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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.

mcu
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1 Answers1

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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): -

enter image description here

Andy aka
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  • So, do you connect the capacitor in parallel with the inductor or the switch? – mcu Dec 06 '15 at 15:28
  • You are trying to protect the switch so it's best across the switch but it works in either position. – Andy aka Dec 06 '15 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 06 '15 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. – Andy aka Dec 06 '15 at 15:42
  • A voltage based vs current based formula would be much handier for inductor energy. – mcu Dec 06 '15 at 15:45
  • Do some work on it yourself - it's not difficult but I've done with this question for now - try googling how snubbers work. – Andy aka Dec 06 '15 at 15:47
  • Wouldn't a cap across a switch provide a current path for AC, in effect bypassing the switch? – mcu Dec 06 '15 at 17:00
  • Yes it could and sometimes the snubber is placed directly across the inductive load. – Andy aka Dec 06 '15 at 17:08
  • So, another way to look at this is that we are applying power factor correction, and if we can bring the power factor close to 1.0, there should be minimal kickback. – mcu Dec 06 '15 at 19:08
  • No, not really. If you can say what the current is in the inductor and you know the inductance you can define the energy stored. Then reverse the process by applying that energy into the capacitor to see what the peak voltage is. – Andy aka Dec 06 '15 at 20:29
  • This is a nice clean solution with the added benefit of power factor correction. I modeled this in SPICE and it looks nice. The only drawback I see is the cost and size of an AC capacitor. – mcu Dec 06 '15 at 20:56