# Capacitor fails to trigger "inductive kick"?

Discussion in 'General Electronics Chat' started by Pleasedonthitme, Feb 9, 2016.

1. ### Pleasedonthitme Thread Starter New Member

Aug 29, 2013
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I've been experimenting with inductive kickback by connecting an inductor to a battery and a multimeter in series - when disconnecting/connection it generates a pulse of at least 20v. However, when I charge up several capacitors (in parallel) and connect them in series (to multiply the voltage) (also in series with inductor and multimeter), there is no pulse and no spark - despite the fact the same amount of current and several times the voltage of the battery should be being discharged.

I'm aware that the "kickback" voltage is proportional to the rate of change of current - however, why is there no spark when I connect the capacitor bank and the current shoots up?

2. ### GopherT AAC Fanatic!

Nov 23, 2012
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I am about to hit you if you don't add a schematic. Tough to help you make changes if you don't speak the language of electronics (or don't try).

3. ### AnalogKid Distinguished Member

Aug 1, 2013
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No inductor value, no capacitor values, no battery value - you don't expect much, do you?

The pulse you are trying to measure is way more than 20 V, probably closer to kilovolts. Why?

An inductive kick happens when the inductor is disconnected from the current source. It is "charged up" by the current through it very similarly to the way a capacitor is charged up by a voltage source across it. When a capacitor's terminal voltage reaches the same potential as it s charging source, charging current drops to zero A. But when an inductor is fully charged up, the current through it increases to the circuit's maximum possible value, set by Ohm's Law. Why?

In your case the inductor is shorting out the capacitors. There is no current flowing in it when the connection is broken because the capacitors have been discharged by the DC resistance of the inductor. If you know the value of the inductor and the capacitors you can calculate how long the two should be connected together for maximum spark. Hint: it probably is a few microseconds. Why?

ak

4. ### Pleasedonthitme Thread Starter New Member

Aug 29, 2013
17
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How could I use capacitors to increase the inductive kick voltage?

5. ### crutschow Expert

Mar 14, 2008
12,537
3,071
You can't.
What is the purpose of this circuit.

6. ### Pleasedonthitme Thread Starter New Member

Aug 29, 2013
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Could I not use capacitors to store up a large amount of charge, and produce a larger current?

7. ### ErnieM AAC Fanatic!

Apr 24, 2011
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No.

At best you can capture most of the energy stored in the inductor when it is turned off. I suggest you do some searches for "boost converter," which is a power supply technique that uses a scheme similar to what you describe.

8. ### MaxHeadRoom Expert

Jul 18, 2013
10,249
2,262
Done all the time with projection stud welders, a large capacitor bank is charged and then dumped in order to weld a stud, bolt etc which has small projections around the edge, it has to be done under pressure of course firing a large SCR .
But I gather you are trying to dump the charge into an inductor?
Max.

9. ### Pleasedonthitme Thread Starter New Member

Aug 29, 2013
17
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Yes, I was hoping this would induce a larger "kickback" from hte inductor - would it be possible to use a bank of capacitors for that purpose?

10. ### Roderick Young Member

Feb 22, 2015
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168
What produces the kick is not suddenly dumping a charge onto the inductor, it's actually having a large current flowing in the inductor, then suddenly opening the circuit. There is an equation for the voltage vs. the current vs. time in an inductor, and you could look that up if you know calculus. But for our purposes, suffice it so say that an inductor "wants" to maintain a constant current through itself. So if a current is flowing, when the circuit opens, the inductor voltage shoots very high in an attempt to maintain current. This is the inductive kick.

In the real world, inductors and batteries have some resistance to them, so an unlimited current cannot flow in an inductor. To get a high current, you want to have an inductor with low resistance, and a battery with low resistance, too. But don't go running out trying to connect a coil with a fraction of an ohm resistance to your car battery. There are all kinds of danger associated with high current. The wire can overheat given the power involved, possibly white hot in an instant. Not to mention damage to the battery.

If your battery can maintain most of its voltage while the inductor is connected, that's the most current you can hope for, and it's limited by the inductor resistance. In that case, for more current, your only choice would be to use a higher voltage battery, or several of your existing batteries in series.

The problem with a capacitor is that the energy storage in it is very small. You could connect a charged capacitor of, say 1000 uF across your coil, but as current starts to flow, the capacitor voltage goes down quickly. The power is dissipated in real-life resistances, and there won't be much of a spark at all, if any. You could use a supercapacitor bank in place of the battery, and achieve high currents that way, if your inductor resistance is low. There is enough energy storage in a supercap so that the voltage won't drop to zero so quickly. However, you might hurt the supercaps if too high a current flows. The 50F Boostcap has a maximum current rating of 27 amps. It will put out far more, but the manufacturer won't be responsible for what happens if you violate their spec.

Is it that you're trying to generate high voltage? What you may want is to generate moderate current, and an inductive kick ("flyback") in the primary of a transformer, then take voltage off the secondary of that transformer, a secondary with many more turns than the primary. Or you could get one of those \$5 electric flyswatter paddles from Harbor Freight, and just harvest the zapping circuit from there.

11. ### Pleasedonthitme Thread Starter New Member

Aug 29, 2013
17
0
Thanks RY (and MH also) that's really helpful!