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I am making a coil gun. I have just ordered some 330uF 200v 105c Radial Electrolytic Capacitor.

I will be using the charging circuit from a disposable camera. I know that two of the same caps in series have double the voltage rating but half the capacitance. I haven't received my caps or camera, but my question is if the charging circuit charges at 300v (which I think it does but I will test when I get the camera) and I have 200v rated caps if I make 2 sets of 2 caps in series then put those to sets in parallel can I charge it at 300v? If I understand this right then my total capacitance would be 330uF and my voltage would be at 400v.

Thanks

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  • \$\begingroup\$ There are tons of coil gun descriptions out there that discuss this into detail. Btw. most disposable camera caps I know are rated at 330V but ymmv. \$\endgroup\$ – PlasmaHH Mar 4 '15 at 16:54
  • \$\begingroup\$ Also consider what happens if one capacitor fails as a short - you then have 300V across a 200V capacitor. \$\endgroup\$ – Greg d'Eon Mar 4 '15 at 17:33
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Yes, you can do that.

To be on the safe side of things I suggest an addition:

Put two 50KOhm to 100kOhm resistors in parallel to the capacitors. These resistors make sure that:

  • The voltage level at the junction between the capacitors is close to 1/2 of the total voltage.

With ideal capacitors the junction would be at 1/2 of the total voltage. The world isn't ideal though and you will get capacitors that have tolerances in capacitance and the internal series resistance. When you charge them they will in practice get a voltage that is somewhere else but not exactly at 1/2 of the total voltage.

I suggest you try this out at home using low voltage (12V or so) and two different 100µF capacitors. You may be surprised how far off the voltage after a charge cycle is.

Adding the resistor voltage divider gives the capacitors a voltage reference.

  • The capacitors discharge over time making your device a bit less dangerous.

Capacitors of that high capacitance and voltage can easily kill you if you touch them. They also hold voltage for quite a long time. Worse: Even after discharging, the capacitors may re-charge on their own due to an effect called dielectric absorption.

Having a resistor in parallel to the capacitors keeps that somewhat in check.

Last word of warning repeated:

The charge stored in the capacitors can easily kill people. If you don't yet know what you're dealing with please carefully read safety rules from the DIY tube-amp community. They deal with with that stuff each day.


Edit: Since the OP asked why such a circuit can kill people, even if it's powered just by a 1.5V battery:

Your disposable flash charger is a circuit that transforms the 1.5V up to some much higher voltage at a much lower current. This current is used to charge up the capacitor. Charging takes a while because the charging current is low, but once the capacitor is charged the energy can be let free instantly and currents of multiple amperes are possible.

Now what happens if you put your fingers across the leads of a 330µF capacitor loaded with 300V?

First thing is, that current starts to flow through your skin. The skin resistance is somewhere between 1KOhm and 100KOhm.

Lets say it's a hot summer day and your skin resistance is on the lower side of things. 10KOhm let's say. You'll get a shock, but this will likely not kill you because the current is not high enough yet. 300V at 10KOhm gives 30mA.

However, something else will likely happen: You get burn marks at the point where the current enters your body. And this is critical: Suddenly the high skin resistance is partly gone and your flesh is in direct contact with the voltage. The resistance will drop down to 1000 to 500 Ohm now.

Part of the energy stored in the capacitor will be consumed now and the voltage dropped down a bit. Let's say it's down to 280V now and your body resistance is at 500 Ohm. How much current will flow? 560mA. OUTCH!

There are different sources how much current is required to kill. It also depends on a lot of factors and differs from person to person. A number that I've picked up on the Internet was 300mA for DC currents.

The capacitor will now rapidly discharge through your body and the current will be down in the safer region after half a second or so.

Will that kill you? The answer is: Maybe. You only got one single discharge cycle, not a prolonged exposure to the current. This is good. If the current passes your heart (easy to do: Just make contact with both hands) the chances are quite high that your heart gets out of rhythm. Have bad luck and you'll drop dead. If you touched the capacitor with a single finger the chances are much lower.

That is by the way the reason why you're often advised to put one hand into your pockets if you're poking around in anything with high voltages. It prevents having current flow through the heart.

