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 Post subject: Repurposing an electrolytic cap
PostPosted: Dec Mon 04, 2017 8:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1579
Location: Shelton, WA
What happens to make the cap work again? Does it "grow" new insulation?
Thanks

billn


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 Post subject: Re: Repurposing an electrolytic cap
PostPosted: Dec Mon 04, 2017 9:09 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10972
Location: Mpls, Minnesota
I think you mean "reforming". This process is described in the following link, for restoring new electrolytics that have have been in storage. On old used electrolytics it is better to replace them as you will have about a 95% failure rate trying to reform them.
http://patriotgaming.com/manuals/gaming ... 0-4341.pdf

Dave


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 Post subject: Re: Repurposing an electrolytic cap
PostPosted: Dec Mon 04, 2017 10:18 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7212
Location: Long Island
In an electrolytic cap, the dielectric layer is a microscopically thin aluminum oxide layer on a pure aluminum foil electrode. The dielectric layer is formed by passing controlled amounts of current through the foil in an electrolytic solution. The voltages and temperatures used in forming are much higher than the cap sees in normal operation.

The process is reversible, so the dielectric layer is always subject to deterioration. When the cap is in operation, the natural leakage current slowly hydrolyzes the electrolyte and the oxygen that is released combines with the aluminum to make new aluminum oxide. When the electrolyte is consumed or evaporates away, there is no more replenishment of the dielectric layer and when it degrades, the useful life of the capacitor is over. There is no way to reform or repair an electrolytic when this happens--no matter what anybody tells you!

But if an electrolytic is placed in long term storage, a different situation occurs. The dielectric layer still slowly deteriorates as before, but there is no leakage current so it is not replenished. Now if full voltage is suddenly applied, the leakage current will be far higher than normal--maybe high enough to damage the cap or other components. So the trick is to apply lower voltages to the cap and allow the dielectric layer to recover gradually until it can withstand full voltage again. Reforming is best attempted by connecting the capacitor to a variable voltage power supply in series with a sensitive milliammeter so the progress of the reforming can be monitored. For most caps, the current should be limited to 2-mA maximum. If the power supply does not have current limiting, a series resistor must be used to limit the current in case the cap breaks down.

One "gotcha" that has to be understood is, reforming is only possible if the cap still has good liquid electrolyte in it. If the electrolyte has evaporated due to faulty seals or storage at extremely high temperatures, or it has become contaminated by internal corrosion, that cap will not reform well--if at all.

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 Post subject: Re: Repurposing an electrolytic cap
PostPosted: Dec Mon 04, 2017 10:26 pm 
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Posts: 31420
Location: Maryland 20709, USA
Chris108 wrote:
So the trick is to apply lower voltages to the cap and allow the dielectric layer to recover gradually until it can withstand full voltage again. Reforming is best attempted by connecting the capacitor to a variable voltage power supply in series with a sensitive milliammeter so the progress of the reforming can be monitored. For most caps, the current should be limited to 2-mA maximum.
I designed a special test fixture to do the reforming automatically.

It's a high-voltage filter power supply followed by a constant-current driver stage.
The current is fixed at 1 milliamp. (It may take some time to reform a large cap.)

A voltmeter is connected directly across the cap. You just watch the voltage rise as it reforms.
When that gets up to full voltage, the charging current starts to decrease.

If it drops below 0.5mA, I consider the reform a success.

- Leigh

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http://www.AtwaterKent.info
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