[ARC5] Reforming Electrolytics attached as PDF

Mon Mar 19 16:15:28 EDT 2018

Yup.  The reforming process is anything but even.  Make me want to build a
mason jar lytic :-)

When I reform a cap I apply a stepped increase in voltage while keeping an
analog meter in series.  It is fascinating to observe the pinhole craters
spiking up the current to be followed by a step function in higher than
normal steady reforming current that eventually reduces to the previous
equilibrium.

Tip: Just like we reduce filament voltage to determine how much emission
margin exists, we can form a cap at a voltage above its rating and observe
the leakage.  Just be careful with lytics rated for 450 to 500V.   Applying
voltages substantially above 500V puts use close to the inherent dielectric
breakdown.

On Mon, Mar 19, 2018 at 12:19 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:

> Some quick comments:
>
> Resistance of any kind -- series or parallel -- diminishes Q. If you shunt
> a capacitor with a 1ohm resistor, its Q will be reduced for certain. It's
> only because ESR frequently dominates that one can usually neglect parallel
> resistance.
>
> Forming also definitely affects leakage, particularly as voltage
> increases. If you monitor the instantaneous forming current, you'll often
> see spikes. Those are pinhole defects being blown away. In the real old
> days when hams built their own electrolytics in Mason jars, you could
> actually see the boundary with the plates sparkle during forming. Sometimes
> these were so numerous that you would perceive only a diffuse glow around
> the plates until forming had nearly completed.
>
> Tom
>
> --
> Prof. Thomas H. Lee
> Allen Bldg., CIS-205
> 420 Via Palou Mall
> Stanford University
> Stanford, CA 94305-4070
> http://www-smirc.stanford.edu
> 650-725-3383 (public fax; no confidential information, please)
>
> On 3/19/2018 11:46 AM, Richard Knoppow 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
>>>
>>
>>
>
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