Relays: When Electricity Controls Electricity

By Cliff Potts, CSO, and Editor-in-Chief of WPS News

Baybay City, Leyte, Philippines — August 14, 2026 — 12:30 p.m.

Last week we looked at switches.

A switch is one of the simplest devices in electronics. It either allows electricity to flow or it does not.

This week we examine one of the cleverest ideas ever developed in electrical engineering.

What if electricity could operate the switch?

That question led to the invention of the relay.

For nearly a century, relays were among the most important components in communications, industrial control systems, telephone exchanges, railroad signaling, and early computers.

Long before transistors and integrated circuits, relays were making decisions.

A relay is nothing more than an electrically operated switch.

When a small electrical current flows through a coil of wire, that coil becomes an electromagnet. The electromagnet pulls on a small metal armature, causing a set of electrical contacts to open or close.

The result is remarkable.

A tiny electrical signal can control a much larger electrical circuit.

That simple idea changed the world.

Relays allowed small control circuits to operate large motors.

They allowed telephone systems to automatically connect calls.

They allowed factories to automate machinery.

Eventually, they even allowed computers to perform calculations.

Every relay depends upon something we have already learned.

Electricity creates magnetism.

The magnetic field pulls the armature.

The armature moves the contacts.

The contacts control another circuit.

Each lesson in this series is beginning to build upon the last.

Resistors control current.

Capacitors store energy.

Magnetism creates force.

Induction produces electricity.

Relays combine all of these ideas into a practical machine.

One of the fascinating things about relays is that you can hear them working.

Click.

Clack.

Every sound represents a decision.

A circuit has changed state.

Power has been applied.

Power has been removed.

Before the silent world of semiconductor electronics, electrical systems often announced exactly what they were doing.

Large relay cabinets filled entire rooms.

Telephone exchanges clicked continuously.

Industrial control panels sounded almost alive.

Electronic Archaeology Note

In 1978, I built a piece of electronic test equipment that depended heavily upon relays.

Looking back today, it was essentially a small relay-based computer. Instead of using software, it used coils, contacts, timing, and careful wiring to determine what happened next.

It worked remarkably well.

Building it taught me something no textbook could.

Relays are not abstract.

You can watch them move.

You can hear them click.

You can follow the current through the contacts.

You can actually see logic taking place.

That experience made electronics feel less mysterious.

Instead of magic, it became mechanics directed by electricity.

Relays also taught me an important lesson about troubleshooting.

If a relay failed to operate, the problem might not be the relay itself.

Was the coil receiving power?

Were the contacts dirty?

Was another switch preventing current from reaching it?

Good troubleshooting means following the entire chain of events instead of replacing parts at random.

That lesson still applies to modern electronics.

One reason relays survived for so many years is reliability.

They tolerate electrical noise.

They can switch high voltages.

They provide complete electrical isolation between control circuits and power circuits.

Even today, many industrial machines continue to use relays because they perform certain jobs extremely well.

Of course, relays also have limitations.

They wear out.

Contacts become dirty.

Mechanical parts eventually fail.

They operate more slowly than transistors.

And they make noise.

Lots of noise.

When semiconductor technology matured, transistors replaced relays in many applications.

A transistor could perform similar switching functions without moving parts.

It operated silently.

It operated much faster.

It consumed less power.

But that does not make relays obsolete.

In fact, modern automobiles still contain relays.

Industrial equipment still uses relays.

Emergency shutdown systems still use relays.

Elevators, heating systems, generators, and electrical substations continue to rely upon them every day.

Sometimes the older technology remains the better solution.

That is one of the recurring lessons of Electronic Archaeology.

Progress does not always eliminate older ideas.

Sometimes it simply gives us more choices.

The relay represents a bridge between the mechanical world and the electronic world.

It is part machine.

Part electromagnet.

Part switch.

Entirely ingenious.

When you hear that familiar click, remember that you are listening to electricity making a decision.

Next week we will examine another remarkable device.

The transformer.

Unlike a relay, it contains no moving parts.

Yet it quietly transfers electrical energy from one circuit to another using nothing more than changing magnetic fields.

If this work helps you understand what’s happening, help me keep it going: https://www.patreon.com/cw/WPSNews

For more from Cliff Potts, see https://cliffpotts.org

References

Horowitz, P., & Hill, W. (2015). The art of electronics (3rd ed.). Cambridge University Press.

Encyclopaedia Britannica. (2025). Relay. Encyclopaedia Britannica. https://www.britannica.com/technology/relay-electronics

U.S. Department of Energy. (n.d.). Fundamentals handbook: Electrical science. U.S. Department of Energy.