Cap reformation question

Scott Stevens chenmel at
Tue Dec 6 18:03:18 CST 2005

> > -----Original Message-----
 >     I've got a system with a couple large electrolytics that 
> > hasn't been 
> > powered up in a number of years.  I have a 5A Variac.  What's
> > the  typical procedure for the reformation process?  Remove
> > all cards (or  maybe leave a junk card in for load?), start
> > the Variac at  0V, increment 
> > by 10V every so often?  How long per step, and/or is a
> > different step  suggested?
> > 
> >     I've seen lots of people say "you'll need to reform the
> >     caps, 
> > first", but never a prodecure for it.
> > 
> >     --jc
> >
On Tue, 6 Dec 2005 08:35:54 -0800
"Robert Armstrong" <bob at> wrote:
> JC,
>   AFAIK the issue with reforming old electrolytics is that the
>   caps will
> have very high leakage currents until they reform.  The goal is
> to limit the power dissipation of the cap to something low
> enough that it won't vent (or explode!) until it's recovered.
>   Lots of people use Variacs for this purpose and they'll
>   certainly do the
> job, but they don't give you any way to measure the leakage
> current.  I prefer the simple technique of putting a light bulb
> in series with the AC side of the supply - when it's bright, the
> current is high and as it gradually dims and goes out, the
> current is low.  
> > maybe leave a junk card in for load?)
>   If it's a linear supply then it's probably happy without a
>   load, and
> there's no sense in wasting even a junk board.
>   If it's a switching supply then it depends on the design, but
>   most likely
> it will require some kind of load on at least some of the
> outputs to maintain regulation. 
> Bob

I would recommend using a high quality Constant Current/Constant
Voltage bench supply.  These have a 'current' and a 'voltage' knob
on them and two meters (someetimes one switchable meter).  The
procedure I use is to set the voltage to the level that you want
the capacitor to reach while disconnected.  Then turn the current
limiting way down and connect the capacitor.  Monitor the current
while turning it up slowly and leave it at some comfortable low
level so the cap can form up.  This process limits the current
while the cap forms up.  You can watch the voltage meter, which
should climb up to what you had it set at, if the cap is any good.

Good CC/CV bench supplies aren't cheap when new, but they're an
essential part of any good test bench, and can be located for a
reasonable cost.

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