[sdiy] Polarized VS non-polarized electrolytic caps

[Robin Whittle]

Hi Harry,

Thanks for this reference, which I found most interesting.

  http://electrochem.cwru.edu/encycl/art-c04-electr-cap.htm

I found it particularly interesting to see the nature of the anode
etching and how it is oxidized to form tunnels.  How this actually
works, I am not sure, since I recall that the aluminium oxide is less
dense per atom of aluminium than the aluminium metal itself, so growing
the oxide in a small but somewhat increasing diameter tunnel would cause
the first-produced oxide to be squeezed into a smaller diameter in the
middle than the metal from which it is formed.  Since this oxide is
alumina - as used in IC packages and grinding stones - I find this hard
to imagine, unless it the remaining metal stretches.  The narrow
interior of these tunnels and their length would surely cause
considerable resistance in the electrolyte which fills them.

I had assumed that the cathode was in direct electrical contact with the
electrolyte, but it seems that they build a very thin oxide layer on
this too, and (implicitly, I think) expect some leakage through this
layer, with no more than a volt across it, in order to provide the
current needed to maintain the much thicker anode oxide which handles
the real voltage difference.  I guess there is no way of making a
flexible metal sheet which does not have such an oxide layer, so they
accept this and make it very thin to get their quoted 200uF / cm^2
cathode capacitance.

Regarding the text you quoted:

> "Non-polar (or bi-polar) devices can be made by using two anodes
> instead of an anode and a cathode, or one could connect the positives
> or negatives of two identical device together, then the other two
> terminals would form a non-polar device."

I interpret this as meaning that the user could get two separate polar
electrolytic capacitors (separate "devices") and wire them together back
to back.

However, you report seeing some non-polar or bi-polar electrolytic
capacitors which were in fact two capacitors in series.  If they
connected the cathodes together, maybe there wouldn't be much trouble
with there being a connection between the two capacitors electrolytes.

  - Robin