Roll your own capacitors – high voltage edition

[Grenadier] tipped us off about his method for building your own high-voltage capacitors. He thought the paper and foil capacitor project was a nice introduction to the concepts, but at the same time he knew he could produce a much more powerful device.

For the dielectric he is using acetate film. This is the material from which overhead transparency sheets are made. He stuck with aluminum foil for the two plates. Just roll the foil flat with a rolling-pin, use thin wire to minimize the air that will be trapped between the dielectric layers, and make sure the foil plates are at least 4cm shorter than the acetate film on each end to prevent leakage. After rolling and securing the capacitor with zip ties you’ll be ready for the 3nF worth of fun seen in the video after the break. [Grenadier] mentions that this can be improved further if you were to vacuum impregnate the device with beeswax.

13 thoughts on “Roll your own capacitors – high voltage edition

  1. @H3xx

    This is a 3nF capacitor according to the article. That is about 1,000,000 times smaller than the ~3mF of capacitance found in coil guns.

    You would have to charge this 3nF capacitor to around 100,000 volts to hold the same amount of charge as a 3mF capacitor at 100 volts

  2. Nice, I guess. But a more refined and useful technique is documented over at the 4HV (For High Voltage) wiki: http://wiki.4hv.org/index.php/Rolled_foil_capacitor_-_60_kV,_3.5_nF

    I posted this reference in the last comment thread too (http://hackaday.com/2011/05/09/roll-your-own-capacitors/#comment-391958) but I guess it went unnoticed.

    Also see their use in a 300kV CW multiplier: http://4hv.org/e107_plugins/forum/forum_viewtopic.php?39162 and how to assemble the HV diodes from 1n4007 strings in pvc tubes in oil: http://wiki.4hv.org/index.php/High_voltage_diode_-_50_kV,_30_A_peak

  3. What is charging the capacitor? How is it coupled? Voltage, frequency, etc? Also, why not a biggish Leyden Jar? e.g. metalized styrofoam cup Leyden jar in proximity to a Wimshurst machine. Maybe I am missing the point.

    My first reaction. Intersted in the beeswax improvement scheme.

  4. Foil capacitors like this only makes sense if you need microsecond range discharges or large continuous AC currents (tesla tank circuits).

    For millisecond range discharges for coilguns you want to use electrolytic capacitors, they have hugely better energy density.

    peteroptional : the surface area of the capacitor with the rolled foil is much larger … and thus also the capacitance.

    Karl : energy per volume and weight both kinda suck … both the Leyden jar and the styrofoam cup are poor in that respect. The rolled foil capacitor is probably the best you can DIY for larger capacitances … ideally you’d use Mylar or BOPP foil though, and putting it in a sealable pipe with oil and then vacuum pumping the pipe to get the air out of the roll would increase it’s reliability.

  5. To achieve decent HV performance, AND decent capacitance, yet minimise the effects of leakage / corona, it’s worthwhile revisiting how capacitor HV bushings are made. Central conductor, with Kraft paper / foil outer layers, the width of which gradually reduces as the outside of the coil is approached. This has the desired effect of managing field distribution, so reduces corona effects.

    The central terminal may be brought out via a standard bushing (not difficult to produce for those with a 3D printer), or you can cast your own bushing using epoxy or acrylic. The outer foil is terminated to ground via the casing (steel tube), with the central conductor isolated via a polyethylene disk. Fill with oil (you’ll need a vacuum pump to ensure best penetration), and away you go.

    Useful resource (HV in general) – http://www.slideshare.net/piyujwr/8-ee7-hve-lab-manual

    Capacitor Bushings – http://www.ohio-brass.com/literature/bushings/PCO-10techpaper_overview.pdf

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