Re: The right capacitor match?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 05:14 PM 8/2/2005, you wrote:
> Greetings,
>
> New coiler here, hoping to find some aid.
>
> I have a 15K/30mA NST which I am building an MMC for.
>
> Given LTR of 1.5* I have come up with a match in the range of .008mF

Here is a very useful chart too:

http://hot-streamer.com/temp/MMCcapSales.gif


> I have had trouble finding HV caps in the lower Farad range. (Any current
> suggestions would be greatly appreciated).

These low values tend to need a lot of the large caps in series.  The 
peak voltage is also about 21,000 volts also suggesting a lot of series caps.


> I have found some Cornel Dubilier (940C30S47K) "Snubber Capacitors" that
> look like they will work. They are not on Terry's known Bad Caps list. They
> are rated at 3kv .047mF
>
> If I have done my math properly, I only need six of these in series to get a
> capacitance of .00783mF (which is in my LTR range) and a voltage of 18,000v
> (which exceeds my NST voltage)

The "peak" voltage is actually 1.4 x 15kV.  The voltage rating on the 
NST is an "RMS" voltage which is the peak voltage divided by the 
square root of 2.  So the MMC will actually see about 21kV.  But 
don't worry, since MMCs can take some over voltage with ease.

What we have to be careful of is peak current.  The 940 series and 
other similar fully metalized caps really hate to have more than 
their rated peak current.  From the chart:

http://www.cornell-dubilier.com/

The 0.047uF caps or only rated at 68 amps peak.  That is TOO 
low...  The ends will burn up where the metal film attaches to the 
end plates.  Unless you already have or can get the caps almost free, 
I don't think it is worth trying.  You will probably want at least 
300 peak amps.


> This seems small and cheap compared to what others seem to need regularly.
> Is it just that my coil is one of the smaller ones out there? Or have I made
> some egregious error?

Your right.  You need high voltage, small value, and lots of 
current.  That falls into sort of an awkward area where you need lots 
of big caps in series.

Cheers,

        Terry


> Your input is golden.  Thank you.
>
> --Ethersmith