Do you happen to have any 15,000 volt capacitors sitting around? [Ludic Science] didn’t so he did the next best thing. He built some.

If you understand the physics behind a capacitor (two parallel conductors separated by a dielectric) you won’t find the build process very surprising. [Ludic] uses transparency film as an insulator and aluminum foil for the conductive plates. Then he wraps them into a tube. He did throw in a few interesting tips about keeping the sheets smooth and how to attach the wires to the foil. The brown paper wrapper reminded us of old caps you might find in an antique radio.

The best part by far, though, was the demonstration of drawing an arc from a high voltage power supply with and without the capacitor in the circuit. As you might expect, playing with a few thousand volts charged into a capacitor requires a certain amount of caution, so be careful!

[Ludic] measured the capacitance value with a standard meter, but it wasn’t clear where the 15,000 volt rating came from. Maybe it was the power supply he used in the video and the capacitor could actually go higher.

The homemade capacitor that results has a pretty low value (in the nanofarads). You could make a lot of them, or increase the surface area to get more capacity. You could also try to find thinner dielectric, although if it is too thin, your homemade cap might not survive the high voltage. If you want lots of capacitance (but perhaps not as much voltage) you can always home brew a super cap.

## 12 thoughts on “Homemade High Voltage Caps”

1. ian says:

Very cool. It would be cool to build one of these in a physics class and calculate the capacitance.

2. Leithoa says:

The voltage rating isn’t measured. It’s assumed, based on the thickness and material the overhead sheets are made from. PET (mylar) has a dielectric strength of 7500 V/ 0.001″, if his sheets are 0.002″ thick that’s where he got 15kV from.

1. Stark says:

I think they are made from PET too, but Wikipedia says cellulose acetate.

1. Keith Reynolds says:

One can use cellulose acetate, PET or a number of other materials that have a high dielectric strength.

2. murdock says:

You do also have to factor in a safety margin because of charge concentration on any pointed parts, or edges of the foil.

3. lightningstalker says:

Recommend polyethylene sheet or plastic or glass from picture frames

3. Sparky says:

At 15kV and 7.5nF, this would store about 0.85J; more than I expected.

Mylar has a relative permittivity of about 3.1, so assuming two sheets are 0.20mm (from first webshop I found) thick, and about 150 x 200mm, the capacitance of two parallel sheets without rolling would be 8.854 * 10^-12 * 3.1 * 0.15 * 0.20 / 0.2 * 10^-3 ~= 4.1nF. Rolling the sheets would approximately double the capacitance, because both sides are used, except for the inner and outermost layers.

4. Jimmy says:

These capacitors are identical to those you find inside spark gap fluorescent light starters for high-frequency high-voltage smoothing, only bigger :D

5. I made some capacitors very similar to these as a teenager. I used them in a 1.5kw spark gap Tesla coil, it taught me that you can build anything if you are willing to MacGyver hard enough and that organic dielectric materials are really flammable.

6. Col_Panek says:

Water makes a great dielectric because its constant is over 80.
Well, it breaks down in milliseconds even if it’s distilled, but you can’t have everything. I worked on a pulse generator that put out 500 KV by switching the voltage on an 8 inch water-filled coax (pulse forming network).

1. Michael Black says:

Fifty years ago I read something about the early days of radio in Alaska. They needed high voltage capacitors, so the radio guy took a scrapped bathtub, filled it with salt water, and used glass beer bottles filled with sea water as the dielectric. Wires connected the segments.

Every so often, a bottle woukd pop, and he’d have to get another one.

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