[time-nuts] Replacing electrolytics - any disadvantages of high temp ones?
cfharris at erols.com
Wed Jun 22 05:16:13 UTC 2011
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.
Bill Hawkins wrote:
> 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>, 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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