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Motors and relays contain a coil of wire. While current flows, that coil quietly stores energy in a magnetic field around it. 🧲

When you switch the coil off, that stored energy has nowhere to go. The collapsing field forces a huge voltage spike in the opposite direction.

This reverse surge is the flyback spike. It can reach hundreds of volts for an instant, far beyond what your transistor or board can shrug off.

That spike slams into your switching transistor and can punch right through it. Over time it fries drivers and corrupts nearby Arduino pins.

The classic cure is a flyback diode placed across the coil. It gives that surge a safe loop to circulate in instead of attacking your circuit.

A diode lets current pass in only one direction. During normal operation it blocks; during the spike it suddenly conducts and drains the energy.

Orientation matters. The diode's striped cathode goes to the positive supply so it stays blocked normally and only opens for the reverse spike.

Wire the diode the wrong way and it shorts your supply the moment power is on. Always double-check the stripe before you power up. ⚠️

The 1N4007 diode is a cheap, sturdy pick for relay and small-motor coils. It handles the brief surge without breaking a sweat.

Good news: most relay modules already include a flyback diode on board. You only add one yourself when driving a bare coil or raw motor.

One cheap diode across every inductive load saves your driver and your board. It is the habit that separates reliable builds from flaky ones.

Where do you place a flyback diode and why?

Switching a coil off makes a damaging flyback spike. A diode across the coil, cathode to positive, absorbs it. Many modules include one already. 🚀