[sdiy] How do old electrolytics go bad?

[ASSI]

On Sonntag, 8. Februar 2009, Roy J. Tellason wrote:
> The actual electrolyte isn't the stuff that's between the plates, 
> which is conductive BTW,  but a thin layer of oxide (?) on one
> plate,  I forget if it's the positive or the negative one.  This is
> applied when the cap is made, and the thickness of the layer
> determines both the capacitance and the working voltage.

The actual electrolyte _is_ the conductive stuff (and it indeed makes 
up the negative plate together with the cathode foil) and the aluminum 
oxide is the dielectric.  The positive plate (or anode foil) is 
usually roughened to provide more surface area.  Modern tantalum 
capacitors usually have a solid electrolyte, BTW.

http://electrochem.cwru.edu/ed/encycl/art-c04-electr-cap.htm
http://www.faradnet.com/deeley/book_toc.htm (from 1938!)

As to reforming caps: any electrolytic that has not been in use for a 
while needs to be reformed (if not outright replaced because its 
electrolyte has dried up).  That's the reason why you should power up 
your vintage gear at least once or twice per year: first very shortly, 
not more than a few minutes; then after a day or so once more, but for 
several hours.  If something's been sitting in an attic for several 
decades you can almost be sure the electrolytics won't survive the 
first power-on without reforming them before.

http://www.vcomp.co.uk/tech_tips/reform_caps/reform_caps.htm



Achim.
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