by Chris Woodford. Last updated: July 10, 2012.
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!
Artwork: An open and shut case: A relay is an electromagnetic device that links two circuits. When power flows through the first circuit (1), it activates an 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. You can see real-life versions of this in John Boxall's superb
closeup photo of a relay
and this wonderful shot of a relay without its cover
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:
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
A relay uses an electromagnet (a coil of iron wire that becomes a
magnet when electricity flows through it) to link two circuits
together. On one 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 alongside. The relatively small
current in the input circuit thus activates the larger current in the
- 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.
- When a small current flows in the input circuit, it activates an electromagnet (shown here as a red coil), which produces a magnetic field all around it.
- 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.
- The output circuit operates a high-current appliance such as a lamp or an electric motor.
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.
Who invented relays?
Relays were invented in 1835 by American electromagnetism pioneer
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
Find out more
On this website
- How to make a relay: A fairly simple 2.5 minute video guide shows you how to wind your own electromagnets and mount them on a board to produce your very own homemade relay.
- How an automotive relay works: This short and simple video explanation talks you through the stuff I explained up above. Same explanation, slightly different words.