Hi Bill,

The electrolyte is kind of like a salt water soaked paper, if you will,
it is very low resistance. Its purpose is twofold: first, it creates
an intimate contact with the aluminum oxide, which is the only
dielectric in the cap, and second, it provides a method of repairing
flaws in the aluminum oxide layer. The electrolyte IS the second plate
of the capacitor.

If the electrolyte wasn't there, (eg. it was paper instead), the
dielectric constant of the paper would dominate and the capacitance
would be similar in magnitude to that of a paper capacitor. It is very
important that there not be any nonconductive material (other than the
aluminum oxide layer, that is) between the plates.

Another way an aluminum electrolytic capacitor could be made would be to
build up the oxide layer on one of the plates, and then coat the oxide
with a liquid metallic layer, such as silver epoxy paint, or "nickel
print" to form the other plate. That should give you the high
capacitance of a wet aluminum electrolytic cap, but the oxide wouldn't
ever degrade.

-Chuck



--exray-- wrote:
Chuck Harris wrote:

Hi Alan,

Because of the physical construction of an electrolytic cap, it MUST
change capacitance if the oxide grows thinner in storage, or thickens
thru reforming...

But, I too notice that sometimes the change is large, and othertimes it
is not.

I suspect that what is happening is the oxide layer thins out only in
spots (probably around impurities) in some caps. These spots are large
enough to readily affect the leakage current, but are small with respect
to the total surface area of the plates. Because they are a small
percentage of the total surface area, they only minimally affect
the total capacitance.

-Chuck Harris


Let me pose a question...not knowing inimately how electrolytics were or
are made.
Seems to me that the 'extra' oxide, ie thicker plates, are taking up
some of the physical space that was formerly the electrolyte (part of
the dielectric, so to speak) thereby leaving the plates closer together.
That would indicate more oxide=more capacitance.
In the case of thin oxide (not holes) I don't see how thick or thin
would relate to leakage as long as there was something there. Running
with the same thought, if the oxide is totaaly absent what makes it
redeposit on the wax paper/mylar?
Maybe the leakage is a result of the metallic compounds being absorbed
by the electrolyte and the reforming process sends them back to the
original metal, albeit somewhat randomly.
Does this make any sense?

-Bill