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

J. Forster jfor at quik.com
Wed Jun 22 15:56:47 UTC 2011


IMO, the issue of reforming is very much alive with electrolytics, old and
new.

In some modern PZT actuator drivers, there is a warning to bring up the
supplies slowly if the unit has been dormant for sx months or more.

-John

================


> 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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