Ideal Polarized Cap Voltage Ratings

[Warren]

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

 

[Warren]

Wimpie expounded in
ups.com:

....
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). ...
Wim
PA3DJS
www.tetech.nl

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

Warren

 

[Rich Grise]

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?

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

Hope This Helps!
Rich

 

[[email protected]]

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

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

 

[Rich Grise]

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

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

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

 

[Pimpom]

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

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.

 

[Uwe Hercksen]

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.

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

 

[Rich Grise]

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.

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

 

[Spehro Pefhany]

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.

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

 

[Warren]

Mike Perkins expounded in

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

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

Warren