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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).

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 don't always turn things on; sometimes they very helpfully turn things off instead. In power plant equipment and electricity transmission lines, for example, you'll find protective relays that trip when faults occur to prevent damage from things like current surges. Electromagnetic relays similar to the ones described above were once widely used for this purpose. These days, electronic relays based on integrated circuits do the same job instead; they measure the voltage or current in a circuit and take action automatically if it exceeds a preset limit.

Other types of relays

What we've looked at so far are very general switching relays—but there are quite a few variations on that basic theme, including (and this is by no means an exhaustive list):

• High-voltage relays: These are specifically designed for switching high voltages and currents well beyond the capacity of normal relays (typically up to 10,000 volts and 30 amps).
• Electronic and semiconductor relays (also called solid-state relays or SSRs): These switch currents entirely electronically, with no moving parts, so they're faster, quieter, smaller, more reliable, and last longer than electromagnetic relays. Unfortunately, they're typically more expensive, less efficient, and don't always work as cleanly and predictably (due to issues like leakage currents).
• Timer and time-delay relays: These trigger output currents for a limited period of time (usually from fractions of a second to about 100 hours, or four days).
• Thermal relays: These switch on and off to stop things like electric motors from overheating, a bit like bimetallic strip thermostats.
• Overcurrent and directional relays: Configured in various different ways, these stop excessive currents from flowing in the wrong direction around a circuit (typically in power-generation, distribution, or supply equipment).
• Differential protection relays: These trigger when there are current or voltage imbalances in two different parts of a circuit.
• Frequency protection relays (sometimes called underfrequency and overfrequency relays): These solid-state devices trigger when the frequency of an alternating current is too high, too low, or both.

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!