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I need capacitors rated for 50 kV. Buying them is an option but the ones with appropriate voltage rating I have seen cost more than 50€ which is far too much.

That's why I want to make caps on my own but the simple aluminium foil - plastic foil - aluminium foil trick won't do it. Such caps can barely resist voltages higher than 5 kV. I plan to stick to aluminium (good idea?) for the plates but I need a much stronger dielectric.

I have heard about several materials to use as a dielectric for voltages that high:

  • Distilled water: It has a breakdown voltage of 65 to 70 MV/m (Wikipedia). The problem is the forming of ions within microseconds after applying the voltage which lead to an immediate breakdown. This can be prevented by keeping the water flowing and deionizing it somehow. Thats rather complicated though as I would need a pump, a filter and a water circuit.
  • Waxed paper: It has a breakdown voltage of 40 to 60 MV/m (Wikipedia). Waxed paper is, as far as I know, basically the same as baking paper. It would be a lot easier to use than water but it comes in thicknesses of about 0.1 mm. Assuming a dielectric strength of 40 MV/m (=40 000 V/mm) it would need to be at least 1 mm thick. I don't think that folding it/putting several sheets on top of each other does the trick.

Do you have an idea on how to achieve my goal? Wikipedia lists a lot of chemical substances with crycptic names that have a high dielectric strength. Maybe one of those is easy to get/produce an can be used by an amateur like me? Hopefully someone is more experienced in this topic than I am.

I need the capacitors for a Marx Generator powered by a flyback transformer which supplies approximately 50kV. Capacitance isn't too important but the higher the better.

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    \$\begingroup\$ Playing with high voltages with DIY parts? What could possibly go wrong here... \$\endgroup\$ – Eugene Sh. Mar 3 at 20:00
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    \$\begingroup\$ Sheets of teflon come to mind : possibly lower cost plastics like Mylar, polycarbonate etc. Home improvement places will sell you a few square feet of polycarbonate (several mm thick) for a reasonable sum : look up the dielectric strength and permittivity yourself. \$\endgroup\$ – Brian Drummond Mar 3 at 20:19
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    \$\begingroup\$ What capacitance do you need? \$\endgroup\$ – JRE Mar 3 at 20:23
  • \$\begingroup\$ How high is your output voltage intended to be? In an \$N\$ stage Marx generator, each capacitor is only exposed to \$\frac{V_{out}}{N}\$ volts. \$\endgroup\$ – JRE Mar 3 at 20:34
  • \$\begingroup\$ People making Tesla coils sometimes use glass plates. If you can get the required capacitance and your demonstration doesn't move around a lot, this could work for you. \$\endgroup\$ – Mattman944 Mar 3 at 20:53
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You forgot to mention how much capacitance you need. You see, this aluminium ruler on my desk and the steel whiteboard 2 m away make an excellently safe 50 kV capacitor. It just has a very low capacitance. You might want to make up your mind on that :)


I don't see how the pump approach would even get you close to the cheapness of two 50 kV capacitors (the situation is not exactly as bad, but 30€ is still money). Also, this sounds like you're neglecting the hard problem here (deionizing water, and making sure all the ducting doesn't introduce ions), and focus on the easy stuff (pumping water).

Waxed paper is, as far as I know, basically the same as baking paper.

If that was the case, your cookies would taste like candle and be stuck to the paper.

It's not; waxed paper is mainly a layer of paraffin (or some other form of synthetic wax with low water content, but paraffin is really the prime choice) applied to a paper "core" for stability.

You can probably buy that or make that yourself, but you'd honestly would want to work very cleanly, very dry, and very uniformly so that you don't get any point in your piece of wax paper that has potential for a breakdown, as lightning shooting through a mixture of wax, a paper wick and high voltage is probably a very good method to start a fire that produces a lot of thick smoke very quickly.

You could at least address that by grossly overdimensioning your wax thickness, but that would reduce the capacitance of your capacitor (that's basically inversely proportional to the dielectric thickness).

Also, remember, including all the effort you put into manufacturing things precisely and all the tools and supply you need for this one-off effort, you're still trying to go below ~100€. I think you'll find this a practical problem once you really sit down to do a rough cost calculation.

Do you have an idea on how to achieve my goal?

Yes:

Wikipedia lists a lot of chemical substances with crycptic names that have a high dielectric strength.

Well, investigate these!

Honestly, waxed paper is probably a good starting point: if you can find a perfectly air-gap and -bubble-free way of constructing a wax-flooded (ie. paraffin-flooded) capacitor, you might get around acquiring or making your own wax paper.

