[ARC5] Reforming Electrolytics attached as PDF

Mon Mar 19 15:44:11 EDT 2018

 There may or may not be some misconceptions floating around.  Perhaps I
can help clarify.

Unlike the other caps with which we familiar, an electrolytic capacitor
*must* "leak."  It's part of the chemistry.  A more instructive term for it
would be equilibrium current because the word leakage carries a negative
connotation.  It's the current that flows due to an ongoing slow re-growth
of fresh oxide to replace existing oxide that has "decomposed."   If zero
leakage is measured it's not a great cap, it's a dry cap.  If too much
(after proper attempts at slowly reforming) then the oxide is simply too
far gone.

Excessive leakage raises the internal temp of the cap.  This accelerates
failure. It doesn't matter if the current is passing through poor oxide or
around the edges of the rolled foil or decomposition of the paper
separator.

Extremely low leakage implies the ESR has probably gone up because the
cathode is actually an ion solution that has now dried out.  This
accelerates failure if the cap is carrying considerable ripple current
because the net affect is also a rise in internal temp. It also means we
aren't getting the filtering expected of the cap in the circuit.  Of course
the best way to determine this is by direct measurement.  The ubiquitous
Power Factor knob on cap checkers is usually sufficient for ballpark
determinations of effectivness in our vintage circuits.

Finally, unless the cap is far gone, proper reforming will indeed reduce
the leakage and thicken up the oxide layer.  The degree of improvement
ranges from slight (toss it) to "it's a keeper."  Then there's a big middle
ground where we use our judgement.

Dennis AE6C

On Mon, Mar 19, 2018 at 11:46 AM, Richard Knoppow <1oldlens1 at ix.netcom.com>
wrote:

>    It should be understood that there is both series and parallel
> resistance in a capacitor. ESR may be less important than parallel
> resistance, which is also called leakage.
>    All electrolytic capacitors have leakage and it is the leakage that
> ruins them when it becomes too low. ESR, also called Dissipation Factor, is
> a measure of the Q of a capacitor while parallel resistance or leakage
> measures the amount of DC is passes.
>    Ageing without voltage may reduce the thickness of the dielectric
> layer. This will lower the effective voltage rating of the capacitor and
> may raise its capacitance, but may not affect its leakage. A high leakage
> capacitor of any kind may measure OK for both value and ESR so both kinds
> of measurement are necessary. The leakage is measured by testing for
> current through the cap. An ideal capacitor should not pass any DC current.
> Practical electrolytics always have some. The old capacitor checkers of the
> sort with a magic eye indicator were leakage testers. They usually have a
> variable DC source for electrolytics and a means, often the eye tube, for
> indicating leakage current.
>     I don't think reforming will help leakage.
>     A typical impedance bridge does NOT measure leakage although it may
> give a value for parallel resistance. These two are confusing because they
> use the same name for different things. An impedance bridge may indicate
> either series or parallel resistance depending on its configuration and
> range but, unless it of the kind that has a DC source for the capactor it
> will not indicate leakage resistance.
>     Leakage may not be apparent with a low test voltage because it may be
> due to break down of the dielectric at some voltage. However, it may show
> up even with a conventional ohm meter when it reaches high value. I have
> bad electrolytics that show bad on a Hewlett-Packard 410-B VTVM ohm range
> and also show bad on a General Radio Megohm meter and also on an old Eico
> capacitor tester. They are just bad.
>     Storage in a capacitor is another way of measuring leakage. ESR will
> not affect the rate at which a charge is lost, that is due to parallel
> resistance. It is partly a property of the electrolytic and partly of the
> insulation of the capacitor body. Good non-electrolytic caps can hold a
> charge for years.
>     There are other properties of capacitors such as dielectric absorption
> AKA voltage recovery, which can be critical in some applications and
> unimportant in others. This is the effect of a capacitor not fully
> discharging when shorted. After the terminals are opened again some
> voltage, often considerable appears across them. This can be very important
> in capacitors intended for timing and low frequency applications. Mica caps
> have a lot of absorption, paper is low in absorption and plastic has very
> little. It is of no importance in the usual applications for electrolytic
> caps.
>
>
>
> On 3/19/2018 10:43 AM, Kenneth G. Gordon wrote:
>
>> On 19 Mar 2018 at 10:06, Bob kb8tq wrote:
>>
>>      His method is to reform the electrolytic VERY SLOWLY, with a B+
>>> source limited to
>>>      FIVE MA., while monitoring the Cap voltage.
>>>
>>
>> This is EXACTLY what that military document I posted says: 5 mA, no more,
>> over a long
>> period of time.
>>
>> Some of us use 1 mA, which may actually be too low. Testing is required.
>>
>> I also have and use an ESR meter.
>>
>> Ken W7EKB
>>
>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
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