I cant seem to gasp why an inductive circuit with RC as a snubber element still produces voltage spikes even when msot online materials suggests that it should be able to solve the issues. The below circuit is used to simulate the spike in LTspice, the inductor is assumed to be perfect with 0 series resistance, C1 is also perfect.

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and this is the waveform it produces at v2:

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One of the possible conclusions I came up with is that when switch S1 opens the current cannot instantly change and therefore although R3 is small there must still be some voltage across it when there is current passing it.

However when i add some resistance in series with the inductor suddenly these spikes disappear:

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the voltage change is expected as the circuit isn't compensated.

The biggest change in the circuit is pretty much where V2 is measured. when measured after R4 (ie in series with inductor) the spikes suddenly disspear. But why is this? Why cant I increase R3 instead of R4 to reduce the inductive kickback.

A side question is, are there any designs where i can completely smooth the voltage kickback without putting in R4? Thanks


1 Answer 1


No, there's nothing you can do within reason other than add a voltage regulator or make R3 extremely small and maybe C1 even bigger. The kickback represents stored energy in the inductor and it has to drive that energy somewhere when the switch opens. In driving that energy somewhere, it inevitably produces a voltage greater than the quiescent voltage.

When you added the series resistor, the spike is still there but woven into the waveform and you have a mixture of regular switching and back emf that when combined are hard to distinguish from each other.


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