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Adam Munich

Fri Sept 24 2010, 01:32AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Mine are 3.5 mil thick, and I used 4 in between each plate. (8 in total). I haven't tried them unrectified yet. A key tip is to pull your zip ties VERY tight. You want as little movement and air as possible.

As for your voltage issue, that fluctuation could very well be the problem. Have you ever thought about getting a gasoline generator? You could also use one of those 12VDC to 120V inverters. They're really cheap now and the NST's don't seem to mid the nonsinusoidal output. It doesn't get more regulated than that.

I'm undecided as to if I may splurge and buy some sheets. But the sparks are certainly impressive. And with a million volts at 2.8nf, that's 1.4kJ! Definitely not too shabby for $60

Edit: I was just playing with these a little more. They hold a charge for quite a long time with minimal hiss (corona). After a 3 second charge they seem to contain the peak voltage, not RMS judging by spark length. I dare not go longer because at full charge the HV from my 15kV NST find it's way into places it shouldn't.

They definitely seem suitably for marx use provided I use 250k resistors. :D

Edit2: I dared to go longer. The HV seems to like to spark to neutral. (small <20uA sparks) This doesn't happen without the caps. It doesn't seem to affect anything in my house lines though, so it's not too big of a problem.

Edit3: Found the problem. I had the NST on a .5inch thick piece of acrylic for insulation, but one HV wire was touching the wood floor. Seems that the charged wood acted a plate, and the NST case was capacitively coupled and a charge formed on it. This charge then sparked to neutral. (If that doesn't trigger the supposed GFCI that I have in my 15kV x-formers as well, absolutely nothing will.)

Edit4: I'm going to buy them. Apparently they're 4 mils thick, so they aren't crap. A 1kJ million volt marx made from "garbage" would be epic on many proportions as well. I have 1.4k in the bank from my summer job anyway, so if the marx is fail, I could use the caps in a tesla coil. They seem reliable enough, and if I use them in a tesla coil putting them under oil won't be a problem.

mnr

Fri Sept 24 2010, 03:45PM

Registered Member #996 Joined: Sun Sept 09 2007, 06:17PM
Location: Canada
Posts: 16

Grenadier wrote ...

I'm undecided as to if I may splurge and buy some sheets. But the sparks are certainly impressive. And with a million volts at 2.8nf, that's 1.4kJ! Definitely not too shabby for $60

Keep in mind the total capacitance of the Marx when erected will be C/n (i.e. a series connection of n capacitors).

Adam Munich

Fri Sept 24 2010, 07:33PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Are you sure about that?

mnr

Fri Sept 24 2010, 07:48PM

Registered Member #996 Joined: Sun Sept 09 2007, 06:17PM
Location: Canada
Posts: 16

Grenadier wrote ...

Are you sure about that?

Yes. From that site: "A common specification is the erected capacitance of the bank, equal to the stage capacitance divided by the number of stages."

Should still be an interesting project though, good luck!

ScotchTapeLord

Fri Sept 24 2010, 07:57PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Take two capacitors, to make numbers easy, 2 farads at 1 volt would be:

.5*(2)(1)^2 = 1 Joule
for each.

Discharged in series would mean sum of the voltages and the total energy of all capacitors.
1 Joule x 2 = 2 Joules
1 Volt x 2 = 2 volts

Solving for capacitance:

.5*(C)(2)^2 = 2 Joules

C = 2/(.5*4) = 1 Farad

So yes, the total C is C1/#.

Another way to visualize it is to realize in its firing stage, a marx gen is just a bunch of capacitors and resistors in series. A string of # series capacitors of equal C charged to Vtotal. If you take one capacitor away and measure it, it is charged to Vtotal/# with its capacitance being #*Ctotal.

Adam Munich

Fri Sept 24 2010, 09:14PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

mnr wrote ...
Yes. From that site: "A common specification is the erected capacitance of the bank, equal to the stage capacitance divided by the number of stages."

Wow how'd I miss that, it's only 16 joules. I had a feeling 1.4kJ would be too good to be true. Man do I feel like an idiot.

.

Irregardless, I'll have a whole bunch more caps done tonight. I'm going to hae to start using earmuffs because these things are LOUD! No video though... I'm going to need to buy a new camera battery charger.

Edit: A whole bunch turned out to be eight. So far I have 15 total, or an estimated 37.5nF of 15kV capacitors. I'll need 55 more (440sheets) to reach 1MV. Pics at 11.

Muttyfutty!

Fri Sept 24 2010, 11:52PM

Registered Member #2915 Joined: Fri Jun 11 2010, 10:41AM
Location: Malaysia!
Posts: 101

Whoops, never mind, everyone makes mistakes

Sorry I didn't make myself clearer, I live in 240V land.
But technically...
15,000V/240v = 62.5v*270 = 16,875 Volts

So, because of mains fluctuation, I'm getting close to 17KV RMS.
That still doesn't really explain why Im getting arc overs, significant corona,
and less capacitance...
Im Very intrigued how you managed to get a higher capacitance than
me with thicker sheets...
I'm getting close to breaking down and buying 'Proper' Capacitors
regards,
mike

Adam Munich

Sat Sept 25 2010, 03:43AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Crap quality sheets? The HP ones, Avery ones, and now the 3M ones seem to hold up just fine.

And sorry, no pics yet. I fell asleep and I'm too tired now. I'll get them in the morning.

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