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A typical relay with its plastic outer case removed, showing the electromagnet and the spring contacts inside.

Relays

by Chris Woodford. Last updated: March 28, 2014.

You might not realize it, but you're constantly on-guard, watching out for threats, ready to act at a moment's notice. Millions of years of evolution have primed your brain to save your skin when the slightest danger threatens your existence. If you're using a power tool, for example, and a tiny wood chip flies toward your eye, one of your eyelashes will send a signal to your brain that make your eyelids clamp shut in a flash—fast enough to protect your eyesight. What's happening here is that a tiny stimulus is provoking a much bigger and more useful response. You can find the same trick at work in all kinds of machines and electrical appliances, where sensors are ready to switch things on or off in a fraction of a second using clever magnetic switches called relays. Let's take a closer look at how they work!

Photo: A typical relay with its plastic outer case removed. You can see the two spring contacts on the left and the electromagnet coil (the red-brown copper-colored cylinder) on the right. In this relay, when a current flows through the coil, it turns it into an electromagnet. The magnet pushes a switch to the left, forcing the spring contacts together, and completing the circuit they're attached to. This is a relay from an electronic, hot-water immersion heater programmer. The electronic circuit in the programmer switches the magnet on or off at preprogrammed times of day using a relatively small current. That allows a very much bigger current to flow through the spring contacts to power the element that heats the hot water.

What are relays?

A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. The heart of a relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it). You can think of a relay as a kind of electric lever: switch it on with a tiny current and it switches on ("leverages") another appliance using a much bigger current. Why is that useful? As the name suggests, many sensors are incredibly sensitive pieces of electronic equipment and produce only small electric currents. But often we need them to drive bigger pieces of apparatus that use bigger currents. Relays bridge the gap, making it possible for small currents to activate larger ones. That means relays can work either as switches (turning things on and off) or as amplifiers (converting small currents into larger ones).

How relays work

Here are two simple animations illustrating how relays use one circuit to switch on a second circuit.

A simple animation showing how a relay uses electromagnetism to link two circuits.

When power flows through the first circuit (1), it activates the electromagnet (brown), generating a magnetic field (blue) that attracts a contact (red) and activates the second circuit (2). When the power is switched off, a spring pulls the contact back up to its original position, switching the second circuit off again.

This is an example of a "normally open" (NO) relay: the contacts in the second circuit are not connected by default, and switch on only when a current flows through the magnet. Other relays are "normally closed" (NC; the contacts are connected so a current flows through them by default) and switch off only when the magnet is activated, pulling or pushing the contacts apart. Normally open relays are the most common.

Here's another animation showing how a relay links two circuits together. It's essentially the same thing drawn in a slightly different way. On the left side, there's an input circuit powered by a switch or a sensor of some kind. When this circuit is activated, it feeds current to an electromagnet that pulls a metal switch closed and activates the second, output circuit (on the right side). The relatively small current in the input circuit thus activates the larger current in the output circuit:

Animation showing how an electromagnetic relay works

  1. The input circuit (black loop) is switched off and no current flows through it until something (either a sensor or a switch closing) turns it on. The output circuit (blue loop) is also switched off.
  2. When a small current flows in the input circuit, it activates the electromagnet (shown here as a red coil), which produces a magnetic field all around it.
  3. The energized electromagnet pulls the metal bar in the output circuit toward it, closing the switch and allowing a much bigger current to flow through the output circuit.
  4. The output circuit operates a high-current appliance such as a lamp or an electric motor.

Relays in practice

Telephone exchange in 1952.

Suppose you want to build an electronically operated cooling system that switches a fan on or off as your room temperature changes. You could use some kind of electronic thermometer circuit to sense the temperature, but it would produce only small electric currents—far too tiny to power the electric motor in a great big fan. Instead, you could connect the thermometer circuit to the input circuit of a relay. When a small current flows in this circuit, the relay will activate its output circuit, allowing a much bigger current to flow and turning on the fan.

Who invented relays?

Relays were invented in 1835 by American electromagnetism pioneer Joseph Henry; in a demonstration at the College of New Jersey, Henry used a small electromagnet to switch a larger one on and off, and speculated that relays could be used to control electrical machines over very long distances. Henry applied this idea to another invention he was working on at the time, the electric telegraph (the forerunner of the telephone), which was successfully developed by William Cooke and Charles Wheatstone in England and (much more famously) by Samuel F. B. Morse in the United States. Relays were later used in telephone switching and early electronic computers and remained hugely popular until transistors came along in the late 1940s; according to Bancroft Gherardi, marking the 100th anniversary of Henry's work on electromagnetism, there were an estimated 70 million relays in operation in the United States alone by that time. Transistors are tiny electronic components that can do a similar job to relays, working as either amplifiers or switches. Although they switch faster, use far less electricity, take up a fraction of the space, and cost much less than relays, they generally work with only tiny currents so relays are still used in many applications. It was the development of transistors that spurred on the computer revolution from the mid-20th century onward. But without relays, there would have been no transistors, so relays—and pioneers like Joseph Henry—deserve some of the credit too!

Photo: Relays were widely used for switching and routing calls in telephone exchanges such as this one, pictured in 1952. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).

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Text copyright © Chris Woodford 2009, 2014. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009) Relays. Retrieved from http://www.explainthatstuff.com/howrelayswork.html. [Accessed (Insert date here)]

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