DIY Pulse Capacitors

You don’t need lots of money to make high voltage capacitors, in fact some pretty decent ones can be made with some cheap and readily available materials. This is because capacitors are very simple devices; consisting only of a dielectric and two plates. Most often a capacitor’s plates are just aluminum foil, and reynold’s wrap is easy enough to obtain, but what about the dielectric?

Enter the overhead projector sheet. Transparencies as they are commonly known as are nothing but acetate film, and while this is not the ideal dielectric for a capacitor it still does quite a good job. Typically a four mil OHP sheet can withstand 14kV before breaking down. As for obtaining them, the cheapest I have found these sheets is $10 for a box of 100, enough for about 16 capacitors.

How you make the capacitors is a rather trivial task, all that needs to be done is some cutting, flattening and rolling. Below I have an image that explains the process. Multiple sheets of OHP sheet are used to increase the capacitor’s voltage rating, and two sets of sheets are used so the capacitor can be rolled up.

Some tips:

• Use a rolling pin to flatten the foil.
• Roll the cap up very tightly.
• Use thin wire to minimize trapped air.
• Secure using zip-ties
• Leave 4cm between the edges of the OHP sheet and the foil to prevent flashover.

If all goes well you should end up with a capacitor that ought to be good for about 50kV. Typically these home-made caps will have a capacitance of about 3nF as can be seen by this LC meter’s screen. That’s definitely not too shabby considering a capacitor such as that would cost you $40 surplus.

Keep in mind that this capacitor is not as robust as a commercial one due to the inclusion of air between the plates and the dielectric. Yet, if you soak the assembly in molten beeswax and simultaneously pull a vacuum the air will be replaced with wax and make a much more robust capacitor. Some sort of modified pressure cooker could do the task, but that’s another story for another day.

Just to prove that these capacitors do in fact work, here is a video of a couple of them placed in parallel with a flyback transformer. They store up the energy and release it near instantly as a bunch of very loud blue-hot sparks.

Remember that they are capacitors and they will store a charge when the power is removed, and being high voltage caps they will give you quite a jolt if you let them bite you. An observant eye will notice that I discharge the capacitors at the end of this video. For those curious, the hum is the from a 48V 6A switch mode power supply.

Uses for these capacitors include filtering the high voltage from a DC flyback, building a large CW multiplier or just annoying the hell out of anyone within earshot. If nothing else they are a fun weekend project. ∎

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  1. I’m thinking you could use an overcut piece of foil to exit the rolled up cap for each conductor sheet (then attached to wire), as long as it was reinforced so as not to tear off easily…or some scrap tabs from laptop battery jumpers.

    I have some dielectric oil instead of wax I could make a huge mess with when I build mine. Maybe put in a piece of capped PVC and wrapped in 67KV splicing tape :) Thanks for the plans!

    • That would actually make for a much better capacitor since it’d reduce the inductance quite a bit.

  2. For those that are into such things, watch the blue and black wires leading to the caps carefully. Every time you draw an arc, you can see them moving towards each other as a result of the magnetic fields. While the forces are always there, it is rare to see them at work so clearly in a video.

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