Flyback Diode

A flyback diode, also known as a freewheeling diode, snubber diode, or suppression diode, is a semiconductor device connected in reverse polarity across an inductive load (such as a relay coil or contactor coil) in a driver circuit. Its primary function is to provide a safe discharge path for the inductive energy stored in the coil when the driving current is suddenly interrupted, thereby preventing damaging voltage spikes (back-EMF) that could harm switching transistors, microcontrollers, or other sensitive electronic components.

When current flows through an inductive coil, it stores energy in a magnetic field. When the driving switch (e.g., transistor) opens, the sudden collapse of this magnetic field induces a high reverse voltage (back-EMF) across the coil. The flyback diode, connected in parallel with the coil but in reverse bias during normal operation, becomes forward-biased by this induced voltage. This creates a low-resistance path for the current to circulate through the diode and coil, allowing the stored energy to dissipate safely as heat over a short period, clamping the voltage spike to approximately the diode's forward voltage drop (typically 0.7-1.2V for silicon diodes).