What is a flyback voltage spike?

When an inductive load is switched off the magnetic field collapses and generates a voltage spike to maintain current flow. Without a clamp this spike can reach hundreds or thousands of volts destroying the switching transistor.

Why do I need a flyback diode?

The diode provides a path for the inductor current to flow after the switch opens. It clamps the spike to approximately one diode drop above the supply voltage instead of allowing an uncontrolled spike.

What determines the spike voltage?

V equals L times dI divided by dt. Faster switch-off time means higher spike. A MOSFET switching off in 100 nanoseconds produces a much larger spike than a relay contact opening in 1 millisecond.

How much energy is stored in the inductor?

E equals one half L I squared. A 50 mH relay coil at 120 mA stores 0.36 millijoules. A 5 mH motor winding at 48 A stores 5.76 joules. This energy must be safely dissipated at turn-off.

Inductive Load Calculator – Flyback Voltage, Stored Energy & Turn-Off Spike

Inductive Load Analysis

Steady-state current, stored energy, turn-off spike, and protection design.

V Supply Voltage

L Inductance

R Coil Resistance (DCR)

t_off Switch-Off Time (optional — for flyback spike)

Analysis Results

Steady-State Current
--
I = V / R

Time Constant τ

τ = L / R

Settle Time (5τ)

Stored Energy
--
E = ½LI²

Steady-State Power

DCR

Inductive Load Turn-Off Spike

When an inductive load is switched off, the collapsing magnetic field generates a flyback voltage spike. The spike voltage depends on the inductance, current, and how fast the switch opens. Without protection (flyback diode), this spike destroys the switch.

V_s — Supply voltage.
R — Coil DC resistance. Sets steady-state current I = V/R.
L — Inductance. Stores energy E = ½LI² in the magnetic field.
Spike — Flyback voltage at turn-off. V = L × dI/dt. Clamped by flyback diode.

Inductive Load Calculator

Inductive loads — relay coils, solenoid valves, motors, electromagnets — store energy in their magnetic field. This calculator analyses the steady-state current, stored energy, RL time constant, and the dangerous flyback voltage spike that occurs when the load is switched off. Every inductive load needs protection circuitry, and this tool shows you exactly why.

What Is an Inductive Load?

Any device that creates a magnetic field to do work: relay coils pull contacts, solenoid valves open fluid paths, motors spin shafts, electromagnets hold or lift. The inductance opposes current changes, causing a slow start-up (governed by the RL time constant) and a dangerous voltage spike at turn-off. RL Time Constant Calculator gives the pure timing analysis.

Key Formulas

I_steady = V / R — steady-state current (at t >> τ)
τ = L / R — time constant
E = ½LI² — stored energy
V_spike = L × dI/dt — flyback voltage at turn-off
V_total = V_supply + V_spike — voltage the switch must withstand

Relay Coil (12V / 100Ω / 50mH)

I = 12/100 = 120 mA
τ = 50 mH / 100 Ω = 0.5 ms
E = ½ × 50 mH × (0.12)² = 0.36 mJ
Spike (1 µs switch): V = 50 mH × 0.12 / 1 µs = 6 kV!

6 kV from a 12 V circuit. This is why flyback protection is not optional. A simple 1N4148 diode across the coil clamps the spike to ~12.7 V (V_supply + V_diode). For the current rise analysis, see the Inductor Current Calculator.

Solenoid Valve (24V / 8Ω / 200mH)

I = 24/8 = 3 A
τ = 200 mH / 8 Ω = 25 ms
E = ½ × 200 mH × 9 = 0.9 J
Spike (10 µs): V = 200 mH × 3 / 10 µs = 60 kV

0.9 joules of stored energy. The solenoid takes 125 ms (5τ) to reach full current. The 60 kV theoretical spike is limited in practice by arcing across the switch contacts, but it still destroys solid-state drivers instantly. For the energy stored, see the Inductor Energy Calculator.

DC Motor (48V / 1Ω / 5mH)

I_stall = 48/1 = 48 A
τ = 5 mH / 1 Ω = 5 ms
E = ½ × 5 mH × 2304 = 5.76 J

5.76 joules at stall current. During normal running, the back-EMF reduces the effective voltage and current is much lower. But at start-up or stall, the full 48 A flows and the stored energy is substantial.

Flyback Protection Methods

Flyback diode — Simplest. Clamps spike to V_supply + 0.7 V. Slow de-energisation (current decays through diode). Standard for relays and solenoids.

Zener + diode — Diode in series with a zener. Clamps to V_supply + V_zener. Faster de-energisation than a plain diode. Used where release speed matters.

RC snubber — Resistor-capacitor across the switch. Absorbs the spike energy. Used for AC loads where a diode cannot be used.

TVS diode — Bidirectional transient voltage suppressor. Fast clamping. Used in automotive and industrial circuits.

Frequently Asked Questions

Why is the flyback spike so high?

V = L × dI/dt. A MOSFET switches off in nanoseconds to microseconds, giving an extremely high dI/dt. Even a small inductance multiplied by a very fast current change produces a huge voltage.

Does the flyback diode slow down relay release?

Yes. The diode allows current to circulate, keeping the relay energised longer. A 50 mH / 100 Ω relay with a flyback diode takes about 2.5 ms to release vs ~0.1 ms without. For faster release, use a zener + diode clamp.

Can a flyback spike damage the power supply?

Yes, if the spike propagates back through the supply rail. Decoupling capacitors near the switch and proper PCB layout help. The flyback diode or clamp should be as close to the inductive load as possible.

Do AC inductive loads also have flyback?

Yes, but the spike occurs at every zero-crossing and when the switch opens. AC contactors use RC snubbers or varistors instead of diodes because the current reverses each half-cycle.

Last updated: March 2026