Electrolytic Cap's Leakage & Voltage Division

[Blake]

I recently read that the leakage current in an aluminum electrolytic
capacitor will increase with higher applied voltge. This seems reasonable.
In fact, I have a vague memory of having measured leakage current and
finding that it rises sharply as the applied voltage nears the rated
voltage.

But I also remember reading that when connecting electrolytics in series,
there is a danger of unequal voltage division which might overstress one of
the caps.

This seems to present a contradiction. If one capacitor takes more than its
fair share of the voltage, wouldn't that cause an increase in leakage
current and a subsequent reduction of the voltage?

 

[Phil Allison]

"Blake"

I recently read that the leakage current in an aluminum electrolytic
capacitor will increase with higher applied voltge. This seems reasonable.
In fact, I have a vague memory of having measured leakage current and
finding that it rises sharply as the applied voltage nears the rated
voltage.

But I also remember reading that when connecting electrolytics in series,
there is a danger of unequal voltage division which might overstress one
of the caps.

This seems to present a contradiction.

** Course it does.

If one capacitor takes more than its fair share of the voltage, wouldn't
that cause an increase in leakage current and a subsequent reduction of
the voltage?

** The rule for connecting two high voltage electros is series is that they
are both the same ( age, type, ratings, temp) plus the combined voltage is
comfortably over (by say 25% ) the actual max DC voltage expected.

The caps will then sort it out themselves.

Ballast resistors are optional.



.......... Phil

 

[David L. Jones]

I recently read that the leakage current in an aluminum electrolytic
capacitor will increase with higher applied voltge. This seems reasonable.
In fact, I have a vague memory of having measured leakage current and
finding that it rises sharply as the applied voltage nears the rated
voltage.

Yes it does tend to do that.

But I also remember reading that when connecting electrolytics in series,
there is a danger of unequal voltage division which might overstress one of
the caps.

Yes, the danger can be very real.

This seems to present a contradiction. If one capacitor takes more than its
fair share of the voltage, wouldn't that cause an increase in leakage
current and a subsequent reduction of the voltage?

Correct. But the caps have to be fairly well matched and have a certain
minimum voltage spec over the working voltage in order for them to
"self balance". So you might get away with it in many cases, in other
cases you could be asking for trouble in the long term, and in some
cases ballast resistor will be essential. Good practice is to put a
ballast resistor in parallel with each cap unless you have a specific
reason not to. The required resistor value can be calculated by:

R=(CapLeakRes * (MaxCapVolt-(Vrail/2)) / (Vrail-MaxCapVolt)) * 2

Dave