Relays: switching big loads safely
8 min read
A relay is a switch moved by an electromagnet. A small signal from your board closes a separate contact, so a low-voltage circuit can control another circuit without touching it.
The three contacts
- COM is the common terminal.
- NO means normally open. It connects to COM when the relay is on.
- NC means normally closed. It connects to COM when the relay is off.
Most projects use COM and NO, so the load stays off if power fails.
Wiring a relay module
- Connect module VCC to the 5 V supply.
- Connect module GND to the common ground.
- Connect module IN to your board's output pin.
- Cut the positive wire of the load and connect the two ends to COM and NO.
Many modules are active-low, so the relay turns on when the pin goes LOW. Test with no load first.
Choosing a relay
The rating printed on the relay is a maximum. Stay well below it. Motors and pumps draw several times their normal current when they start.
Safety: students should switch only low-voltage DC loads such as a 12 V pump or lamp. Mains wiring can kill. Any mains connection must be done, checked and enclosed by a qualified electrician, with the power off.
Relay or MOSFET?
Use a MOSFET for fast DC switching such as motor speed control. Use a relay when the load must be fully isolated. Relays click and wear out, so never use them with fast PWM.
Quick quiz
1. With no power, which contact is joined to COM?
2. Who should make a mains connection?
3. Which is better for fast motor speed control?