Electronic – Clamping Inductive Kickback in AC Circuits

inductorkickbackmovsafetytransient-suppression

I want to eliminate/clamp the inductive kickback that would result from turning off a large inductor, such as a relay or a transformer.

In a DC circuit, I would use a diode to flywheel the current. But what do I use in a line voltage AC circuit?

Is an MOV an acceptable device to use? Should I connect it parallel with the inductor or parallel with the switch? MOVs fail closed circuit, so this could be a fire hazard. Fire safety is a very important consideration.

Or should I use a TVS or something else?

Please describe how the suppressor device should be connected to the circuit.

Best Answer

You can use a capacitor - the peak current of the coil implies an energy level in that inductor and, when the supply is disconnected that energy flows into a capacitor to charge it up (rather than form an arc). This cycles back and forth between coil and capacitor and eventually dies down as all the stored energy becomes dissipated as heat in the coil's dc resistance.

The peak voltage on the capacitor can be derived from Energy = \$\dfrac{CV^2}{2}\$

The energy in a coil = \$\dfrac{LI^2}{2}\$ for reference.

This is the basis of a snubber except a snubber has extra R in series with the capacitor to dissipate the energy a tad more quickly. Here's a typical snubber for use with a triac and inductive load (see circuit note): -

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