Ideal Polarized Cap Voltage Ratings

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

Reply to
Warren
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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

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Wimpie

Wimpie expounded in news: snipped-for-privacy@s4g2000yql.googlegro ups.com:

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

Warren

Reply to
Warren

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

Reply to
John Larkin

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

Hope This Helps! Rich

Reply to
Rich Grise

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

Reply to
krw

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

Reply to
Rich Grise

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.

Reply to
Pimpom

Pimpom schrieb:

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

Reply to
Uwe Hercksen

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

Reply to
Rich Grise

  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

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

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.

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Mike Perkins
Video Solutions Ltd
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Reply to
Mike Perkins

I've just realised how old this thread is!! Thanks for Thunderbird!!

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Mike Perkins
Video Solutions Ltd
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Reply to
Mike Perkins

Mike Perkins expounded in news: snipped-for-privacy@bt.com:

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

Warren

Reply to
Warren

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