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Re: Capacitor Help

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Malcolm Tesla

At 10:26 AM 12/12/2005, you wrote:

Hi everyone,

I'm almost done building my first tesla coil.  I used a 15kv 30mA neon sign
transformer as my source.  I have a spark gap made with two nuts welded to L
shaped brackets with bolts threaded in.  I can adjust the gap by threading
the bolts back and forth.  There is a very powerful 110v squirrel cage fan
cooling the spark gap.


Good!



My concern is the capacitor.  I'm trying to purchase something and just not
sure what to get.  I've been searching google.com and looking at sites until
my eyes can't take it anymore.

I can tell from everyone's sites that Polypropylene or Polyethylene is the
way to go.  I can get those from mouser.com but they are only 1k to 3k and
the just look so darn tiny compare to the pictures I see of everyone else's
on the net it's got me worried I'm getting the wrong thing.
Good to ask about this!! You want polyproylene ONLY. The value should be according to the chart: 

0.008uF or 8nF

http://hot-streamer.com/temp/FormulasForTeslaCoils.pdf

The voltage needs to be 21000v or greater.



If anyone can help me out with capacitor values I need, where to purchase,
etc. I'd be very grateful.  I did find one at amazing1.com but it was $750.
Wow


That is about 20X too much ;-))

Most people are using the Cornell Dubilier 942 series caps

http://www.cde.com/misc/h942.htm

These can be obtained from Richardsen electronics with a part number search:

www.rell.com
Many people are using 12 of the 0.1uF 2000V types in series (942C20P1K) That gives 8.3nF at 24kV.
Rell has them for $1.99 but you have to buy 67 of them :-p If you are really lucky, someone may have some extra they could sell you ??? 

http://www.mouser.com

Has them for $4.28 but you can buy any quantity (12 x 4.28 = $51.36).
DigiKey has the 940 series now!!! We will have to pester them to start carrying the 942 series too!!! 

Part number 338-1175-nd  (0.1uF  2kV) is a possibility at $3.30 each

www.digikey.com
The peak current is 171 peak amps for the 940 types as opposed to 288 peak amps for the 942 types. So you would have to figure out the peak primary current: 

Ipeak = 21000 / SQRT(Lp / 8nF)

http://hot-streamer.com/temp/FormulasForTeslaCoils.pdf
For 171 amps, that gives Lp greater than 120 uH (Fo<163 kHz). For the 288 amp caps Lp should be larger than 42uH (Fo<275kHz).
So it sort of depends on the Fo frequency of your coil. You can "push" the current spec about 50% on the 942 series caps, but that IS the limit. The 940 series does NOT like to be pushed much at all.
People have run 15/30 NST coils with 7 DigiKey part number P10156-nd caps (the original MMC caps) 

http://hot-streamer.com/TeslaCoils/MMCInfo/mmcinfo.htm
They are over current and spookely over voltage too, but the cost is only $22 and I don't think any ever blew up. I have a bunch extra and you give you 7 for free if you want to "test" that theory and let us know ;-)) I might have the 0.1 uF CDE caps but I would have to dig around...
Be sure to use safety gaps (or nice NST protection filter) and bleeder resistors in any case. 

Cheers,

        Terry





Thanks
Malcolm