damages when charging capacitor?

[CoolGod]

I have a couple of 100v 10000 uf capacitors used for a coilgun. I need a 100v powersupply charger but i can't get the voltage to be at 100, it's usually about 105ish, if i use 105v to charge the capacitor but stop charging at 98 ish does it damage the capacitor on the inside? Sorry I am new to these things, thank you.

[mariushm]

Re: damages when charging capacitor?

Well, technically it's not a good idea to even get close to 100v as that's the maximum rating. You don't know the tolerance, maybe the caps can withstand 110v or more, maybe they'll fail prematurely if you keep charging them over 95v...
Maybe you're lucky and the manufacturer made the same caps for 120-150v rating and re-labeled them to 100v because there were more orders for the 100v one.

Anyway, if you really want to see less than 100v from the charger, I guess one idea would be to use a few diodes in series. As each diode drops about 0.6-0.8v, you can insert a few diodes to get the voltage below 100.

[CoolGod]

Re: damages when charging capacitor?

i was thinking maybe a voltage divider to change the voltage? I need my coilgun to be consistent thus i need the voltage in the capacitor to be the same each time.
I'm currently using a transformer that is suppose to drop ac main 118-120v ish to 108-110v. That is connected to a bridge rectifier which converts it to ac. I need my coilgun to be precise and charging to be not too slow. I'm thinking if i use diodes and ac main voltage flucates my coilgun will be affected. I have 3 35w rheostat, do you think it's possible for me to hook that up and fine tune the voltage to be 95-100v ish?
I'm worried about the rheostat heating up, do i need a load or limit resistance on both circuits coming out of the rheostat? I've actually melted one b4, it was glowing bright red.
Another general question about capacitors, i see my capacitors after charging to 100v starts self discharge itself pretty quick usually down to 84v ish in under a minute, then it drops very slowly. Is this a sign of old capacitors? Should I reform them to fix this issue?

[999999999]

Re: damages when charging capacitor?

A voltage divider would be very lossy, does not seem a good option.

Using diodes makes it fluctuate no less than using a resistive divider, it's still a ratio you're dealing with and a ratio that changes because you have no regulation.

You could use a rheostat to adjust voltage but again it will fluctuate based on input voltage, you'd have to adjust it every time and odds are, the adjustment would be too fine an increment to do easily by hand unless it's a several-turns rheostat which most aren't in this wattage range (AFAIK).

Yes you could resistively limit the rheostat current, but it doesn't solve your problem to use the rheostat(s). If your caps are old then yes reforming would be a good idea. Once they are in a condition as good as factory new (or close enough), a 100V cap should have enough margin built in to tolerate 105V - but it is not guaranteed.

What you really need is a regulation subcircuit set to a value slightly under 100V. Put some resistors in series for current limiting as it'd blow a (cost effective) few transistors the regulator IC circuit uses otherwise. Though many of the common regulator ICs don't handle 100V, you can use one that has a floating ground so the voltage rating of the IC is merely the rating for the difference it can handle between input and output voltage. For example LM317 IC could take 105V input and regulate it to 98V, but use it in conjunction with one or more transistors as you'd see in a typical boosted current circuit for LM317 since charging the caps would be FAR beyond the current LM317 can handle alone, even with resistive current limiting it would take a while to charge the caps with only the LM317 handling the current.

[Jack Crow]

Re: damages when charging capacitor?

CG,
Just a thought.

When using caps for such applications they may not last long or at all.
Depends on the designe.

When your coil gun fires, there is a huge demand for current, and the plates may warp or distort, destroying the cap.

Heard of this happening in broadcast transmitters when someone trips the interlock while the high voltage is on.
The plates in the filter caps vaporize.

There are 'tesla' coil clubs around here that have a good feel for what parts work and what won't.

At my last real job we were playing with a Marx bank. Eight 30pF caps charged up to 30kV each, then discharged in series into a high preformance spark gap. This was an electronic weapons application.

So please show a care.

Jack Crow

[Evil Lurker]

Re: damages when charging capacitor?

Easiest way to do it would be with a 555 timer or mc34063... preferably the latter. Even though the mc34063 is only designed to handle 40 volts max, if you set it up in step up configuration modified to drive an external NPN mosfet it is possible to hit a regulated output of over 200 volts. The trick to pulling it off is to use a PNP transistor, like say maybe a 2907A (commonly found on atx PSUs primary side) a resistor and perhaps a 1n4148 diode on the FET's gate to actively pull it down during Toff. I also would recommend using a small uf value cap and series resistor on the output going to the capacitor bank so that you could use a smaller amperage rated inductor and power supply... perhaps using a zener or avalanche diode for overvoltage protection.

Look up nixie tube drivers and battery desulfators for ideas on how to design the circuit.

[budm]

Re: damages when charging capacitor?

You should use proper type of cap for high discharge current application. Google
"Low Inductance High Current Pulse Discharge Capacitor"