A van der Graaf machine with real welly

I want to build a van der Graaf machine with real welly. Not for any criminal purpose I assure you.
I want it to be a portable machine, run off 12 volt batteries.
I want to use a generated voltage to put the charges onto the belt, so I don't rely on the tribo electric effect at the roller and I can change the polarity of the final voltage. I will put the voltage onto a conducting but insulated lower roller, pulling the charges off a comb at earth voltage. I would welcome suggestions as to where to get such high voltage low current generator.
I would also welcome any design suggestions about the van der Graaf machine itself. Something from easily available materials, which will be a pleasure to build and look at... dream on!
The voltage finally reached will be the balance between the number charges carried to the upper sphere (which I can vary by changing the speed of the belt) and the rate of leakage. A more controlled voltage would be welcome, but I don't see how to achieve it.
I don't suppose such a machine is available buy, but I am open to the possibility.
Michael Bell
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Michael Bell wrote:

How about the HV generator in an old TV set. IIRC they're this sort of thing http://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator .
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David Billington wrote:

The OP might find this of interest. It is based on the HV transformer from an old TV set, but runs on 12V or so:
http://www.powerlabs.org/flybackdriver.htm

No, they are simple transformer supplies.
They look complicated in TV circuit diagrams because the designers tend to use them for lots and lots of different things at once (thus lowering part count), but the basic HV supply part is just a step-up transformer with a diode.
Sometimes they use a few diodes rather than just one, as eg 3 x 8 kV diodes were (are?) cheaper and more reliable than one 24 kV diode. Sometimes the diodes are built into the transformer, sometimes external.
The CRT itself is used as a capacitor to store and smooth the output voltage.
Especially if you are looking for a bit of power, up to 100 kV or so a transformer is usually the best way to go, C-W and similar voltage multiplier generators are usually pretty low power unless they are huge and expensive.
Both can still be lethal though.
-- Peter Fairbrother
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Michael Bell wrote:

Have you seen how well a good self-excited machine performs? Pick the right materials and you might not need the HV generator.
I have built a Van de Graaff machine in the past (it's sitting on my filing cabinet here). Sometime I might replace the belt and lower roller, because the one thing I wish I'd spent more time on is materials selection.
Best wishes,
Chris
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But I want to be able to change the polarity and to do that with certainty I need an HV generator to put charges of the sign I want onto the belt.
That aside, have you any recommendations for the rest of the design?
Michael Bell
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Michael Bell wrote:

Fair enough.

You want the greatest swept area of belt per second. That means a belt width the same as the roller diameter and the highest rotational roller speed possible. Crowned, well-balanced rollers permit a machine to run at a higher speed. You need some kind of adjustment mechanism so that you can ensure the two shafts are parallel. Avoid any sharp edges on your spherical terminal, unless they are inside. Some machines have variable speed, which can be useful, but isn't essential.
Good luck. You can always e-mail me at chris AT ruggedmachines DOT com if you have questions.
Best wishes,
Chris