[time-nuts] Replacing electrolytics - any disadvantages of high temp ones?

paul swed paulswedb at gmail.com
Wed Jun 22 15:42:15 UTC 2011


Oh to add further information.
I love old caps. They go bad and I get my test equipment for cheap.
That said I do measure the caps I am going to put in on a old style HP cap
meter that can apply up to 100 volts to the cap. I look for leakage. What I
see in quite modern caps that have been around for a while (Surplus you get
at hamfest approx 3-5 years) is that there is a higher leakage current that
does settle down after a while. So I sense the forming effect still exists.
Am I wrong about this??
Regard
Paul
WB8TSL

On Wed, Jun 22, 2011 at 1:16 AM, Chuck Harris <cfharris at erols.com> wrote:

> Hi Bill,
>
> I agree with your forming information, as applied to older caps,
> but not your temperature information.  The 105C high temp caps
> are just as happy, or unhappy really, with low temperatures as
> the 85C caps.  Basically the difference between the two is water.
> The 85C caps have an electrolyte with a significant amount of water,
> that boils dry at high temperatures.  The 105C caps don't.  Kind
> of like the difference between an antifreeze and water solution,
> and straight antifreeze.  Both seriously run out of capacitance
> when they get below freezing.
>
> The loss of capacitance can really bite you when you use integrated
> low overhead voltage regulators in automotive temperature ranges.
> The regulators will oscillate if they don't have enough capacitance
> on their input terminals... which can happen if you specify an
> electrolytic capacitor that is right around the 100uf needed.  When
> it gets to 0C, and becomes a 10uf capacitor, the regulator takes off
> and burns up your load.
>
> -Chuck Harris
>
>
>
> Bill Hawkins wrote:
>
>> Group,
>>
>> During my days of interest in antique radios, I learned that
>> the dielectric between aluminum plates was formed by passing
>> current in one direction to build up an oxide coating on the
>> plates, which became the dielectric. The thickness is directly
>> proportional to working voltage and inversely proportional to
>> capacitance. As we learned from reforming old caps, the oxide
>> thins when there is no voltage on the cap, but can be restored
>> by passing several milliamps through the cap. Applying rated
>> voltage before it was formed would destroy the cap by welding
>> spots of the plates together.
>>
>> I'm not sure that this applies to modern caps.
>>
>> As to the temperature rating, a high temp cap run in a cool
>> environment will be as unhappy as someone transplanted from
>> Miami to Minneapolis in the winter. It may work, but it will
>> be very unhappy - so it depends on your empathy for the cap.
>>
>> There ought to be a way to work precision time into this
>> thread, but I can't think of one.
>>
>> Bill Hawkins
>>
>>
>> -----Original Message-----
>> From: Poul-Henning Kamp
>> Sent: Tuesday, June 21, 2011 2:40 PM
>>
>> In message<4E008A73.50701 at erols.**com <4E008A73.50701 at erols.com>>, Chuck
>> Harris writes:
>>
>>  and yet, I find that some electrolytic
>>> capacitors that have been run at lower than normal voltage improve
>>> markedly
>>> when "reformed" by applying  rated voltage through a 10K resistor for a
>>> couple of hours.
>>>
>>
>> I noticed in a datasheet at one point, that the capacity only was
>> warranted above a certain percentage of rated voltage.  No explanation
>> was given.
>>
>>
>>
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