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What is a Relay and Why Are They So Important?

Posted on by Judy Johnson


Converting a small electrical input into a high-current output is no easy feat, but this task is necessary to efficiently operate a wide range of standard appliances and vehicles.  Many circuits achieve these conversions through the use of relays, which are indispensable in all kinds of electronic equipment.

What Is a Relay?

Relays are electric switches that use electromagnetism to convert small electrical stimuli into larger currents.

These conversions occur when electrical inputs activate electromagnets to either form or break existing circuits.

By leveraging weak inputs to power stronger currents, relays effectively act as either a switch or an amplifier for the electric circuit, depending on the desired application.

Why Use a Relay?

Relays are highly versatile components that are just as effective in complex circuits as in simple ones.

They can be used in the place of other forms of switches, or they can be specifically designed based on factors such as required amperage.

Switching Level of Current

One of the most common situations that require the use of a relay occurs when an application needs to switch from high to low current (or vice versa) within the same circuit.

For example, the temperature sensors that power HVAC units require levels of amperage that vastly exceed the capacity of their wiring.

Relays provide the necessary amplification to convert a small current into a larger one.

Complex Applications

Relays are not limited to transforming single inputs into single outputs at single points in the circuit. In other applications, a single relay can activate multiple circuits, allowing one input to initiate many other effects.

Similarly, relays can be used in combination with one another to perform Boolean logic functions that, while possible to enact using other components, may be more cost-effective when implemented using relays.

What’s more, specific relays can perform more advanced functions than other electronic components. Time-delay relays, to name just one category, allow systems to run only for a set period of time or to start only after a set period of time.

This introduces more sophisticated possibilities for constructing electronic systems.


Even when an application doesn’t specifically require a relay, using one can prove beneficial.

Relays can reduce the need for high-amperage wiring and switches, which are expensive and take up space.

Therefore, switching to relays in your electronic systems can reduce the size or weight of a casing, for instance, or allow manufacturers to fit more functionality into a space of the same size.

How Does a Relay Work?

Relays differ in their size, capacity, and corresponding uses. However, although they may differ in these respects, all relays function in essentially the same way: one circuit is used to power another.

The specific manner in which this occurs depends on whether the relay is normally open (NO) or normally closed (NC).

Normally Open Relays

Most relays are normally open; that is, the second, larger circuit is in the off position by default.

In a normally open relay, power flows through an input circuit, activating an electromagnet. This generates a magnetic field that attracts a contact to join with the second, larger circuit, allowing current to flow through. When the source of power is removed, a spring draws the contact away from the second circuit, stopping the flow of electricity and turning off the end device.

Normally Closed Relays

The fundamentals of an NC relay are the same as an NO relay: there are two circuits, with the second being larger, and an electromagnet moves a physical contact between two positions.

But in the case of an NC relay, the default states are reversed. When the first circuit is activated, the electromagnet draws the contact away from the second circuit. As such, NC relays keep the larger circuit in the on position by default.

How to Identify a Faulty Relay

Though generally reliable, relays can fail like any mechanical component. Luckily, it’s relatively easy to identify a faulty relay using a multimeter.

To do so, you must first locate where the circuits enter and exit the relay, an area typically marked by pins. After identifying this location, you can use the multimeter to measure the voltage at each point.

Use the following troubleshooting steps:

  1. Check for voltage at the point where the relay plugs in. If there is none, check the fuse or switch for defects.
  2. If there is voltage at the connection point, use the continuity function on the multimeter to ensure a good ground connection on the opposite side of the relay.
  3. If Steps 1 and 2 do not reveal the source of the problem, check the voltage at the point where the relay connects to the battery or other power source. If there is no voltage here, there may be a problem with a fuse or circuit breaker.
  4. Finally, ensure that a proper connection exists between the relay and the component by using the continuity function of the multimeter. If the connection exists, and if the prior steps did not suggest another malfunction, then it may be time to replace the relay.

Why Use A Relay

Types of Relays

Many types of relays exist, each bringing unique functions to a variety of applications. Some of the broader categories include:

Time-Delay Relays

Time-delay relays are useful in any situation that requires components to be powered for a set length of time, or when the component must power on or off after a specific delay. These relays have time-delay functionality built in, making them desirable for a range of time-based applications.

This category encompasses several types of time-delay relays, each with its specific uses.

Most time-delay relays can be sorted into one of two broad categories:

  • On-delay timers begin timing when the input is applied, powering the second circuit after a set wait time. This can be used to stagger the powering of multiple components, preventing power surges, or for applications like alarms and warning systems.
  • Off-delay timers wait for a trigger after the input is energized. Once the trigger is removed, the output is energized, and then it de-energizes after the time delay. Reapplying the trigger resets the delay. These relays can be used to power devices for set intervals, such as in washer and dryer cycles or amusement park rides.

Other patterns of start and delay are possible with flashers, one-shot timers, or repeat cycles, and each of which allows for the component to be powered at different repeating intervals. This makes flashing indicator or warning lights possible and also enables certain types of timed cycles.

Sequential Relays

Sequential relays can be used to power multiple components in turn, typically in a set order. A common application for this type of relay involves powering multiple systems or sets of lights one after another, such as in runway lights or power supply sequencing.

Automotive Relays

Relays have almost unlimited uses in automotive applications, and these applications encompass many of the relay types discussed. Many automotive relays allow manufacturers to implement advanced safety features and modern electrical conveniences.

To name just a few examples relays to power the following systems in standard cars or trucks:

  • Gas Valves
  • Headlights
  • Windshield wipers
  • Interior lighting
  • Alarm systems
  • Warning systems as used for weight limits, seatbelt usage, or hazard-sensing

Where to Find Your Next Relay

Because relays are an integral part of circuit design, it’s essential to source high-quality relays of the type and size necessary for your application.

Amperite offers a wide range of relays and other electronic components designed to help you save time, money, and energy.

We also specialize in manufacturing custom products to address your individual concerns.

If you would like to learn more about our electronic applications and solutions, contact us today!

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