Reforming capacitors (technical description, not politics)

David Gesswein djg at
Sat Aug 1 17:19:33 CDT 2015

On Fri, Jul 31, 2015 at 06:43:44PM +0100, Robert Jarratt wrote:
> From the slides you posted it looks like you have some kind of automated set
> up. Can you explain what you have there?
The only automation was the data collection. For the reforming I set the
lab supply to the current limit I think is safe and set the voltage to
the final reform voltage and just let it go. If your reforming several 
capacitors at once if you put a resistor in series with each cap you can use 
to the measure the leakage.  If you use a 100 ohm resistor you get 100 mV = 
1 mA which makes it easy to convert the meter reading.

Stuff I found wasn't consistent for reforming current. I though
5 mA was safe for a capacitor so used that for the entire bank. The
most strict reference said to use the rated maximum leakage current for
reforming. If one capacitor is worse it may take most of the current. The 
supply I had wasn't entirely stable regulating at that low of a current so 
you can see the steps where I readjusted it when it started to creep up.
I used the maximum continuous voltage on the cap label as my reforming voltage.

I just do the reforming run until the current stops dropping. You can see
that in the graph. I then verify that the leakage on each capacitor is 
reasonable. For coke can capacitors I want them each to be under 1 mA. I 
had forgotten to put the series resistors but since my entire bank was .5 mA 
I didn't worry about not measuring each one though the variance would 
have been interesting. My capacitance meter didn't measure that high and my
ESR meter didn't measure that low so I made the measurements using the other
methods. The slides seem to have gotten a little confusing when I split
some into backup. The capacitance was measure by setting the supply to
a reasonably high current limit and timing how long it takes to charge.

The automation was logging the data. I had a digital scope with USB to
a laptop that I used to measure the voltage on the bank and the voltage
across the series resistor.

> The limited reforming I have done
> has been done manually. I also notice that you reform over quite a few
> hours, but those are big caps. Not sure I have seen any suggestions over how
> long to reform for, and whether it is a function of the capacitor's nominal
> capacitance.
As others have said the capacitor and storage conditions are the driver.
If your not in a hurry just let it go until its not changing much.

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