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Old 17th August 2012, 05:48 PM   #1
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Default Elecrolytic capacitor reforming

Hello, everyone!

I have come across a number of old aluminum can capacitors rated between 20-170 uF at 200-500V. Some of them were salvaged from old equipment, some of them are NOS. I have checked them for leakage using a Heathkit IT-11, which does not function very well and is not very reliable. I trashed some of the caps, but I also kept about 20 of them that checked OK. I am now trying to reform them using one of those simple circuits found on the web with a few mods. I added a milliampermeter in series with the capacitor and a voltmeter across it. Therefore, when I charge the capacitors I can measure the current through them at different voltage values. I am getting currents between 40-500 uA, depending of the capacitor value and the test voltage.

Here comes the BIG QUESTION!!!

Could any of you, please, give me an idea what the allowable current through these old reformed capacitors might be? By “allowable current” I mean something like how far can I go before the caps don’t filter well and I get hum in my tube circuits. Of course, I could do it by trial and error, but it would be nice to know ahead of time what I should expect.

Thank you in advance for you input.
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Old 17th August 2012, 06:04 PM   #2
TheGimp is offline TheGimp  United States
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The only spec I can find is for the United Chemi-Con KMH series cap which is rated at :

I = 0.02CV (A) or 3mA, whichever is smaller, after 5 minutes at20C.
Where I = Max. leakage current (A), C = Nominal capacitance (F) and V= Rated voltage (V)
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Old 17th August 2012, 06:21 PM   #3
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TheGimp,

Thank you very, very much. This is exactly what I was looking for. Your input is of great help to me.

Thanks again.
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Old 18th August 2012, 02:07 AM   #4
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Honestly don't even bother trying to reform old caps. They may work fine or they may short out and take some other parts with it. Caps have a finite lifetime so spend the extra $5 and get some new ones.
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Old 18th August 2012, 04:34 AM   #5
Charm is offline Charm  Russian Federation
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How to treat old capacitors?
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Old 18th August 2012, 06:18 AM   #6
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@astouffer
Yes, you are correct as far as the caps are concerned. The cost however, it's a totally different story. I need a whole bunch of multisection 350-475V caps. If I could get them for the extra $5 bucks, I would. I haven't found a place that sells them that cheap.
Thanks for your thoughts.

@Charm

I'm not sure what's the question. If you are asking how to reform old electrolytics, I would suggest a search on theweb. There are procedures described by people much more knowledgeable than me.
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Old 18th August 2012, 06:36 AM   #7
AJT is offline AJT  Philippines
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as long as there are no visible dents, seal craks and can corrosion i would be willing to give them another chance.....CAPACITOR REFORMING Reforming Electrolytic Capacitors
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Old 18th August 2012, 11:37 AM   #8
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Look at the production date, or estimate depending on the product and catalogs. Over 20 years on pro equipment, bad rubber seals. Over 5 years on cheap consumer grade equipment, bad seals. To test, put at 75 deg C for a couple of days and measure if leakage is the same. '
On salvage caps for temporary experiments, I usually "reform" e-caps with an ohmmeter, then switch to 2000 mv scale to see how fast the voltage drops. More than 80 mv per scan (about 3/4 second on my meter) ->trash. On caps over 100 uf. If that is okay and cap is over 20 WV rating, I charge through 47k resistor off a 12 VDC battery charger (about 18v peak)
Best source of 400 V rated caps is a variable frequency motor drive (3 phase) from a food plant dumpster. The SCR's are usually blown up by a water hose at cleanup. The caps are usually fine, and Siemens AllenBradley or TB Woods have extremely high quality ones. (Semi brand has **** ones). The heat sinks and MOS supressors are nice for SS amp projects, too.
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Last edited by indianajo; 18th August 2012 at 11:42 AM.
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Old 18th August 2012, 12:59 PM   #9
DF96 is offline DF96  England
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Deliberately heating an old cap could be an example of destructive testing.

To reform a cap you need a DC voltage about the same as the cap's working voltage, not much higher and not much lower. Connect via a highish value resistor to limit the current, and use either a series current meter or a parallel voltmeter to monitor how things go. It can take many hours to reform an old cap, depending on how long it has been without a charge, so don't give up too quickly and don't try to hurry it too much.

One test I do (with a parallel voltmeter) is to disconnect the voltage source for a few minutes, from time to time. See how quickly the voltage reduces. Reconnect, and see how fast it rises up again. A good cap, being reformed well, will quickly rise to about the voltage it reached before then noticeably slow down.
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