On Thu, 25 Nov 2004, John Ferrell wrote:

> I believe you are saying an electrolytic capacitor could be used as a 
> rectifier. Is this the reason I occasionally see a pair of them in 
> series with - to - in audio schematics? It always leaves me with some 
> doubt.

You can use an electrolytic as a rectifier ;-) But only for a short time. 
Once upon a time (for a short time) there were electrolytic rectifiers. 
Very inefficient.

The antiseries connection uses this fact. Reverse leakage is higher than 
forward leakage so by putting two electrolytics in antiseries (minus to 
minus or plus to plus) you get one with half the capacitance but 
unpolarised, and of the same voltage rating as each of them. The caps must 
be identical (same value, same batch if possible).

The higher reverse leakage (by a factor of 10 or more) causes the voltage 
drop to appear on the cap that is polarised 'right' for the polarity 
applied at the time. When the polarity changes the caps change roles. Each 
cap is either a cap or a 'resistor' with a cap in parallel. ESR is more 
than double that of each capacitor and using these for pulse duty is a bad 
idea. Evil tongues hold that distortion is also higher for this circuit 
and they may be right, as the polarity reversal causes something akin to 
what happens in a rectifier.

Peter


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