100nf rolled foil capacitor

[AndreiRS]

I made two of these in the hope they will work good for my Tc since I can't find cornell dubilier around here. Each one used to be 215-220nF. But then that was too high for my transformer. So I "unrolled" them a bit. Now each one is around 104nF. Which in series, doubles the voltage rating and gives me 52nF, close to the 50.1nF I need. Are these good for Tesla Coil? I also dipped them in electric motor varnish.

[profdc9]

High voltage capacitors are hard to make.

The problem is that if there is air in the capacitor, you get this effect called "partial discharges."  Basically its tiny little sparks between the layers of dielectric that slowly eat away at the dielectric through oxidation because the air is ionized because of the high electric field.  The capacitor will be fine for a while, but after awhile it will arc through.  Capacitors are often filled with oil or wax to prevent this.  A vacuum needs to be applied for the air bubbles to be removed as well so there is no air left in the capacitor.

That said, I made a capacitor out of stacks of aluminum roof flashing and wrapped them in 3 mm thick multiple layers of polyethylene painters tarp and it continues to work.  I put the stack in a vise and clamped on it very hard to try to push the air out as much as I could.  Furthermore, I carefully trimmed the roof flashing to have rounded corners so there would no points that serve to concentrate the electric field.  But I used a lot of painters tarp, had very thick layers, and only got 26 nF capacitance.  It probably would have been cheaper had I just bought a MMC when I got done with it, but like a lot of things I just like to try things out.  My guess is that the dielectric you have is thin because the capacitance is high.  The breakdown field is going to be at best 20 kV/mm and probably a whole lot less once partial discharges start.   If you're using aluminum foil as your conductive layers, its probably crinkly, and the creases will act to concentrate the electric field.   So I'm guessing your capacitor may not last long.

You could try to get mineral oil and fill the pipe with that, or maybe even vegetable oil would have a high enough dielectric strength.  Heat the oil to get as much water and air out of it as you can before pouring it in, pour in it in to make sure no air gets in the pipe, and then use a plastic screw perhaps as a plug so that no air is introduced into the pipe when you seal it.  If you use oil, you may just want to use paper insulation instead, and let the oil be the dielectric.  But after all of that trouble, I think most opt for just putting together a MMC if that is an option.

Dan

[AndreiRS]

Thanks for all the info. Ah they don't look soooo bad after reading your post. I have done the varnish, oil thing. The only problem is the aluminum foil, it is very crinkly as you said. I will have to wait untill the mail service here works again and buy capacitor from another country.