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Old 14th December 2006, 07:36 PM   #1
AndrewT is offline AndrewT  Scotland
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Question dodgy PSU capacitor

Hi,
I just noticed one of my new smoothing caps looks a bit bulged on the top. I cannot recall what it looked like when I installed it two weeks ago.

I have disconnected it and charged it up through a 100k resistor.
The charge current has fallen to 32uA (3.2V across 100k) and the charge voltage is now 60Vdc on a 63Vdc 15mF capacitor.

Do I scrap it?
or
does the low leakage value prove it is OK?

Is there another test or tests I should do before re-using it?
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Old 14th December 2006, 07:41 PM   #2
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Default Re: dodgy PSU capacitor

Quote:
Originally posted by AndrewT
Hi,
I just noticed one of my new smoothing caps looks a bit bulged on the top. I cannot recall what it looked like when I installed it two weeks ago.

I have disconnected it and charged it up through a 100k resistor.
The charge current has fallen to 32uA (3.2V across 100k) and the charge voltage is now 60Vdc on a 63Vdc 15mF capacitor.

Do I scrap it?
or
does the low leakage value prove it is OK?

Is there another test or tests I should do before re-using it?

Andrew,

I'm not sure about the bulge, but aren't you cutting it a bit close with 60V across a 63V cap? What happens if the line voltage goes up by say 5% temporarily?

Jan Didden
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Old 14th December 2006, 07:46 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi Janneman,
I'm OK, this is just a test to see what leakage I get as it approaches EMF (63.2Vdc).

The working voltage is 49Vdc +-tolerances. That is what it has had periodically during the last two weeks and most of that time either off load or off completely.
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Old 25th June 2011, 04:28 AM   #4
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32uA is suspiciously low leakage current for a 15mF cap at 60V- check their datasheet but 0.5-2mA (after 2 minutes) is what I would expect.
What's odd is it pressured up enough to bulge but didn't pop... possibly heated up during high ripple currents (10kHz w/amp at full load), vented and dried out.
Next test would be capacitance (RC time) or simply charge and short it, see if it has a bit of spark. I use a cap meter with dissipation factor readout to test.
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Old 25th June 2011, 09:13 AM   #5
AndrewT is offline AndrewT  Scotland
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thanks for the feedback.
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Old 25th June 2011, 09:27 AM   #6
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I've got a couple that the plastic jackethas a bulge but you can press the bulge and it behaves like the top of an opened jar. Are you sure the cap has bulged or is it just the plastic jacket ?
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Old 1st July 2011, 04:39 PM   #7
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The bulge is the sign of trouble. Voltage is not the only concern, temperature is the other. The temperature can be measured during operation with thermocouples and a temperature meter. Not having those, a temperature rise can be calculated from the Voltage ripple and the ESR of the cap to get an idea of the power dissipation. From the ripple, the difference between the max and min operating voltage, and the ESR, you can get the RMS power being dissipated in the cap. V^2*R = Watts. Most manufacturers will specify a temperature rise against ripple that can help you too.

I've seen many a cap overheat and blow it's lid. The bulge you see is caused by the formation of oxygen gas inside the cap as a result of the reversal of the process to create the electrolyte.

Solutions to this problem include low ESR caps, a higher value cap, or a higher temperature cap.

Cheers
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Old 1st July 2011, 07:44 PM   #8
DF96 is offline DF96  England
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Maybe it has sat on a distributor's shelf for a few years since manufacture? If so, it might have reformed itself rather too quickly when you first switched on and built up some gas. If leakage and capacitance value seem OK then it should be fine.
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Old 1st July 2011, 11:51 PM   #9
Enzo is offline Enzo  United States
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I think you should answer Andy5112405's post. I often see caps that are perfectly good, except the plastic end cover has domed. The aluminum can under it has not bulged, though it looks like it.

While it is possible the part has failed, it could merely be a reaction to warmth in the amp warping the plastic around the cap.

So if the cap really has bulged, it has to come out and be replaced, so it won't matter if you remove the end cover plastic. If the cap is OK, but the end cover plastic has warped, it won't hurt the cap to be without it, so remove the end piece and find out if the underlying cap has bulged.

And if there is no end cover plastic, please ignore this post.

And is that 15 millifarads or 15 microfarads? "mf" is old school for microfarads, and I have no idea how old you might be.
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Old 2nd July 2011, 09:03 AM   #10
AndrewT is offline AndrewT  Scotland
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I am from the old school that tries to abide by the rules.
When I am corrected, I do try to stick with the new information.
That for me means milli = 10^-3 and micro = 10^-6.

The cap was removed as soon as the "fault" was noticed. Then tested for leakage and that result prompted my question. I may still have it and could retest and strip the casing.
But, this was over 4years ago.
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