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  • \$\begingroup\$ I just mentioned the water capacitor for completeness, I never really planned to do that. On the other hand one could set up such a system and have multiple caps in it at the same time. That might be cheaper in total but i won't try it. Thank you for correcting my belief about waxxed paper. English isn't my first language and the translators gave me several different translations. I will have a look at the different waxxed papers out there. Thank you @MarcusMüller \$\endgroup\$ – hanslhansl Mar 3 at 21:53
  • \$\begingroup\$ one "x" in wax ;) I really don't think the water thing is even remotely realizable for you, to be honest. \$\endgroup\$ – Marcus Müller Mar 3 at 21:53
  • \$\begingroup\$ You are probably right. It just sounded extremely cool to have this circuit standing around in my room with constantly floating water and sudden flashes between the caps when the Marx Generator fires. \$\endgroup\$ – hanslhansl Mar 3 at 21:59
  • \$\begingroup\$ YOu really still need to define a minimum capacitance. As my initial example illustrates, your statement "capacitance doesn't matter" is false, and doesn't work. So, do a rough calculation on how much capacitance you'll need and add that number to your question. \$\endgroup\$ – Marcus Müller Mar 3 at 22:04
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    \$\begingroup\$ @Marcus Müller: thanks for the comment. Sure - I left it up to the original poster to find some data on dielectric strength and I linked to the basic capacitor formula: permitivity * (A/d). And in the PDF on Mylar that I linked in my response, you can find a note and a chart detailing the unfortunate fact that as the layer of dielectric gets thicker, its nominal volumetric dielectric strength tends to gradually decrease :-( so it's not a simple linear proportion. You lose more in terms of capacity than you gain in isolation strength... \$\endgroup\$ – frr Mar 4 at 21:05
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The end of fun with static electricity comes exactly where you start speaking capacitors :-( Because E = 1/2 * CU^2, and because what hurts you is the current, and I = U / R.

Years ago, I've built a simple Van de Graaf style generator of static electricity: just a piece of 160mm sewerage PVC tube (about 200 mm long), mounted on a shaft with a crank, a piece of synthetic fur to charge the surface and an improvised soft copper brush as a collector. And I used that to charge a small leyden jar, made of the plastic "bucket" used to house a "cake" of CDR media (100 pcs I guess - about 15 cm deep). The wall is about half a mm thick, and the material is probably polyethylene.

The point then was to simulate accidental static discharges into some exposed grounded points on a vehicle-mounted PC, that was being RMA'ed to us, with a problem description that it's susceptible to static when inserting a USB dongle...

When the air was dry, the generator + leyden jar combo was able to arc at maybe 3-4 mm, has helped us reproduce the problem and take countermeasures, and has taught me a lesson when I accidentally touched the live pole of the jar: it gave me quite a shock, even though I had barely moved the crank (since the last discharge of the jar), the tinfoil "charge indicator" was perfectly limp, and there was no spark when I touched the sweet spot. What shook me more was the horror upon thinking what it would feel like if I touched the jar when fully charged. Where I live, you need a fairly high degree of training (and tough exams) to be legally allowed to work with high voltage.

Speaking of materials, I recall that Mylar sheets of A4 size used to be sold as xerox transparencies. They're probably not exactly cheap, unless you can find a stack unused and forgotten somewhere (as nowadays we use digital projectors. Mylar transparencies are so last century.) Very similar to the Xerox transparencies, only cheaper, are office binding covers - also fairly tough, glossy clear plastic (vinyl/PVC/polyester), in the simplest form just a 100pack of 8-10mil A4 sheets (10 mil = 0.25 mm). The binding covers are not heat-resistant - it used to be a popular mistake to put a binding cover into a xerox or a laser printer :-) Maybe avoid materials that are nominally anti-static. At home we have some plastic "workspace pads", just over A4 size, about 0.3mm thick. The pads are apparently made of nylon or some such resistant material (probably not teflon though). They probably weren't dirt cheap... generally in stores selling paper, office and school supplies etc., I recall various folders and school aids consisting of sheet plastic.

If you're after thickness of 1 mm or more, I've seen sheets of various plastic materials sold in hobby shops and "materials for craftsmen" shops around here: polyethylene, polypropylene, acrylic plastic glass, polycarbonate, polystyrene glass. Plastic sheets are also used for signmaking / advertisement production. I recall buying a sheet about 1x2m of white "toughened polystyrene" (probably AKA "high impact polystyrene") from one such supplier.

Some of the sheet materials are available in black or other solid color - especially with the black variety I'd be afraid of the pigment being conductive (carbon). For your purpose, go for the "natural" variant (no pigment = the sheets are often clear or white or just slightly tainted).

Or you could of course use a stack of flat glass :-)

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