Inductors and RL circuits: the voltage spike at turn-off
An inductor resists sudden changes in current. Why a large voltage spike appears when a relay or motor is switched off, and what to do about it.
When current flows, an inductor stores energy in a magnetic field. Try to change the current and it resists by producing a voltage in the opposite direction.
V = L × di/dt
Current does not change suddenly
If 100 mA is flowing in a relay coil and a transistor cuts it off in 1 µs, then with a coil inductance of 10 mH
V = 0.01 × 0.1 / 0.000001 = 1000 V
In reality the transistor is destroyed first. This is inductive kickback.
What to do
- Flyback diode: a diode fitted in reverse across the coil gives the current a way out. An essential part for relays and solenoids.
- TVS diode: clips the voltage at a set value when you need to turn off fast.
- H-bridges and inverters: the body diode inside the MOSFET plays the same role.
RL time constant
τ = L / R. The time for the coil current to rise to 63%. The electrical time constant of a motor winding sets how fast the current control loop has to be.
Where it is used
Energy storage in switching converters, filters that block noise, and the windings of motors and transformers are all inductors.