What is an Auxiliary Relay?

Alexis W.

An auxiliary relay is a relay that assists another relay or device in performing an action. It does this when its operating circuit is opened or closed. These relays are used in nearly all electronic devices to assist them in functioning correctly. Essentially, it is as simple as an action done to the relay causing a circuit to be opened — completed — or closed and not allow power to travel through it.

Most car ignition switches include an auxiliary setting.
Most car ignition switches include an auxiliary setting.

One example of an auxiliary relay in action can be seen when a light is turned on in a home. The auxiliary relay is the switch on the wall. Once the relay is opened by flipping the switch to turn the light on, a signal is sent to tell the light to turn on. When the relay is closed, or the light switch is turned in the opposite direction, a separate signal is sent to kill the power to the light.

When the user turns the key in the ignition to the auxiliary position, everything running is powered by the car's battery.
When the user turns the key in the ignition to the auxiliary position, everything running is powered by the car's battery.

Another example of such a relay is a television power button. When a viewer presses the power button to turn the television on, he is opening the relay. A signal is then sent to the main power to turn the television on. When he presses the power button again, he is closing the relay and the power will shut off.

Yet another type of auxiliary relay can be found in a phone. When someone is calling or an individual picks up the phone to call someone, the relay is opened, allowing the call to be transmitted or received. When the users have completed the phone call, the relay will close until the phone is used again.

Computers also have these types of relays. When the computer is powered off and the user presses the power button, this opens the auxiliary relay and tells the computer to come on. If the user presses the power button again, the relay would be closed, indicating to the main electrical sources in the computer to shut off or power down.

Auxiliary relays are also used in automobiles. When the user turns the key in the ignition to the auxiliary position, this indicates to the electrical system in the car that everything running, such as the radio, will be powered by the car's battery. In this case, the auxiliary relay is in the open position. Once the key is turned in the ignition and the car is started, the auxiliary relay is closed and everything running will operate by the alternator.

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Discussion Comments


@Charred - That’s fascinating. I suppose in your business the purpose of the relays is to act as fault protection, to trip a circuit closed in case of a spike or surge of some sort.

I learned about electrical relay technology way back when I was a kid, playing with electronics kits. It’s a simple principle, true enough, but it’s used in so much of our current technology. A relay is basically an open or closed gate, as they call it in the electronics.


I work for a software company that serves the utility industry. Our primary focus is relay software. The article is correct in its basic explanation of relays, but of course, they can be a little more complicated; not the principle, but its implementation.

For example there are hundreds of electric relays that service the utilities from a wide variety of manufacturers. We have GE relays as well as units from German manufacturers, and the relays run the gamut from simple electromechanical relays to microprocessor relays.

The microprocessor relays are all computer controlled and offer more settings. All relays have what’s called a nameplate where basic settings information is entered but you can also add other settings as well.

I develop software which facilitates the reporting of relay tests. These are important for government audits. If you’re a utility and your relay doesn’t work right, you can be fined millions of dollars.

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