Re: Cap AC voltage ratings

Hi Gary, all,

>Original Poster: Gary Lau  19-Mar-1999 1622 <lau-at-hdecad.ENET.dec-dot-com>
>It's encouraging that you were able to AC-operate your PP caps at their
>DC voltage rating.  But I'm not so sure though if I agree with your
>technique to determine their breaking point.
>While a cap's capacitance value is a tightly controlled parameter, it's
>breakdown voltage is not.  {numbers that follow are speculative:}
>If a manufacturer sells a cap marked 1000V, I suspect that if you
>a hundred of them and destructively tested them to the point of
>breakdown, you might find values from 1500V to 10,000V.  The 1000V is a
>guaranteed minimum.  There is no maximum.  If the test string that I
>happened to be built with some of the higher-valued breakdown voltage
>devices, I will get a very optimistic "max" voltage rating.  But then
>when I build the big array of 200 of these, I will have many of the
>voltage parts mixed in too, and when these break down, the voltage
>the remaining parts goes up, then they fail too.

I think we are misunderstanding each other, here. Let me use an
example of what I mean. Except for the values, this is basicly what
I did.

10kV xformer
100nF caps rated at 1.4kVdc
(I am using these numbers to make it easy).

Our 10kV xformer put out a peak voltage of 14kV (roughly). So for peak
AC=DC we would need 10 caps, right? To test the "Tesla properties" of
these caps, I would build a test string of 30-40 in series. This would
give me a 30-40kVdc rated cap. Measure the capacitance of each cap
and the total capacitance of the string. Now add this MMC to your tank
circuit and fire up the coil (tune for longest spark). Assuming the MMC
survives, short one cap, re-measure and compare the capacitance
values of the remaining caps. Also take note of the new total
capacitance of the string. Fire up the coil again, re-tune for longest
spark. Repeat this process, keeping close tabs on the single cap
capacitance. Once you reach peak AC volts = DC RATED voltage,
STOP (in our example this would be a string of 10 caps). Going any
further is too risky, because (like you said) you might be overloading
some caps. (if the string breaks down before you reach peak AC=DC,
stop there, of course). Now you know the maximum voltage that your
MMC can take. For added safety I would add between 30 and 40%
safety margin. This means for our example (assuming they survive
down to peak AC= rated DC) I would go for a string of 13-14 caps.

I hope this made things a little clearer. You should NEVER go any
higher than peak AC volts = DC volts (doing so is playing Russian
Roulette with your caps). Keeping close tabs on capacitance is
important, because any degradating of the dielectric will show up as
a loss of capacitance. Sorry, if I didnīt make this point clear enough.

Iīm not sure I can agree with your first statement in the second
paragraph. If we say that the capacitance is a tightly controlled
value (agreed), then the breakdown value has to have similar
tolerances. Why? Well, the capacitance is the result of the dielectric
K and the spacing between the plates (i.e: dielectric thickness). So,
if the capacitance is to be uniform on a batch of several thousand,
the spacing (K is uniform for virgin material) has to be pretty exact.
This means the breakdown voltage will be similar for all caps (made
to "x" specs), because the v/mil breakdown rating is pretty constant for
virgin material. Assuming a certain "x" 1kVdc cap will fail at 3kV
(numbers are just spectulative), then the weakest might fail at 2.8kVdc
and the strongest might fail at 3.5kV. I donīt think the failure voltage
has such a huge variation as you thought (1-10kV).

Using PE foil as a homebrewer you WILL have such huge variations,
but this is due to non-virgin materials, handling of the material with
your hands, entrapped air, non uniform material thickness, and
enclosed dust (due to non "clean-room" conditions). All of these
derating factors DO NOT apply to the MMC, because you are using
the high tech and knowledge of cap manufacturers.

>I think only time and experience will tell if we really can push these
>PP caps to operate so far beyond their published ratings.  If we can,
> and the testimonials I've seen so far seem to indicate that we can,
>then we should be cautious and realize that that success is only valid
> for that particular cap vendor and model, and maybe even date code.
>I should be able to add to the experience base for this in a few

Sure, only experimentation will give us valid data. Because of the
unknown variables involved (true breakdown voltage, cap construction,
etc), the only real way to fit the MMC to oneīs specific needs is to
build a small array and actually test it in TC usage. However, this is
true for ANY homemade TC cap. Once we have collected enough data,
the MMC HAS one advantage over any other type of homebrew caps:
The tolerances (i.e.: how far can you push it) are much, much smaller
than for other homebrew constructions, because the critical factors
and work have already been done for us at the factory and they donīt
vary (e.g.) as much as the actual (forget the printed values) PE
thickness does in the home rolled PE construction.

BTW: Did you get the XLS sheet and my private mail?

Coiler greets from germany,