|Jochen's High Voltage Page|
|Some homebrew caps. From left to right: PVC tube, yoghurt container, plate stack. Larger version in extra window|
One of the simplest possibilities is a 50cm long, 10cm diameter PVC tube (2-3mm thick walls) with aluminum foil both inside and outside. Leave about 10cm plastic uncovered at each end. This type has quite a high capacity and is usable with up to 30kV. The tube could withstand even more, but the edges of the alu foil will begin to spray then (i.e. show corona). Use this cap to get bright, high-current discharges from a TV-cascade (especially when it's driven by a simple mains transformer).
A more elaborate version (sealed airtight, no corona, no arcing) is the following:
Get three identical plastic yoghurt containers (PE or PP and rather thick; "Zott Sahnepudding" serves well and tastes o.k.:-). Also get adhesive aluminium tape (7cm wide). You may find this in DIY markets, it's original purpose is to seal heat-insulations on hot-water pipe. Put it on the inside and outside of one container. Fix some kind of terminal on each of the two others. Connect the terminals to the inside resp. outside alu layer with a narrow strip of alu foil. The three containers must still fit into one another! Fill some polyester resin (with hardener of course) into the "outer" and "inner" container and press all three into one another. Expect the polyester resin to splash out :-) When the resin has hardened (some hours), the cap is ready and can be tested. With a not too thin container, it should easily withstand 30kV. However, the capacity is low due to the small conducting surface (maybe some 100pF).
Important: When polyester resin reacts with the hardener (forming a polymer), it produces heat. If you use more hardener, the resin will harden faster (sounds logical, doesn't it?). However, it will also develop the same amount of heat in shorter time, and this means higher temperature! Actually, the thing can get so hot it melts the plastic containers. So better be patient.
The capacity of the above "yoghurt container" cap is by far too low for many purposes. This plate stack cap is intended to be used in a tesla coil circuit and a marx generator. It is designed to withstand at least 30kV DC or 20kV AC with a big safety margin. The dielectric is polyethylen (PE), therefore losses should be low even at higher frequencies. The conducting plates (aluminium foil) are all in parallel, resulting in low internal resistance and inductance. Both low losses and low resistance are important for tesla coil and pulse discharge use.
The stack consists of 20 insulating layers (each one in turn consisting of 10 sheets of 100microns PE foil) between 21 sheets of aluminium. I used the standard PE foil available in DIY markets (watch out for the thickness!), which is rather bad quality. Therefore you should have a close look at every sheet you cut, and reject those with visible flaws (grains, small holes). I rejected nearly every fourth sheet. The dimensions should be taylored to your needs (e.g. the container you plan to use). It is important to have the PE sheets somewhat larger than the aluminium ones - about 1cm at each edge, 2cm at the connection strips. When you cut the sheets, you will probably have most of them a mm or two shorter than planned. You can even out this by stacking every other PE sheet shifted a little. See the figures for the dimensions I used. With these dimensions, the cap will have about 2nF capacitance.
|Sheet dimensions (left) and stacking scheme (right) for my PE plate stack cap. Note that this is merely an example. Choose your own dimensions according to your needs.|
The complete stack must be tightly compressed to keep it in place and to remove air and increase capacitance. I achieved this by putting the stack between two slightly larger PVC plates, which were then tightened together with four screws at the corners. This proved to be not quite the perfect way, as PVC is too soft and bends.
To suppress corona and arcing over the foil edges, the stack is immersed in mineral oil (kerosene for its low viscosity, or special transformer oil). The biggest difficulty is to find a suitable tight container for this. I found PE sandwich containers useful. Unfortunately, they are very difficult to glue, so I "welded" the lid tight using a soldering iron. You have to be quite careful doing this, as you have to melt both lid and container for a good connection, but don't want to burn a hole. After sealing, make a small hole and fill with oil - leave as less air as possible. The small hole is easily welded tight afterwards (some extra PE to fill the hole). The PE sheets will soak with kerosene and expand somewhat, becoming a little uneven. This does not seem to have any undesirable effects. The PE container is not absolutely tight as well, as there was a constant smell of kerosene wherever I stored the caps.
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