Inductive Kickback

Observe the large switch-node voltage created when a switch interrupts an inductor current. A small capacitance makes the voltage finite.

The 12 V supply feeds R1 = 100 Ω and L1 = 100 mH. SW1 starts closed and opens at 5 ms. C1 = 100 nF represents capacitance across the switch. It gives the interrupted inductor current somewhere to flow and makes the predicted voltage finite.

$$I_0\approx\frac{12}{100+0.1}=119.9\,\mathrm{mA}$$ $$E_L=\frac12 LI_0^2\approx0.719\,\mathrm{mJ}$$ $$\Delta V\sim I_0\sqrt{\frac{L}{C}}\approx120\,\mathrm{V}$$

After opening, Vswitch rises far above the supply and the LC energy exchange produces a damped oscillation. The estimate gives the scale of the excursion; winding resistance reduces its actual peak. Icoil remains continuous at turn-off and may reverse as the ideal LC network rings.

Try it. Change C1’s Capacitance to 200 nF. The excursion becomes smaller and the oscillation slower. Restore 100 nF. Compare with Blocking Inductive Kickback to see a flyback diode limit the excursion.

Read the chart. Vswitch is green and Vcontrol red. The blue Icoil trace measures the actual winding branch. The pulse source changes from 5 V to 0 V; the switch’s thresholds are 4 V and 1 V. The view includes the energized state, the 5 ms opening and the decay.

Model limits. Switch off resistance is 100 MΩ. The model omits avalanche, arcing, saturation and device damage. The diode, when present, is a generic junction model; real protection parts need suitable current, energy and reverse-voltage ratings.