Re: Electrolytic caps?

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 02/26/05 

Date: Sat, 26 Feb 2005 14:57:21 -0800

Jim Thompson wrote:

> On Sat, 26 Feb 2005 21:51:38 GMT, Joerg
> <notthisjoergsch@removethispacbell.net> wrote:
>
>
>>Hello Watson,
>>
>>
>>>Yes and yes, but it depends on how old those caps are. Over five years
>>>old, and they're not 'fresh'; ten years and they're 'stale'.
>>>
>>>
>>
>>Nah. I have some 40+ year old radios where the electrolytic caps are
>>just fine. A couple times I actually did a resistor charge and leakage
>>test just to see. They were nearly as good as new.
>>
>>But sometimes when they had been in storage for decades they may have to
>>be "formatted" slowly to get them used to the job of "being a capacitor"
>>again.
>>
>>Regards, Joerg
>>
>>http://www.analogconsultants.com
>
>
> Hi, Joerg. I do, literally, have some 40 year-old aluminum
> electrolytics in my part bins.
>
> What's the best way to "format" them?
>
> ...Jim Thompson

Its "forming" (technically "reforming" since it was already done once at
the factory long long ago). You can do a web search on electrolytic
forming.

Basically you feed it rated voltage through a big current limiting
resistor -- what you're doing is restoring the aluminum oxide layer on
the foil. There are a number of techniques for verifying that the
forming is complete (vs. the cap being bad) -- basically when you think
you're done you test with more than the rated voltage to see if the
thing leaks -- if so, you scrap it.

I've never done it, but it's a common thing with tube-equipped boat anchors. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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