Ideal Polarized Cap Voltage Ratings

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I recently read somewhere some net wisdom about the ideal
voltage ratings for polarized caps. Some hobbyists have said "as
long as the rating is higher than your max operating voltage".

But someone else has suggested some reasons to temper that by
suggesting the ideal operating max should be about 66% below the
cap's maximum rating, for longer life etc.

Maybe I even read it here :)

Any advice/pointers from the experts on this?

Warren

Re: Ideal Polarized Cap Voltage Ratings
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Hello Warren,

For low voltage capacitors (say below 100V), voltage is not in the
lifetime calculations (or a negligible factor with respect to the
others). So you can use them up to their rated voltage (but surges/
peaks should also be within the rated voltage).

In case of high voltage capacitors, the higher leakage current will
lead to increased depletion of the electrolyte, so in that case
derating may increase the service life (see for example  par. 2.9.4.
of "Application Guidelines for Aluminum Electrolytic Capacitors" from
Nichicon).

You may visit the technical data (with regards to lifetime expectancy)
for various manufacturers (Panasonic, Elna, Nichicon, etc).

With kind regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely

Re: Ideal Polarized Cap Voltage Ratings
Wimpie expounded in
ups.com:

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...
..

Thanks for that info. I'll read it on my commute home.

Warren

Re: Ideal Polarized Cap Voltage Ratings

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I use wet aluminums up to rated voltage. MnO2 tantalums are more
complex. If a tantalum is in a timing or filter circuit where current
is limited, full rated voltage is safe. If there's potential for lots
of dv/dt charging current, derate as much as 3 or even 4:1, or use
something else.

I've done limited testing on polymer aluminums. They typically fail
hard at around 2x rated voltage, so 1x or maybe a bit less is prudent.

Haven't tried polymer tantalums, or the niobium things.

John


Re: Ideal Polarized Cap Voltage Ratings

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FWIW, my rule of thumb is to derate everything by about 20 percent.

Hope This Helps!
Rich



Re: Ideal Polarized Cap Voltage Ratings

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That's a good plan, but with tantalums you have to go at *least* 50%.

Re: Ideal Polarized Cap Voltage Ratings
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OK, fair enough, but I've never "designed"[1] anything that tight, and maybe
I've just been lucky, but I've never ever seen a tantalum cap explode (or
exploded) either in my own "designs,"[1] or by the pros.

[1] I don't actually design stuff, I just lift other people's designs and
connect the dots. But I did slap together an astable once with a pot between
the bases, to get a duty cycle from about 5% to about 95%, but it needed
a knob or slider, so not amenable to voltage control.

Cheers!
Rich


Re: Ideal Polarized Cap Voltage Ratings
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Same here. One caveat though: I read somewhere a long time ago
that using an aluminium electrolytic at voltages too much below
its rated voltage could result in reduced capacitance. This may
not matter depending on the application. The literature didn't
give numbers but I got the impression that it was talking about
voltages that are a small fraction of the rated voltage, not a
few tens of percents below. I haven't verified this personally,
but it makes sense to me. Running a 400V cap at, say 25V would
tend to degrade the electrode formation.



Re: Ideal Polarized Cap Voltage Ratings


Pimpom schrieb:

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If the 400 V electrolytic cap is running with only 25 V, the oxide layer
on the surface of the electrode will get thinner by the degradation.
This will increase the residual current, but it will increase the
capacitance too, it will not reduce it. Think about the formula for the
capacitance of a condensor, especially the thickness of the isolating
dielectric. If you compare electrolytic caps with the same capacitance
but different voltage ratings, the ones with less voltage are much
smaller than those with large voltages.

Bye


Re: Ideal Polarized Cap Voltage Ratings
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Along these lines, I've seen ridiculous tolerances spec'd for
aluminum electrolytics, whose application is presumably power
supply filtering, like -20/+100%.

Cheers!
Rich


Re: Ideal Polarized Cap Voltage Ratings
On Thu, 28 Apr 2011 15:34:50 +0530, the renowned "Pimpom"

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1. Deterioration really doesn't happen with modern capacitors
  (reforming is an artifact from the past). One could guess that
   very long term operation of an ancient cap at 1/10 of rated
   voltage, followed by operation at 100% might be similar to the
   reforming thing of years past.. perhaps the origin of this
   old wive's tale.

2. Even if it did, why would that _decrease_ the capacitance?

3. Manufacturers say nothing of the kind in their applications
   data.  

4. Many low capacitance capacitors are currently used at a small
   fraction of their rated voltage. That's not where failures
   occur- they mainly happen to stressed parts (large ripple current
   and high Ta).




Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Ideal Polarized Cap Voltage Ratings
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I have always factored in a x2 in voltage and rms current specs and
chosen a cap rated at 105 degs C or better, with a lifetime quoted at
5,000 hours or more.  For the simple reason that specified lifetime of
electrolytics is so short, sometimes 1,000 hours at rated temperature
and voltage.

The only time I have ever come unstuck, as did many other manufacturers,
is the electrolytic disaster a decade or more ago, where after 18 months
of use, many spontaneously failed.

It seems according to other posters here, my choice is perhaps too
conservative, but I do feel electrolytics are probably the most likely
to fail component in any equipment.

--
Mike Perkins
Video Solutions Ltd
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Re: Ideal Polarized Cap Voltage Ratings
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I've just realised how old this thread is!!  Thanks for Thunderbird!!


--
Mike Perkins
Video Solutions Ltd
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Re: Ideal Polarized Cap Voltage Ratings
Mike Perkins expounded in

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...

No problem-- I was just surprised to see it again. :)
Thanks for your comments.

Warren

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