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Re: [TCML] Looking for pulse cap

Dave -

Here's another possible option.
Several manufacturers offer polypropylene film capacitors specifically designed for use as IGBT snubbers, with very high pulse current ratings and low inductance, at moderate voltage ratings. Also, these caps show up constantly at reasonable prices on eBay.
For instance, in the Aerovox RBPS series, the 2.0uF, 1KV cap is rated at 900A peak current, 22.5A RMS current. Ten of these in a string will give you .2uF at 10KV, with a 900A current rating. Five of these strings will give you 1.0uF at 10KV, with a 4500A current rating. These caps have wide, heavy strap terminals, so it's easy to directly connect them together in low-inductance strings.
At one time I bought a bunch of brand new ones on eBay for $0.85 ea, and I see "used" ones all the time for $2.00 - $3.00.
I've got some of mine assembled into a tank cap for a SISG coil, but I haven't had a chance to finish the coil yet, and can't demonstrate their suitability for this application. Spec-wise, I can't see why these caps wouldn't be good candidates for DRSSTC tank use.
Even at $3.00 each, your total cost for 50 (surplus) caps would be around $150. 

Regards,
Herr Zapp
----- Original Message ----- From: "Dave Leddon" <dave@xxxxxxxxxx> 
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Tuesday, September 15, 2009 1:25 PM
Subject: Re: [TCML] Looking for pulse cap



Phillip, Steve,
Thanks for your insight. You both responded almost simultaneously with the suggestion that I raise the value of my tank cap to 1 uf which has several advantages, not the least of which is that it reduces the capacitor count down to something almost manageable. Using the 940C8W2K-F caps ($5.99 at rell.com) I could get by with 9 strings of 17 each and given that I already have 38 of these on hand my out of pocket expense would only be, ka-ching $690. Still a bit more than I can justify. So if anybody knows where I can score 1 uf at around 10,000 volts for less money, please let me know. 

Thanks,
Dave
Pleasanton, ca

At 11:20 AM 9/15/2009, you wrote:

Herr Zapp,
The lower voltage ratings you may have seen were for 2 reasons: less current 
and secondly, they were over stressed.
I used to run my MMC caps with peak AC voltages at the DC rating, and after 
enough time, one by one, the caps would fail.  Testing shows that the cap
makers arent fibbing about the VAC ratings, stick with that and it will last 
for a really long time.

Dave,
Id suggest going up higher than .5uF, perhaps to 1uF or so (i didnt specify this in our correspondence). Im running 1.25uF on an experimental setup at 1500A, but a lower frequency. While you get less amp-turns in the primary, 
you do arrive at the higher peak current level faster, which translates to
achieving a certain top voltage on the toroid faster, which is more
efficient (in my data i saw a 20% efficiency improvement going from 8 cycles 
to 5 cycles of operation by increasing the tank cap value).  You have
current to spare with those big CM600s, so you should have no problem
producing well over 10 foot sparks (id guess in the 14-16 foot range,
secondary coil withstanding).

Steve



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