A mechanical relay is a switch whose contacts are actuated by a coil. It allows controlling a load with a separate circuit within the specified isolation limits.
When the coil generates a magnetic field, it attracts an armature, overcomes a spring, and changes the contacts. The click confirms movement, but does not guarantee that the contact has good continuity.
What’s inside the little blue box?
If you break the plastic casing of a typical relay (like the blue Songle ones that come in Arduino kits), you’ll see two completely separate parts:
- The control circuit (the coil): it’s a copper wire wound around an iron core. When we apply its rated voltage (e.g., 5 V), current flows and it becomes an electromagnet.
- The Power Circuit (The Contacts): These are two small metal strips. One is fixed and the other is movable (the armature).
- The Spring: Keeps the strips apart (or together) when the magnet is off.
The three contacts: COM, NO and NC
This is the part that confuses beginners when wiring. A relay usually has 3 output pins, but you’ll only use 2 at a time.
- COM (Common): This is the movable contact. It doesn’t have to be the center pin: always identify the pinout on the diagram or with continuity. In electrical mains, which conductor should be interrupted depends on the design and applicable regulations.
- NO (Normally Open):
- At rest (relay off), it is disconnected from COM.
- When you activate the relay, it connects.
- Use: To turn things on (lights, motors). It’s like a normal switch.
- NC (Normally Closed):
- At rest (relay off), it is connected to COM.
- When you activate the relay, it disconnects (opens).
- Use: Safety systems. Imagine a magnetic barrier that must sound an alarm if someone cuts the wire. Or an emergency stop button.
Mnemonic Rule: NO = Normally Open (no current flows until you activate it). NC = Normally Closed (current flows until you activate it).
The click
When you activate the coil, the electromagnet pulls the movable plate with force. When metal hits metal, the characteristic “CLACK” sound is produced.
That sound confirms the armature has moved, not that the contact has continuity or that it correctly supports the load.
The hidden danger: the flyback diode
Switching a coil with a transistor generates a voltage spike if we don’t provide a path for the current.
We’ve already seen that coils don’t like having their current cut off abruptly. They have magnetic inertia.
You have the relay activated (5V flowing through the coil).
You suddenly turn the relay off.
The coil, annoyed, tries to maintain the current and releases a reverse voltage spike (inductive kickback) that can reach -100V or -200V.
That spike enters your Arduino’s pin and destroys it.
The solution is the antiparallel diode
In a DC coil with unidirectional switching, a diode is usually placed in parallel, reverse-biased during normal operation (cathode to positive).
- When normal current flows, the diode does nothing.
- When you cut the current and the coil releases the “reverse kick,” the diode absorbs that energy and recirculates it through the coil itself until it dissipates, protecting the transistor.
Many DC relay modules include a transistor and a diode, but you should check the schematic. If you use a bare relay, design the control stage and its protection.
