Skip to content
LearnTronics menu

LearnTronics

Inductive Flyback in Depth

Tutorial Index

Recipe #37 — inductive flyback in depth

The earlier relay-driver circuit used a flyback diode. Now the reason for that diode can be examined directly: a magnetic field contains stored energy, and an inductor will generate voltage to keep current flowing when its circuit is opened.

INDUCTIVE FLYBACK — WHAT HAPPENS WHEN CURRENT STOPS WITHOUT A CLAMP+9 VL1 / RELAY COILOPEN SWITCH → current tries to continue → voltage can rise sharply WITH FLYBACK DIODE+9 V D1OPEN SWITCH → coil current circulates through D1 and decays safely Stored magnetic energy: E = ½LI²Induced voltage follows v = L(di/dt)
The key rule: inductor current cannot stop instantly. When the normal path opens, the magnetic field collapses and the coil generates whatever voltage is necessary to keep current moving until its stored energy is dissipated.
E = ½LI²     and     v = L(di/dt)

Why the diode works

During normal DC operation D1 is reverse-biased. When the switch opens, the coil reverses its terminal voltage as needed to keep current flowing. That forward-biases D1 and creates a local recirculation path.

The diode changes release speed

A simple diode clamps the voltage very low, so coil current decays relatively slowly. That can make a relay release more slowly. Zener or TVS clamps permit a higher safe voltage and can release stored energy faster when quicker mechanical release matters.

Try these yourself

Why can an opened inductor circuit generate a high voltage?
A rapidly forced change in inductor current produces voltage, and an open path can require a very large voltage to continue current.
When is the ordinary flyback diode forward-biased?
The diode is normally reverse-biased, then conducts when the coil generates reverse voltage at turn-off.
What tradeoff comes with a simple low-voltage flyback clamp?
Low clamp voltage lets current decay gently, which often lengthens relay release time.
< RL Transient Response   |   Simple Boost Converter >