So best case it will hurt a lot. If worse comes to worse you'll drop dead from from a 1.5V battery.

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    \$\begingroup\$ If you add resistors I recommend putting one across each capacitor. Make sure they're rated for >1/2W to be on the safe side \$\endgroup\$ – DerStrom8 Mar 4 '15 at 17:27
  • \$\begingroup\$ In response to the edit: Keep an eye on the value of the resistors, the wattage is very important. If you have 400v across a 50K resistor then that gives you 8mA, and the power dissipated by the resistor (worst-case) would be 400v*8mA = 3.2 watts. A common 1/2W resistor won't be able to stand that \$\endgroup\$ – DerStrom8 Mar 4 '15 at 17:29
  • \$\begingroup\$ @derstrom8 you're absolutely right with your comments. In my tube-amp I'm dealing with just 50µF at 260V, and I'm running a series resistance of 130kOhm at 5Watt across the main capacitor. \$\endgroup\$ – Nils Pipenbrinck Mar 4 '15 at 17:32
  • \$\begingroup\$ Those'll do the trick! =) \$\endgroup\$ – DerStrom8 Mar 4 '15 at 17:34
  • \$\begingroup\$ Actually, at the resistor values involved, I doubt that they will. Remember, he's going to use the charge circuit from a camera. I very much doubt that those things will provide current over a long duration (>2 or 3 seconds) without overheating and dying. \$\endgroup\$ – WhatRoughBeast Mar 4 '15 at 17:55
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Yes, if you make two series strings of two 200v 330uF capacitors and connect them in parallel then you will have a capacitor bank rated for 400v and a capacitance of 330uF. However, you're not going to get much energy from that. Let's assume you charge up the capacitors with the maximum 400v. Your energy will only be E=1/2*C*V^2 = 1/2*(330*10^(-6))*(400)^2 = 26.4 Joules. You might be able to fire a staple if you're lucky, but I would even doubt that. You're going to want MUCH more capacitance (i.e. more strings of capacitors in parallel).

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  • \$\begingroup\$ well.....¯_(ツ)_/¯ guess I will have to do something else.....it would be ok though cause this is for a school science project and it would make a better experiment trying different caps and such. \$\endgroup\$ – codegeek511 Mar 4 '15 at 22:43
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Charge very slowly.

Consider this - one capacitor is actually not 330uF but 400uF and the other is not actually 330uF but in fact 260uF. Together (if in parallel) they would look like 2x 330uF. But, in series they are 157.6uF.

If you put 300V across the series connection of those two caps one will charge to a significantly higher voltage than the other. Sure, the stabilizing resistors mentioned by others will work to equalize the voltages across each cap but if the charge time is a couple of seconds, I can see a scenario where one cap might reach it's 200V limit whilst the cap with larger capacitance is still at about 100V.

I can't say how quickly the 300V charging circuit might charge these caps but I suspect it might be too quick even if you have balancing resistors. I'd be tempted to modify the circuit to charge to 180V and charge them all in parallel.

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  • \$\begingroup\$ How would you suggest to mod it? I was thinking of finding a transformer that worked would be the best. \$\endgroup\$ – codegeek511 Mar 10 '15 at 19:22
  • \$\begingroup\$ @codegeek511 Sorry dude you've accepted someone else's answer. You should have waited before doing that. Maybe speak to him or reconsider the answer you accepted!! \$\endgroup\$ – Andy aka Mar 11 '15 at 9:12
  • \$\begingroup\$ Didn't know I could only get answers from one person....his was the best one so I chose his I can't chose multiple ones. If you don't want to help that's fine, but if you did you could have just wrote a suggestion of what to do in the comments. I don't really understand your logic, if it would benefit me and possibly other people reading this what's the harm? \$\endgroup\$ – codegeek511 Mar 11 '15 at 18:31
  • \$\begingroup\$ @codegeek511 you shouldn't have accepted so early maybe. It's a demotivator both for further questions and for those who don't bother reading the question because they see it's answered. In simple terms if the caps were resistors in series but not close in value would you see equal voltage across them? \$\endgroup\$ – Andy aka Mar 11 '15 at 18:37

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