[sdiy] Polarized VS non-polarized electrolytic caps

[ASSI]

On Sunday 22 January 2012, 18:18:43, Robin Whittle wrote:
> 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.

Oxidation also consumes metal, but yes, the outer hole diameter gets about 
as much wider than the inner hole diameter gets narrower, so the initial 
hole diameter has to fall into the right range.  If you look at Fig.5 again, 
you'll see that they carefully included that effect.  However you can 
electrochemically reduce the aluminum oxide, so judicious cycling between 
reducing and oxidizing conditions will produce the desired "tunnels" almost 
by itself.  You can do the same thing with silicon to get porous structures, 
btw.

> 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.  

Aluminum is very ductile...

> The narrow
> interior of these tunnels and their length would surely cause
> considerable resistance in the electrolyte which fills them.

That's one reason why the ESR of an Al electrolytic capacitor is large.  
But, even though each individual tunnel has high resistance, these are all 
connected in parallel.  If you now figure that there should be an optimum 
between diameter and density of the tunnels, you'd be right.  Capacitor 
manufacturerers already spent man-centuries trying to get there.  :-)

> I had assumed that the cathode was in direct electrical contact with the
> electrolyte,
[...]

Electrochemistry tells you that this won't work unless you use an inert 
electrode (a noble metal, mercury or carbon).  Even then you get an 
electrical double layer, which is incidentally what an ultracapacitor uses 
to obtain its high capacity per volume.

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

You can do that, but you have to bias the connection between them or live 
with the consequences of not doing that.  The intermediate node will self-
bias through leakage (*), but in some cases the formation of the anode will 
deteriorate.  Depending on whether you connected the anode or cathode, you 
have to bias this connection to the highest positive or negative voltage in 
your system.  If you can do that easily, the lifetime of the capacitors is 
greatly increased (**), so there's one reason not to select NP/BP capacitors 
if all other things are equal.

(*) In a normal electrolytic capacitor, the cathode-electrolyte interface is 
very leaky and the cathode capacitance very high, so the electrolyte 
potential is almost the same as the cathode potential.

(**) You can of course increase the lifetime by just using a larger 
capacitor, rated at higher voltage and ripple current.  But that single 
capacitor probably uses up more space.  A Single NP/BP capacitor has less 
series inductance, however.

> 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.

If you read the article again, you'll see that the cathode is usually just 
the same thing as the anode, only that the forming process has not been 
done.  If you do forming on both the anode and cathode metal, the result is 
a non-polar capacitor with half the capacitance (that's described in the 
article as using "two anodes").  The caveats about some bias conditions 
being detrimental to the electrode forming apply, so sometimes you find a 
third electrode that allows you to apply bias to the electrolyte (the case 
generally is at electrolyte potential, so for instance for motor start 
applications it has to be isolated).

Generally non-polar should refer to a capacitor with two anode foils and bi-
polar for the arrangement of two separate capacitors IMHO, but there doesn't 
seem to be any consensus on that and these terms are often used 
interchangeably.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Factory and User Sound Singles for Waldorf Blofeld:
http://Synth.Stromeko.net/Downloads.html#WaldorfSounds