Relay Technology

A relay is an electrically operated switch. An electromechanical relay uses a small coil that, when energised, becomes an electromagnet and pulls an armature to open or close a set of contacts. Because the coil circuit is electrically separate from the contact circuit, a relay lets a low-power signal control a much larger current or voltage, and it provides isolation between the two. This makes relays useful for switching mains-powered loads from low-voltage control electronics.

A contactor is essentially a heavy-duty relay built to switch high currents, such as those drawn by motors, heaters and lighting circuits in industrial and building installations. Contactors have larger contacts and are designed to interrupt the arc that forms when a high-current circuit is broken. Relay and contactor contacts come in normally-open and normally-closed configurations, so a single device can both make and break circuits as its coil changes state.

Mechanical switches, including relay contacts and push-button and toggle switches, suffer from contact bounce. When the metal contacts meet, they spring apart and reconnect several times in a few milliseconds before settling, producing a burst of rapid on-off transitions. To a slow load this is harmless, but a fast digital circuit may read the bounce as many separate presses. Debouncing removes this effect, either with a simple resistor-capacitor filter and a logic gate that ignores the rapid transitions, or in software by waiting a short interval and re-reading the switch after it has stabilised.

Frequently asked questions

How does an electromechanical relay work?

Energising its coil creates a magnetic field that moves an armature, opening or closing contacts; the coil and contact circuits are electrically isolated from each other.

What is the difference between a relay and a contactor?

A contactor is a robust relay designed to switch high currents, such as motor loads, with larger contacts and provision for quenching the switching arc.

Why do switches need debouncing?

Contacts bounce open and closed for a few milliseconds when operated, which a fast digital circuit can misread as several presses; debouncing filters out these spurious transitions.