What Is a Relay?
A relay is an electromechanical switch that opens and closes a much larger circuit with a small control signal. It electrically separates the control and load circuits from each other.
How Does It Work?
When current is given to the relay, a coil creates a magnetic field; this field pulls a lever to close the contacts. When the current is cut, a spring returns the contact to its former position.
The Isolation Advantage
The coil and contact are physically separate; so a 5 V MCU signal can safely switch a 220 V mains load. It's the basic method of controlling high power with low power.
| Contact | Function |
|---|---|
| NO | Closes when the coil is energized |
| NC | Opens when the coil is energized |
| COM | Common terminal |
Flyback Diode
When the coil is switched off it produces a reverse high voltage; this spike can burn the driver transistor. A flyback (freewheeling) diode connected in reverse-parallel with the coil safely dissipates this energy.
Where Is It Used?
Home appliances, automation, automotive (headlights/motors), power distribution and MCU-controlled high-power switching.
Coil Voltage and Triggering
Relays are chosen by coil voltage (like 5V, 12V, 24V). Relay modules can also be "high-trigger" or "low-trigger"; that is, whether the pin must give a 1 or a 0 to drive the coil differs. A wrong voltage or trigger logic makes the relay never pull in.
Common Mistakes
- Forgetting the flyback: the reverse voltage damages the driver.
- Insufficient current: an MCU pin can't drive the coil directly, a transistor is needed.
- Contact rating: the load current must not exceed the contact rating.
Frequently Asked Questions
Why is a relay driven with a transistor?+
What's the difference between a relay and an SSR?+
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