Decreasing the Leakage Current of Idle Electrolytic Capacitors

[Kerim]

More than two decades ago, I bought many hundreds of 470u/50V capacitor then I forgot about them (the project for which I bought them was dropped). Lately, I needed them, but I found out that their leakage became rather high (low Rpar).

Usually, I refresh such capacitors manually. But since their number is rather big, I built the basic circuit below.

Here, the voltage applied on the capacitor under test (Ctst) is close to 50V. When its leakage current (represented by Rpar) becomes relatively low, the LED turns on, almost continuously. R5 limits the discharging current. The resistance of R9 determines the lowest leakage current that turns on the LED (the higher the resistance, the lower the leakage current to turn on the LED). There are three DC supplies which are connected in series. The first/bottom one (here 15V) supplies the opamp (LM358 or LM324 could be used). The third/top one (here 10V) supplies the timer NE555. The second/mid one (here 25V) is added to let the voltage on the capacitor under test be close to the voltage of interest (here 50V).

Sorry, since my private business is somehow paralyzed lately due to many factors, I try passing my free time by sharing here some of my works while I understand that no one around here may have a good reason to be interested in any of them.

Cheers,
Kerim

 

[rjenkinsgb]

I don't quite see the point of the complex circuit?
Charge/discharge is not needed, just time at an appropriate voltage.

All you need is a resistor to feed each cap from a suitable voltage source and leave them for a day or two for the electrolytic layer to reform. You could do them in large batches.

In fact with 50V caps I doubt they could flashover so even the resistors are probably not required, unless you were using them for monitoring?

(Large high voltage ones such as used in massive industrial servo drives need a stepped voltage over a few hours, rather than applying 600 - 700V instantly - but not small low voltage ones).

 

[Kerim]

All you need is a resistor to feed each cap from a suitable voltage source and leave them for a day or two for the electrolytic layer to reform. You could do them in large batches.

I didn't hear (or think) of this method yet to refresh old capacitors. But I think it works since you say it.

Unfortunately, on my side, I can't apply it. Since a decade ago, my personal workplace had to be just a small desk (small as the actual apartment in which I had to live ). On this desk, I try to test new designs and the new boards (assembled by my assistants), besides repairing or updating a board which could be received at an unexpected time from one of the various products I sold (many thousands).

I assembled the circuit above on a breadboard. So, when I was on the computer desk, I was able placing it on it too and monitor, once a while, the end of refresh which may take 30 to 60 sec (unless the capacitor is defective). I admit it is rather a primitive way but it is also better than refreshing the capacitors manually due to the limited space.

 

[rjenkinsgb]

This is an example for high voltage caps in industrial gear:

SIOS

support.industry.siemens.com

The stepped voltage is only needed if full DC voltage is sufficient to risk excess heating or insulation breakdown in the caps.

 

[Tony Stewart]

Alum. e-caps are etched Aluminum foil electrode with paste electrolyte caps that can be quite physically large bigger than hand and store high energy with very low ESR. This poses a safety risk if they short out on a power supply just as reversing the voltage supply might on good caps.

my understanding since the ‘60’s was if the rated leakage R is computed Vmax/ I leak, this is a good value to use to reform idle ecaps such as ones we used then for 100W/ch audio supplies with 100k uF 63V caps.

As leakage current reduces in reforming, the voltage across the E-cap naturally rises which you can easily measure.
my understanding since the 60’s was…

This flaw is much like the dendritic short circuits that form in old lead acid batteries and under-charged Lithium Ion cells when left alone for many hours. **Thus reforming is the safe practice of burning off the partial short-circuit conducting solids in a liquid paste.**. In solid e-caps it was from crystals grown from contaminants that might cause excess leakage but less likely.

here is a more recent technical explanation. TL;DR I could be wrong

https://www.nmr.mgh.harvard.edu/~reese/electrolytics/tec1.pdf

Rap on Replacing Electrolytic Capacitors

 

[unclejed613]

i once worked at a TV shop that had a bench set aside for reforming caps. it had several power supplies, and a selection of resistors appropriate for whatever caps were being reformed. one of the "tricks" my boss used, was if he needed (for instance) a 10uf/100V cap, he would take a 25uf/50V cap, and reform it for 100V (12.5uf is close enough). as the oxide layer thickens, the capacitance drops.