Mechanical Relay
A relay (electromechanical, coil-actuated) lets a small control current switch a much larger isolated load. Energising the coil pulls a hinged armature against return-spring force; the armature carries the moving contact onto the stationary contacts, completing the load circuit.
Current through the coil magnetises a soft-iron core; the resulting field attracts a hinged armature against a return-spring force. The armature carries one or more contact bridges that slam onto fixed terminals, switching the load circuit. When the coil de-energises, the spring returns the armature; a flyback diode across the coil clamps the inductive kick.
In plain terms
A small electromagnet with a hinged iron flap; energise the coil and the flap slams onto the contacts; cut the coil and a spring throws it back.
Why designers use it
- Switching mains loads from a low-voltage microcontroller GPIO with full galvanic isolation.
- Latching power and motor circuits where a contactor's snap is a feature.
- High-impulse switching tolerant of arcs and welds (e.g. 16 A motor starts).
Best for
- Mains switching
- Galvanic isolation
- High inrush
Key specifications
- Coil voltage: 3, 5, 12, 24 V common
- Coil current: 20 – 200 mA
- Contact rating: 2 – 30 A
- Operate time: 5 – 15 ms
- Lifetime: 10⁵ – 10⁷ cycles
When not to use it
- Where switching speed matters — mechanical relays take milliseconds; an SSR is microseconds.
- In high-cycle automation — contacts wear; an SSR has no moving parts.
- When the package size is critical — even a tiny SMD signal relay dwarfs an equivalent MOSFET.
Common mistakes
- Forgetting the flyback diode across the coil — back-EMF spikes destroy the driving GPIO or transistor.
- Driving the coil from a marginal supply — at brownout the relay chatters.
- Switching DC near the AC contact rating — the contacts arc and weld because there's no zero-crossing.
Where you will find it
- An HVAC thermostat uses a 24 VAC coil relay to switch the 240 VAC compressor: the relay isolates the low-voltage thermostat wire from the mains contactor coil and survives years of compressor inrush.
- Automotive starter relays carry the 200 A surge from a 12 V battery to the starter motor solenoid — the dashboard ignition switch only carries the relay's 100 mA coil current.
- A home-automation board uses a 5 V SPDT coil relay to switch a kitchen appliance circuit: the ESP32 GPIO drives a transistor to the coil, and the relay's NO contact handles the 10 A load.
A short history
The electromechanical relay was invented by Joseph Henry in 1835 as a means to extend telegraph signals across long lines; he demonstrated that a weak distant current could energise a local coil and snap a strong local contact, in effect amplifying the signal. Samuel Morse incorporated the relay into his 1837 telegraph system, and by the 1840s relays had become standard repeating elements on intercontinental telegraph cables. Through the 20th century the relay remained the backbone of telephone exchanges, factory PLCs, and protective electrical equipment, and although the transistor displaced it in low-current logic, coil relays still dominate high-current isolated switching.
Good to know
- A clicking relay is the original sound of automation — every elevator, telephone exchange, and pinball machine ran on relays before the 1980s.
- The Harvard Mark II computer was running on relays in 1947 when Grace Hopper's team logged the first 'computer bug' — a moth taped to a relay contact.
- A dropping coil voltage near the release threshold causes 'chatter' — the armature buzzes between open and closed at hundreds of hertz.