Relays

Last updated: December 4, 2009.

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: Relays and other assorted electrical switchgear. Photo by Фигушки, published on Flickr in 2006 under a Creative Commons Licence.

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. Think of it as a kind of electric lever or see-saw: switch it on with a tiny current and it switches on 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).

Relays in practice

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.

Relays were widely used in telephone switching and computer equipment until transistors were invented in the late 1940s. These tiny electronic components can do a similar job to relays, working as either amplifiers or switches. Transistors switch faster, use far less electricity, take up a fraction of the space, and cost much less (but they 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.

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).