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4hv.org :: Forums :: General Science and Electronics
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popping noise on capacitor bank

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rp181
Mon Dec 08 2008, 02:16AM Print
rp181 Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
On my Capacitor bank, I keep hearing a popping noise coming from the bank. I saw small sparks jumping to the acrylic from the capacitor, and sparks going to the SOT package diode.This Is odd for two reasons:
1) Its only 330v.
2) It only occurs when i turn on the bleed switch. I usually turn this on after discharging into a wire, magnesium, etc,so the bank voltage is much less than 330v.

A picture of the bank:
Link2

That is a old picture without the resistors. The resistors are just 2 1kohm 300W wirewround resistors in parallel.
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El_Roberto
Mon Dec 08 2008, 04:35AM
El_Roberto Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Posts: 135
Dunno what it is, could be that too much current has damaged them? Do they still hold their charge?
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Proud Mary
Mon Dec 08 2008, 09:15AM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
One or more of your electrolytics is probably failing - a dielectric puncture leading to local arcing perhaps. The popping may be gas from an arc being vented.

You'll just to have demount them all - charge them up individually to a known voltage - and see how well each holds its charge over a few minutes. That's a simple way of detecting a dud or duds if you don't have too much equipment.

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rp181
Mon Dec 08 2008, 12:41PM
rp181 Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529
Ok, ile do that today. Will the voltage drop be dramatic, like more than 50v?

EDIT: If it way, why would it be only when discharging.
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Proud Mary
Mon Dec 08 2008, 01:00PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I'd expect it to be quite obvious, yes.

Try them, say, after 1 min, 2 min, 3 min, and 10 min and see how any that are down compare with the best.
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Proud Mary
Mon Dec 08 2008, 01:14PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
El_Roberto wrote ...

Dunno what it is, could be that too much current has damaged them? Do they still hold their charge?

In pulse capacitor operation, it's quite common for there to be a big reverse voltage swing - the negative-going half of a rough sine-wave going downward from the trailing edge of the pulse. This will, of course, wreck polarized electrolytics, which is one of the reasons why they are not usually thought suitable for pulse operation.
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rp181
Mon Dec 08 2008, 01:26PM
rp181 Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529
two capacitor's are in a voltage doubler configuration. That diode protects somewhat from reverse spikes.

Why would this only happen when i am bleeding the bank through resistors?
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Proud Mary
Mon Dec 08 2008, 01:29PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Look to see if there any duds, or excessively leaky ones first, then work out why afterwards.
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KLH
Mon Dec 08 2008, 08:42PM
KLH Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
From what I see in the pictures of your capacitor bank, I think you got those from The ElectroStore, and I doubt whether they sell good parts at all. The ElectroStore sells those exact same capacitors (3900uF 400V U32D series), and they say that they were made anywhere from 1999 to 2004. If these capacitors are still usable at all, you should try fixing them by charging them to the rated voltage through a 1K resistor. But from your description of the problem, I think that there is no solution.

I may be wrong about where you got those capacitors, so if you got them from somewhere else, you should just try buying other ones.
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rp181
Mon Dec 08 2008, 09:37PM
rp181 Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I got the capacitors from Alltronics. and many people have said they are good quality (for price).

Is this the type of problem that can be reversed by slowly charging it to repair the dielectric?
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