Re: Maxwell Capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hi Justin

On 10/27/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: Just Justin 
<<mailto:rocketfuel@xxxxxxxxxxxxx>rocketfuel@xxxxxxxxxxxxx>

I picked up a Maxwell pulse cap rated thusly:

Voltage: 35kV
Cap. (Meas.): 30.7uF
E.S.L: ~0.02uH

I would like to use it with my first coil, which is
a full wave rectified 12kV NST (~17kV peak DC).

I'm not planning to use a charging reactor at this time.
I will pursue that avenue when I move up to MOTs (otherwise
I'd be looking a tank cap voltage of 35kV!).

I just thought I'd ask if anyone knows of any gotchas that
I should be aware of that could possibly raise the primary
circuit voltage above the 17kv DC coming out of the PSU?


Only "gotcha" is diode failure usually.  Be sure to use an RC pass 
filter on the output after the diodes, just a simplified "Terry 
filter" is what you are after.  Make sure the diodes are rated for 
maybe 1.5X that 17kVpk, if not more.


I know with AC there is the issue of resonance in the primary
circuit that can raise primary voltage through the roof, but
since my PSU is DC, that seems like it should be less of an
issue.


Or, non-existant.

 On the other hand, there will be some ripple (or a lot,
if I can't find some cheap filter caps!) which I imagine could
end up resonating with the cap as well.


No, the resonance occurs between the tank cap and inductance of the 
transformer.  When you insert diodes (as in, a full-wave bridge) you 
do not allow resonance to occur between the inductance and 
capacitance.  You must have AC to have resonance (as far as i know).

  Am I correct in assuming
one would use 120Hz as the frequency in calculating the resonant
capacitance of the NST-Tank Cap system?  Does it even apply here?


Well, in your case, you can really size the cap however you want.  I 
find that 120bps is best for small and medium coils, so just use the 
2.8X Cres of your NST, or something similar to that.  Basically, you 
want to select the biggest cap that you can still charge up to nearly 
17kV on one AC quarter-cycle (you only charge the cap when the diodes 
are forward biased, so as the transformer voltage falls below the max 
V, it no longer supplies charging current).  If your cap is too 
large, it wont charge to 17kV in a half-cycle, and if its too small, 
you wont use the full capability of the transformer.  You could also 
run down at 60bps and use a really big cap (twice the bang 
energy).  There has not been enough work done at these lower break 
rates i think, though for most medium coils i dont think 60bps is 
optimal for streamer growth (it may be just fine with large TCs).

Steve

Thanks!

Justin in Austin