Finding high voltage capacitors can be tricky. Sure, you can buy these capacitors, but they are often expensive and hard to find exactly what you want. [RachelAnne] needed some low-value variable capacitors that would work at 100 kV. So she made some.
Instead of fabricating the plates directly, these capacitors use laminations from a scrap power transformer. These usually have two types of plates, one of which looks like a letter “E” and the other just like a straight bar. For dielectric, the capacitors use common transparency film.
As you might expect, she had to strip off the insulating varnish where the contact was to be made with the plates. The moving mechanism uses a toy car tire as a handle to make sure it is insulated. The screw moves one set of plates in and out while the other side remains fixed.
For more capacity, it would help to use a thinner dielectric. The transparency film is about 100 microns thick and there are several sheets in between each plate. According to the post, there are 400 microns of dielectric and 600 microns air gap, so that’s a millimeter gap between each plate. Of course, the voltage capacity depends on the gap and the dielectric constant, so too thin and you might need a better dielectric to handle the high voltage.