Relay
A relay lets a low-power control signal switch a much higher-power load while keeping the two sides electrically isolated. Useful for mains, motors, and anything you want to control safely from a microcontroller.
In a mechanical relay, a small coil pulls in an iron armature that closes contacts. In a solid-state relay, an opto-isolated triac or MOSFET does the same job silently and without moving parts.
In plain terms
A doorman who opens a heavy gate when you press the buzzer. Your finger never touches the gate itself.
Why designers use it
- Switch mains-voltage loads from low-voltage logic.
- Isolate a sensitive controller from a noisy load.
- Provide a satisfying physical click in industrial gear.
- Handle inrush of a pump or heater that a transistor would baulk at.
Best for
- Mains control
- Inductive loads
- Galvanic isolation
- High-current switching
When not to use it
- Fast switching — relay coils take milliseconds to settle.
- Battery-powered designs where coil current would dominate budget — use a MOSFET.
Common mistakes
- Forgetting the flyback diode across the coil.
- Driving the coil straight from a microcontroller pin without a transistor.
- Sizing contacts for steady current and forgetting inrush.
Where you will find it
- Your car's starter motor needs hundreds of amps the moment you turn the key — a beefy relay (the 'starter solenoid') is what physically routes that current; the key only switches a small wire to its coil.
- Every smart plug audibly clicks when you turn it on from your phone — that click is a relay coil pulling its contacts closed to feed your lamp without exposing the firmware to mains voltage.
- Solid-state relays inside industrial ovens switch heater elements millions of times without wear — they look like silent cousins of mechanical relays but use a triac instead of moving parts.
A short history
The electromagnetic relay was invented by Joseph Henry in 1835, who used an electromagnet controlled by a weak current over a long wire to switch a heavier local circuit—the principle later applied to the electric telegraph. Samuel Morse also utilised relay stations for long-distance telegraphy. The relay's ability to electrically isolate the control circuit from the switched circuit (providing both signal amplification and galvanic isolation) made it the dominant switching device for a century, until the transistor replaced it in most electronic applications. Mechanical relays remain indispensable in power systems, automotive fuse boxes, and safety-critical circuits where their fail-open behaviour and optical isolation are required.
Good to know
- The 1830s Wheatstone telegraph relay is the direct ancestor of every digital logic gate — Bell Labs built the first electromechanical computers from rooms full of them.
- Reed relays — a glass tube containing two thin ferromagnetic blades — switch in 200 µs and last for 100 million cycles, beating most solid-state switches for sub-volt analog signals.