Capacitor AC vs. DC Voltage Ratings

What's the magic for a cap's AC voltage rating vs. DC? I see 630VAC caps that are rated at 2kVDC -- that seems to be more than just sqrt(2)*630. Is there some consideration of current at 60Hz, an estimate of the size of spikes you'd see on an AC line? Some regulatory razz-matazz?

I may be helping out on a CDI circuit, and it looks like choosing a robust and cheap storage cap is going to be a good part of the magic.

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Tim Wescott
Wescott Design Services
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Tim Wescott
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"Tim Wescott"

** No magic - but plenty of cap makers play fast and lose with AC voltage specs.
** Might just be true.

** Mains rated caps are either class X or class Y types - with suffixes 1 or 2 as well.

So X1 or X2 or Y1 or Y2 is printed on the caps.

These are specially made types * proven * to withstand continuous use at 50 or 60 Hz and tolerate a range of hefty spike voltages too.

** Then choose a cap " designed " for steep pulses.

Polypropylene film /foil is one look at.

Oil filled caps film /foil /paper may be OK too.

..... Phil

Reply to
Phil Allison

y

The problem with AC ratings in capacitors is corona dischages inside the capacitors.

If the voltage differences inside the capacitor are high enough, you can get avalanche breakdowns within small air bubbles inside the capacitor. This creates ozone, and the ozone eats away the aluminium metallisation. There's a minimum voltage to produce avalanche breakdown (the Paschen minimum)

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so some high voltage capacitors are constructed as a bunch of series- connected capacitors to keep the inter-electrode voltage less than this limit.

Capacitors with aluminium foil electrodes aren't troubled by this effect - there's too great a thickness of aluminium in the foil for the ozone to get through - and oil-filled capacitors don't contain bubbles of air (if they've been made properly).

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

Tim Wescott schrieb:

Hello,

I found examples of 400 V DC, 200 V AC and 640 V DC for testing only, not longer than 2 seconds. Another example 400 V AC and 1720 V for maximal 2 seconds. Third example 250 V AC, testing 3 kV DC or 2 kV AC for maximal 2 seconds.

You should check if the 2 kV DC are continous or only for a short limited time.

Bye

Reply to
Uwe Hercksen

y

I think all the voltage specs are saying, is that low-ripple DC usage is much kinder to film caps than low-ripple AC usage.

Those specs don't seem particularly relevant to me because these caps are used in snubber networks and the like. There's a "real world" derating factor in snubber networks that's a very large number, typically 5 for voltage ratings.

If I compare/contrast the CDE 940C series, note that the cap lifetime is 60000 hours at rated VDC and 30000 hours at rated VAC, even when VAC is only a sixth of VDC.

CDI =3D Capacitor Discharge Ignition? Can't be Capacitive DeIonization, that's only a couple volts :-).

Tim.

Reply to
Tim Shoppa

In DC operation, the cap is usually largely fully charged. In AC operation, the cap swings constantly both on the source side, and the load side, and the thing physically 'flexes'. It will 'dry out' faster it will fail more often. Essentially yielding a much lower claim number on operational capability/life span. They have an internally "rougher street life".

Reply to
The Great Attractor

tory

r

In pulse operation I can often hear audible noises (both "clicking" and "crinkling") from snubber capacitors being operated near their limits. I've always attributed this to high currents although I suppose it's possible that the crinkling is a slower mechanical deformation and relaxation due to voltage.

Soakage on high voltage capacitors can be quite surprising. (Meaning, yowza, where did that two inch spark come from? I just discharged everything a minute ago.) I wonder if it's partly (or mostly?) mechanical relaxation.

Tim.

Reply to
Tim Shoppa

Reply to
Tim Wescott

There is a phenomenon called Partial Discharge which degrades the dielectric.

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With AC voltages dielectric absorption can theoretically lead to 2X the peak applied voltage appearing locally within the dielectric. Art

Reply to
Artemus

I believe some of the motor type AC capacitors are built with thin fusible sections of their foils so that if a section shorts it will take itself out of the circuit and leave most of the capacitor running. If you routinely discharge it at high currents you might end up blowing a good part of those fuses and be left with not much capacitor. I know of some folks who tried to make a storage bank for a pulsed magnetic field out of some cheap AC motor capacitors and they weren't successful. I never heard the details, just that they weren't getting the current they wanted, by a lot.

Paul Probert

Reply to
Paul Probert

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