagnet, a relay functions as an electromechanical switch that connects or disconnects two circuits. Unlike manual switches, relays work without the need for any manual input.

Each electrically powered relay consists of

  1. Mechanically movable contact
  2. Electromagnet
  3. Switching points
  4. Spring

A copper coil twisted around a metal core creates an electromagnetic in an electromechanical relay. The coil’s two ends are linked to the relay’s two DC supply pins.

To connect large ampere loads, two additional links, referred to as due to diversity, are often present, along with a single contact to connect the switching points. 

The contacts are designated as commonly used (COM), typically open (NO), and typically closed (NC) connections.

Relays can accommodate all AC and DC circuits; however, with AC relays, there is a risk of ongoing circuit disruption since the relay demagnetizes at every present zero state.

To solve this problem, unique mechanisms that sustain constant magnetic, such as circuit design layouts or shaded coil mechanisms, are incorporated into the design of AC relays.

How does a Relay Work?

  • Through electromagnetic induction, the relay functions. 
  • The electromagnet produces a magnetic field whenever electricity is applied. 
  • A switch is used to deliver DC to a load in a relay, as shown in the image above.
  • The iron core and copper coil of a relay act as an electromagnet.
  • The energizing of the relay is the process of applying a DC to the coil, which attracts the contact, as shown in the image. 
  • This de-energizing of the relays is when the contact returns to its initial position after the supply is shut off.

Some relays operate reversely, with their contacts initially being closed and then opening when power is applied.

 Solid-state relays, on the other hand, feature a device that senses the input voltage and employs opto-coupling to activate the output.

Types of Relay Contact 

Relays act as switches, as was previously mentioned, and are classified to use the “poles and thrown” terminology according to the number of connections and circuits they switch.

We must first appreciate the poles and throws of a relay switch to understand the categorization of relay contacts.

· Poles & Throws

Each switch on a relay is known as a pole, and it can regulate one or more circuits. The throws of a relay show how many circuits it connects, categorized according to their throws and poles, which include

· Single Throw

A single pole, single throws (SPST) relay has only one output and controls one circuit. Applications that need the On or OFF state make use of it.

· Double Throw

A one-pole double throw (SPDT) relay, commonly known as a changeover relay, links one input circuit to one of 2 output circuits. 

Even though SPDT has two output places, depending on the design and application requirements, it may include more than two throws.

· Double pole and single throw

A dual pole single throws (DPST) relay connects two endpoints of a single path simultaneously and has two poles or a single throw. For instance, it can connect the load’s phase and neutral terminals simultaneously.

· Double pole and double throw

The double pole, double throw (DPDT) relay consists of two poles, each with two throws, for a total of 4 throws. It is frequently employed in motors direction control for period or polarity reversal.

These relays execute the switching motion between contacts whenever the coil is energized.

Types of Relays


Relays can be categorized according to their use, structure, functioning, etc. Here are a few prevalent kinds of relays.

  • Latching
  • Electromagnetic
  • Non-Latching
  • Electronic
  • Reed
  • High-Voltage
  • Time delay
  • Small Signal
  • Multi-Dimensional
  • Thermal
  • Distance
  • Differential
  • Automotive
  • Frequency
  • Rotary
  • Polarized
  • Sequence
  • Moving Coil
  • Safety
  • Buchholz
  • Supervision
  • Ground Fault

Please see our comprehensive guide on the Classes of Relays for more information on the many forms of relays.

Applications of Relay 

Relays protect the electrical system and lessen the risk of overcurrents or overvoltages harming connected equipment. They are used to safeguard the linked equipment.

They are used to regulate the lower voltage signal that powers the high-voltage power circuit in power amplifiers and some types of modems.

These relays are used in applications like car starter solenoids to control extensive current connections with a lower power signal. In electricity transmission and distribution networks, they can also find and isolate issues. Relays typically find use in the following contexts:

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

Importance of Relay

Relays are adaptable parts that work well in both simple and complex systems. They can be used in place of other switch types or to meet specific specifications like amperage restrictions.

Switching the Level of Current

Relays are frequently employed when an application transitions from high to lower current or simultaneously inside the same circuit. For instance, HVAC temperature sensors need more amperage than their cabling can supply. To increase the current from a small one to a larger one, relays amplify it.

Complex Applications

A single fact in the circuit is not the only place relays can change a single input into a single output. In some applications, they can turn on several circuits, allowing a single input to start various effects. Relays can also be used in conjunction with each other to execute Boolean logic operations that may be less expensive than using other components. 

Furthermore, specific relays are more advanced than other electronic components. For instance, time-delay relays let systems operate for a predetermined amount or start after a predetermined amount of time. This gives rise to more advanced design alternatives for electronic systems.

Benefits of Relay 

Even though a relay is not technically necessary for an application, using one can still be helpful. High-amperage cabling and switches may require less money and room if relays are used. 

Relays in electronic devices allow manufacturers to pack more functionality into an amount of area while using thinner, lighter housing.

How to Identify a Wrong Relay?


Although they are often reliable, relays can malfunction like other mechanical parts. However, using a multimeter to find a lousy relay is not too difficult. 

To find a bad relay, find the wires where the connections enter and leave the relay. Then, using a multimeter, examine the voltage of each of those points.

1. Check to see whether there is voltage in which the relay is connected. If there’s no voltage, look for flaws in the fuse or switch. 

2. To verify a solid ground relation on the opposite side of the relay, use the multimeter’s continuity function if there are volts at the connection point.

3. Examine the voltages where the relay attaches to a battery or another power source if Steps 1 and 2 are unsuccessful in identifying the issue’s root cause. When there’s no voltage at this location, a fuse and circuit breaker may be faulty. 

4. To confirm that the relay or the component is connected correctly, use the multimeter’s continuity function. It might be necessary to replace your relay if the connection is present and the previous stages did not indicate a different problem.


We looked at the definition and operation of a typical electromechanical relay in this overview of relays. We also introduced a few different relay types and gave examples of how they might be used in various situations. In addition, we covered severe testing techniques.




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