[SI-LIST] Fw: Re: Re: Reset problem with power supply
- From: olaney@xxxxxxxx
- To: olaney@xxxxxxxx
- Date: Sun, 7 Dec 2008 19:21:00 -0800
On all the military design projects I've worked on recently, you could
not use an aluminum electrolytic cap without a long and painful
justification process. You could get a yes if there was no other way to
get there, but basically electrolytics were verboten. Allowable
temperature range was an issue even if the cap was all but hermetic
against leakage. Electrolytic caps use wet chemistry and if they freeze
it causes damage. Also note that electrolytic voltage ratings don't
derate well. If you operate a 50 volt rated electrolytic cap at 25
volts, over time it will become a 25 volt capacitor. It's inherent in
the chemistry between the aluminum and electrolyte, which is not stable
like a polymer dielectric. You could put the worn cap on a current
limited supply and build it back up to 50 volts, but the general advice
for electrolytic caps is to choose one rated not much higher than the
working voltage of the circuit, e.g. 6.3V rated caps for 5V applications.
This raises another issue: the chemistry that maintains the dielectric
barrier on the surface of the aluminum is an active process. As is
typical of many chemical processes, the tiny amount of leakage current
that results is noisy. This doesn't matter for bypassing, but for low
level coupling I go for tantalum caps, film or ceramic if I can get away
with the cost and size.
I know this discussion makes aluminum electrolytics seem like an
invention of the devil, but it is a very successful technology where the
limitations are not constraining.
Miltary equipment is designed for long service life and can be quite old
before it is discarded, sooner only if replacement parts cannnot be had
or new technology offers advantages that cannot be ignored. Other than
that, the stuff keeps going. The B-52 is one of the few planes in our
active defense inventory that is older than the pilots flying them, but
the cockpit electronics get refreshed somewhere in the 10 to 20 year
range.
Orin Laney
On Sun, 07 Dec 2008 02:19:34 -0800 Hal Murray <hmurray@xxxxxxxxxxxxxxx>
writes:
>
> weirsi@xxxxxxxxxx said:
> > Electrolytic caps come along with some engineering requirements.
> The
> > first is long term product life: electrolytics have a typical
> shelf
> > life of 5-10 years for cheap to very high quality. This is a
> function
> > primarily of the seals. However it is not that unusual to find a
> box
> > with large body computer grade electrolytics going for more than
> 20
> > years. The big issues: are initial conditioning, adequate
> > temperature, voltage derating, and absolutely : NO EXPOSURE TO
> > HALOGENS. A 10 year life is readily attainable, and with a lot
> of
> > derating 20 years can be had in large body parts. Miniatures are
> > really constrained to about 10 years no matter what is done.
> > Consumer products contain only miniatures these days, and are so
> > fiercely price sensitive that no one pays for the kind of
> derating
> > needed to see long service lives. As a result, the electrolytics
> > rank #1 to #2 for failure rates in consumer electronics ahead or
> > behind of the power semiconductors.
>
> Thanks.
>
> I assume the halogens mostly come from cleaning. Has that been
> solved by the
> save-the-ozone efforts? Do assembly houses know about that? Is it
> in the
> data sheets?
>
> I don't remember hearing it before, but I could easily have not paid
>
> attention. I just scanned one handy data sheet. I didn't see
> anything about
> cleaning. It's probably buried off in an app note or such.
>
> I did see that they are only rated for 2000 hours. I hadn't paid
> much
> attention to that before. That's under 3 months at 24/7. Ouch.
> Thanks for
> the heads up. I'll have to find the temperature re-rating specs.
> Sigh.
>
>
> What do military or other high reliability folks do?
>
> I expect a lot of military gear is still in service after 10 years.
> (B-52s
> are still flying.) Does the electronics turn over fast enough to
> avoid this
> problem?
>
>
>
>
> --
> These are my opinions, not necessarily my employer's. I hate spam.
>
>
>
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