[ARC5] NOS Electrolytics

Mon Jun 2 14:28:53 EDT 2014

John and all,

I think your reforming process is very sound.  A resistor and fixed voltage
that keeps the power dissipation within the cap to <500mW (for the large
caps) is sufficient.  Better quality cap testers can be used to reform too.
 There is really no excuse for not doing it because it is easy.

When I reform I like to put a current meter in series (as you do with your
HP supply) to observe what is going on.  As the reforming takes place the
current does not undertake a steady and smooth decline.  It is often
erratic.  Others may have noticed this too.  This is because the process of
regrowing the oxide is not uniform across the roll of layers.  You can even
observe the current increasing after a decline and then declining once
again only a fraction of a minute later.  Old caps tend to be more erratic.
 Once formed this behavior stops, so as long as the residual leakage is
reasonable vs the number of uF you probably now have a good cap.  A check
of the dissipation factor (i.e. ESR) wouldn't hurt but is not really
necessary.

Many of you already know the following but it bears repeating for those
readers who don't.  Leakage in a 'lytic is a normal part of the dielectric
maintenance process.  The cap is losing and regrowing atomic layers
continuously as part of a self-repairing equilibrium state.  This is normal
and healthy as long as the amount is proportional to what one would expect
for the uF's and voltage rating.  In fact, if you don't have some leakage,
then you don't have a functioning electrolytic capacitor.  EIA has
published standards for electrolytic leakage and modern cap datasheets give
ranges (*Electric Radio* published a chart a number of years back but I do
not remember the issue).  For example, a 47uF 400V cap has an allowance of
about 800uA.  1500uF at 100V has an allowance of 2.3mA.    If you're at or
under the allowance, the cap is deemed OK.  Like John found, it seems to be
the rule rather than the exception that old NOS (even only 10 to 20 years
old) benefit from reforming or one risks heat damage from a prolonged
current surge at the hands of the equipment's circuitry.

Dennis AE6C

On Sun, Jun 1, 2014 at 3:08 PM, J. Forster <jfor at quikus.com> wrote:

> The supply I use is regulated w/ adjustable current limit (HP 6209B).
>
> You can use any old adjustable supply and a DMM and current limiting
> resistor.
>
> I set the supply voltage to the rated DCWV of the cap and set the current
> limit to keep the dissipation <0.5 W.
>
> If you're doing the resistor setup, monitor the current and jack up the
> voltage intermittantly.
>
> These caps are about 2" long, and 7/8" OD, axial lead. After a while the
> DO get very slightly warm.
>
> IMO,it's a very good idea to keep the power limited to avoid any
> overheating the guts of the cap.
>
> -John
>
> ==================
>
>
>
>
> > John:
> >
> > What voltage should be used to reform caps?
> > I've got a lot of them with label voltages between 10 and 330 V.
> >
> > I have a table showing measured ESR on a bunch of caps at:
> > http://www.prc68.com/I/Capacitors.shtml#Tbl
> > On the page is also a procedure for measuring super caps
> >
> > I sell a combined ESR & Capacitance meter that's made in Russia.  See:
> > http://www.prc68.com/I/ESRmicro.shtml
> > I've made a number of repairs using this meter where there was no
> > documentation.
> >
> > Have Fun,
> >
> > Brooke Clarke
> > http://www.PRC68.com
> > http://www.end2partygovernment.com/2012Issues.html
> > http://www.prc68.com/I/DietNutrition.html
> >
> > Kenneth G. Gordon wrote:
> >> On 1 Jun 2014 at 14:12, J. Forster wrote:
> >>
> >>> I want them to reform some before judging.
> >> OK.
> >>
> >>>>> I've not measured the ESR or C as yet.
> >>>> Oh. OK. I have at least two ESR meters here and have found them
> >>>> invaluable.
> >>> I've never been a real fan. I can measure it on a GR bridge.
> >> Well, the two I have, a Dick Smith kit, and one I bought off eBay, work
> >> very
> >> well, are quick, and have other uses too.
> >>
> >> I have a couple of bridges, including a really beautiful military one,
> >> but they
> >> take too long to set up for a single use.
> >>
> >> If I were going to use it to test a bunch of caps, I would use it, but
> >> otherwise it
> >> is too much trouble,
> >>
> >> Ken W7EKB
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