Inductor Behaviour

Inductor Discharging

When the current flowing through an inductor is interrupted, the magnetic field surrounding the winding begins to collapse. The energy stored in the magnetic field is released back into the circuit, producing a voltage that attempts to keep the current flowing. This process is known as inductor discharging. It is an important consideration in relay circuits, solenoids, motor controllers, switch-mode power supplies and many other electronic systems.

Inductor Discharging

What Happens When the Supply Is Switched Off?

An inductor resists sudden changes in current. When the supply voltage is removed, the magnetic field collapses and generates a voltage known as back electromotive force (back EMF) or inductive kickback.

This voltage attempts to maintain the original current flow until the stored energy has been dissipated.

Stages of Discharging

Stage What Happens
Supply Removed Current cannot stop instantly.
Magnetic Field Collapses Stored energy is released.
Back EMF Generated The inductor produces a voltage to maintain current.
Energy Dissipated Current gradually falls to zero.

Current Decay

The current decreases exponentially according to the RL time constant. 


Ļ„ = L / R

The larger the inductance or the lower the resistance, the longer the current takes to decay.

Current Remaining After Switching Off

Time Remaining Current
0 100%
1Ļ„ 36.8%
2Ļ„ 13.5%
3Ļ„ 5.0%
4Ļ„ 1.8%
5Ļ„ 0.7%

After approximately five time constants, the current has effectively fallen to zero.

Energy Released

The energy stored in the magnetic field is given by: 


Energy = Ā½ Ɨ L Ɨ IĀ²

During discharge, this energy is transferred to the surrounding circuit or dissipated as heat.

Back EMF

Because an inductor tries to keep the current flowing, the voltage generated during discharge may become much higher than the original supply voltage if no safe discharge path is available.

This high voltage can damage transistors, MOSFETs, relays, integrated circuits and other semiconductor devices.

Protection Methods

Protection Device Purpose
Flyback Diode Provides a safe discharge path for DC relay coils.
RC Snubber Reduces switching voltage spikes.
TVS Diode Clamps excessive transient voltages.
MOV Protects AC-powered inductive loads.
Active Clamp Circuit Controls energy release in switch-mode power supplies.

Applications

Application Importance
Relay Drivers Protects switching transistors.
Solenoid Valves Controls magnetic field collapse.
Motor Controllers Protects power semiconductors.
SMPS Transfers stored magnetic energy.
Ignition Systems Generates high-voltage pulses.

Worked Example

An RL circuit has: 


L = 200 mH

R = 50 Ī©

Calculate the discharge time constant. 


L = 0.2 H

Ļ„ = L / R

Ļ„ = 0.2 / 50

Ļ„ = 0.004 s

Ļ„ = 4 ms

After approximately 20 ms (5Ļ„), the current has effectively decayed to zero.

Testing Inductor Discharge

  • Observe the voltage waveform with an oscilloscope.
  • Measure current decay in an RL test circuit.
  • Check flyback diodes for correct operation.
  • Inspect switching devices for damage caused by voltage spikes.
  • Verify the discharge time against the calculated time constant.

Common Mistakes

  • Assuming current stops instantly when power is removed.
  • Ignoring the need for flyback protection.
  • Confusing magnetic energy storage with capacitor charge storage.
  • Using an underrated protection device.
  • Ignoring transient voltages during troubleshooting.

Interesting Facts

  • The back EMF generated during discharge can be several times higher than the supply voltage.
  • Automotive ignition coils intentionally use magnetic field collapse to generate tens of thousands of volts.
  • Every relay coil generates a voltage spike when switched off unless protection is provided.
  • Many switch-mode power supplies rely on controlled magnetic energy release during every switching cycle.
  • The stored energy increases with the square of the current, making high-current inductors capable of releasing significant energy.

Key Points

  • When current is interrupted, an inductor releases its stored magnetic energy.
  • The collapsing magnetic field generates back EMF.
  • Current decreases exponentially according to the RL time constant.
  • Flyback protection is essential in many inductive circuits.
  • Proper protection prevents damage to switches and semiconductor devices.

Next Lesson

Continue by learning about Flyback Diodes, including how they suppress inductive voltage spikes and protect electronic circuits.

Next Lesson ā†’ Flyback Diodes