forming caps?

[W. Edwin Lambert, Jr. K1ZOK]

Capacitors are devices that pass alternating current and withstand direct
potential....  up to a specific limit depending on the configuration and
electrolyte.  The chemistry of the capacitor actually shows the
electrolyte to have a very high resistance (lots of megohms) between the
terminals.  The voltage maintains this high resistance.  As a capacitor
sits around unused, without voltage applied, the electrolyte changes in
that it's resistance decreases.  The longer the capacitor is inactive,
the less the resistance.

Now, when a voltage is first placed on a capacitor, it looks like a
resistor until the electrolyte "forms" and turns into a very high value
again.  The amount of current that flows at the instant of voltage
application is dependent on the amount of voltage and the resistance.  In
some cases, the internal resistance is so low that the capacitor heats up
and short circuits.  This causes a drastic increase in current and can
actually cause the capacitor to explode.

When any set using electrolytic capacitors is turned on after a long
period (years) of inactivity, the voltage should be brought up
slowly.....  how slowly is a matter of discussion and you will find
varying opinions all over.  The intent is to prevent a large surge of
leakage current from causing any damage to the unit.  The lower the
applied voltage, the easier it is for the capacitor to change chemically
and increase it's internal resistance....  thereby reducing the current.
The idea is to change the applied voltage in steps (or slow varying) to
make this happen.  The length of time is determined by the time it takes
for the chemistry to happen, and this varies from capacitor to capacitor.
 The optimum solution is to have a milliamp meter in series and limit the
leakage current to under 10 mils or so at any time.  When the current
drops to zero (or a very low value), the voltage can then be increased
again to draw 10 mils.  At some point, the rated voltage is reached and
the capacitor now functions correctly.

I, personally, run in steps of 20%, 40%, 75%, and 100 % leaving the unit
at each step for a minute or two.  Once this is done, the set can be
turned off for a reasonable period of time (months) and not lose it's
capacity.

The capacitor chemistry, aluminum electrolytic, shows this trait, while
the tantalum variety does not (AFAIK)

Ed
K1ZOK

On Tue, 14 Dec 1999 14:09:44 -0500 Ken Ketner <b9oky at TTACS.TTU.EDU>
writes:
> From
> time to time I hear rumors about reforming el caps. Does anyone have
> the
> straight info on this? Does it work? How does it work? Is it a lot
> of
> trouble? If it is, why not just clip in a new one in 60 seconds and
> be
> done?
>       Humbly enquiring, es 73
> --
>

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