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Re: Small-ish polyprop. caps



> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
>
> Hi Sam,
> 	I am experimenting with smaller poly caps too.  I just got a batch today
> and am wiring them together tonight (160  0.1uF 630V to give 0.01uF at
> 25000v).  Richard Hull and Bill Richards of TCBOR also experimented with

I browsed through some local surplus sellers' lists, and one of them had a
couple of caps I thought were usable. Siemens 180 nF, 1000 VDC polyprop. caps,
at 2 FIM ($1 = 5.07 FIM nowadays) and Rifa 100 nF, 660 VAC impregnated,
metallized paper caps at 1 FIM.

But alas, the Siemens one had only 150 V/us rated dV/dt... So, it would have
made the cap unnecessarily large (and expensive!) 

So, the cheaper one. I found the datasheet, and it was an X2-rated capacitor
(EMI/RFI suppression, for constant connection between phase and ground even
here in Europe where it is 230 V), so they boast self-healing properties and
long life at overvoltages. dV/dt is 600 V/us, very good. The only thing I'm
not sure about is dissipation, will it stand up to constant abuse at higher
frequencies without overheating. Anyway, it seems to be a cheaper option than
the big, mean, expensive 1.5 uF/1600V caps I was thinking about in the first
message, don't know about better (except in that I couldn't afford the bigger
one anyway :-))

> first since it would be much easiler if it works.  There was a thread last
> week about this.  Contact me if you missed it and I'll resend the posts to

I've seen most of what's been sent after Oct.30, so I've seen that thread 
(was it "Cheap 20nF 20kV pulse caps - If it would work??" ?) 

> you.  I am putting 10 Meg resistors across each cap to be sure stray or
> unbalanced voltages can't build up in individual caps over time and blow

Same here, the surplus place has cheap 10 Mohm, 1.6 kVDC caps (Philips VR25),
so I'll be getting more of them at a time and making a high-voltage meter too.

> them out.  I'll report what happens to my experiment soon.

Me, too. When I get my caps, at least. Although my NST doesn't have that much
muscle so that it could break the cap anyway (4 kV, 35 mA), but all the better
for me. 

Oh, by the way, here's something on my coil : Diameter 10.7 cm, 2 mm thick
cardboard tube with 0.315 mm enameled copper wire wound around it for a
40.5 cm
length, that makes about 1260 turns. When I pop it in the JavaScript Tesla
Coil Calculator, it says the resonance frequency should be 176 kHz. But, I
measured it using a RF generator - I pumped "in" through a 5-turn primary
a signal that was 1 V rms into a 75-ohm load (I had to put a resistor in 
series with the generator, to not blow the output stage...) and I got across
the coil 100 V rms, but at negligible power, of course. Resonance seemed to
happen at 190 kHz, without a top load, of course then it will creep somewhat
lower.

Just for you, not the list, unless you consider it "worthy" :-)