Reference 01 Inductive loads — relays, solenoids, motors ↑ Contents Built from Lesson 01

Driving an inductive load

The equations, the datasheet numbers, and the one trade-off that every clamp choice sits on.

The two equations

Voltage across a coil V = L × (dI/dt) Volts are set by the rate of change of current, not by the current. Constant V → current ramps linearly.
Energy stored in a coil E = ½ L I² What has to be dissipated somewhere at every turn-off.

Time constant  τ = L / R. Current reaches ~99% of final in 5τ.

The one rule

Current in an inductor cannot stop instantly. At turn-off it will flow somewhere. The coil raises the switch-node voltage until something conducts.

You are not choosing whether current flows. You are choosing what conducts. If you don't choose, the coil picks your transistor.

The trade-off

Decay rate is proportional to the voltage the clamp holds:

dI/dt = Vclamp / L
  • Low clamp → gentle on the switch, slow release.
  • High clamp → fast release, more stress on the switch.

Every clamp type is a point on this one line. There is no free option.

Clamp options, ranked by clamp voltage

ClampNode sits atRelease
Plain diodeVS + 0.7slowest
Diode + resistorVS + 0.7 + IRfaster
Zener / TVSVS + VZfast
Active clampset by designfast
Nothingswitch breakdownfastest — once

Detail and sizing: Lesson 02.

Diode orientation

Across the coil, cathode to the positive rail, anode to the switch node.

Sanity check: while the load is on, the diode must be reverse-biased and carrying nothing. If it conducts when the load is energised, it is backwards and you have built a short circuit.

Reading a coil datasheet

  • Coil resistance → steady current = V / R.
  • Inductance is often two numbers — armature on and off. For turn-off analysis use armature on (the larger one). The relay is pulled in at the moment you switch it off.
  • Many relays publish no inductance at all. Omron's G5V series does.
  • Values are typically ±10% at 23 °C. Copper resistance rises with temperature, so a hot coil draws less current.

Worked example — Omron G5V-2, 12 V

VS12 V
Rcoil288 Ω
I41.7 mA
L (armature on)0.74 H
τ = L/R2.57 ms
E = ½LI²640 µJ
Decay, 0.7 V clamp7.4 ms
Decay, 60 V clamp0.57 ms

Order-of-magnitude checks

  • Node rise time to clamp ≈ Cstray × Vclamp / I. Tens of nanoseconds — too fast for a multimeter to ever show you.
  • Unclamped “pressure” if current died in 1 µs: L × I / 1µs. For the G5V-2 that is ~31 kV. It never gets there — the number tells you how hard the coil is pushing, not what you'd measure.
  • A signal relay stores hundreds of µJ; a big solenoid or motor stores hundreds of mJ. The arithmetic is identical, the consequences are not.

Failure signatures

  • Transistor shorted after N cycles → unclamped, or clamp too slow to catch the edge.
  • MCU resets when the load switches → the transient is coupling into the supply or a signal line.
  • Relay drops out sluggishly → clamp voltage too low. This is the plain diode's cost, not a fault.

Sources