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Re: MMCs AC to DC...



Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Johnathon,

A DC rating of 2000 volts does mean that we can run them to 2000 volts "peak"
in Tesla coil applications.  This is due to small duty cycles and relatively
short life expectancies (days instead of decades).

When polypropylene has AC placed across it, the dielectric will ionize at about
350 volts AC.  I glows a pretty blue color as ions bombard the dielectric. 
This is a fairly destructive process that eats away at the poly.  This is why
poly caps have low AC ratings of 300 or 640 VAC.  They use series plates in
them to get to the higher 640VAC number (two caps in series for double the
voltage).  All polypropylene caps will have these limits since it is a physical
property of the material.  This is not really a factor in DC applications. 
This is all called the "ion inception voltage" (web keyword) if you want to
look into it more.  

Since homemade poly caps (remember those days ;-)) where only a single layer,
they had to use very thick poly since the full say 20000 volts was across it. 
Ionization probably acounted for the failures rather than 3X voltages.

I should point out the 2000 VAC is typicaly an "RMS" value where the peak is
really 2000 x 1.4 or 2800 volts peak.  So to keep the peak at 2000 volts the AC
voltage is really 2000 / 1.4 = 1429 volts.

I should also point out the MMC caps are darn tough voltage wise and "I" run
them at about 125% peak of the DC voltage.  So I run my 1600 volt caps a little
over 2000 volts peak.  RMS current seems to be the thing to really watch.

Cheers,

        Terry


At 10:44 PM 4/20/2001 -0400, you wrote: 
>
> Hi all, 
>       Ok, I know that for examle, A 2000VDC cap can't take 2000VAC (can 
> it?), the VAC rating is lower.  Is there any formula or guessing you can do 
> to find the VAC rating on a DC cap? 
>
> Jonathon Reinhart