# Clamping Inductive Kickback in AC Circuits

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.

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): -

• So, do you connect the capacitor in parallel with the inductor or the switch?
– mcu
Dec 6, 2015 at 15:28
• You are trying to protect the switch so it's best across the switch but it works in either position. Dec 6, 2015 at 15:30
• Can you provide the formula for peak capacitor voltage as a function of AC voltage, coil inductance and capacitor value? Thanks.
– mcu
Dec 6, 2015 at 15:32
• I've explained that in my answer. Whatever the coil current is when the switch opens implies a stored energy in the coil. The energy transfers to the capacitor - I've given the formulas in my answer. Dec 6, 2015 at 15:42
• A voltage based vs current based formula would be much handier for inductor energy.
– mcu
Dec 6, 2015 at 15:45