platecap

This capacitor is constructed by alternately stacking sheets of aluminimum (kitchen) foil and polyethylene. The aluminium foil sheets were approximately 12x14 inches active area. The 14 inch side was extended to enable the plates to be paralleled together and to be attached to the terminals. This was achieved by two steel clamping strips with through bolts at each end. To connect to the terminals a short car starter motor cable was attached to one of the through bolts at each end. The polyethylene sheets were cut to allow an overlap of 1.5 inches around the edge of each aluminium plate. I have seen it recommended that at least two inches of overlap should be allowed but we have had no problems so far.

The capacitor is formed by connecting two smaller caps of equal value in series. This was done so that each capacitor would be subjected to less than 7500 volts. This value was suggested as the value at which corona becomes a problem. When the two capacitors are connected together in series together the total dielectric thickness was 0.090 inches, 0.045 inches from each capacitor. This was to allow us to run at 10KV to 15 KV with some safety margin. Sufficient aluminium plates were stacked to give a total capacitance of 0.015 micro farads. The whole assembly was sealed into a heavily varnished wooden box (twice as it leaked the first time!).

A vacuum pump was attached to the box and the assembly was evacuated for about 20 minutes to remove as much air as possible. The vacuum was provided by a very tired Speedy Vac pump. The box was then back filled with transformer oil while still under vacuum. We could tell that the inside of the box was at reduced pressure by the way the large sides of the box were pushed in. When the plates were covered with oil the box was held at reduced pressure to degas the oil for another 20 minutes or so. It was then sealed and left alone for several weeks before it was tested.

When building one of these darling devices it is worth noting that it is a whole lot of work for a little capacitance. Before you finish you will wish you had not started. The finished article is heavy and bulky. It was however an enormous step up from the beer bottle capacitor we started off with (but not as much fun collecting the materials!).

I have not given complete details on how to arrive at the capacitance value we needed. This is for two reasons;

i/ You really should read up on this in the Tesla List archives (www.pupman.com) to read about others peoples experiences, dielectric strength etc, etc.

ii/ I cannot lay my hands on my design notes at the moment.

If I were going to do this again I would not bother!!! It is a much better idea (smaller, cheaper and less aggravating) to build a Multi Mini Capacitor or MMC.