Electrolytic Capacitor question.

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alibear

alibear

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Joined 2009
Hi, not sure if this should have been placed on the power supply forum, but here goes. I have some Vishay 100,000uf 40v screw terminal capacitors which I purchased about eight years ago.. They have never been used. The question I am asking is do they deteriorate in storage. If so is re-forming required? Or anything else.
Thanks
Alan
 
Elvee

Elvee

Member
Joined 2006
alibear said:
The question I am asking is do they deteriorate in storage. If so is re-forming required? Or anything else.
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Capacitors from reputable manufacturers (Siemens, Rubycon, etc) do not deteriorate.
Others can (litterally!) crumble. I won't give any names, but some US and British manufacturers fall into that category.

For decent caps, reforming will do no harm, so why not, but for myself, I don't bother anymore with it, for two reasons:
1) Experience: I regularly activate NOS caps having code dates of '75 and earlier, without absolutely no precaution whatsoever, and I never encountered a single incident.
2)If you understand the chemistry and physics of these caps, it looks (mostly) unnecessary: reforming plugs the "holes" in the dielectric, and this requires a definite amount of Coulombs, resulting in a definite amount of gas discharge and heat generation (in Joules or calories).
The speed of reforming has very little effect on the total amount of moles to convert (in fact, it is slightly lower when done fast), so there is no major inconvenient in doing it very fast, except of course that beyond a certain threshold, the amount of heat and gas generated could lead to failure.
The oxygen generated serves to repair the dielectric layer, but hydrogen is freed, and eventually leaks out because of its nature, but if too much is generated in a short time, the accumulation could lead to overpressure.

As I said, I never encountered such a failure, BUT: if I first power a vintage equipement, typically a tube one from the forties or fifties, I never do it brutally: first, the caps are far from NOS, and second, they may not be from a decent reliable manufacturer like Siemens.

If you are unsure of your caps, gentle reforming is the solution, but decent caps seem to have ~infinite shelf life.

I will try to dig out counter-examples: crap, crumbling ones.
They are so obvious that the question doesn't even need asking...
 
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