Rolled foil capacitor - 60 kV, 3.5 nF

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The capacitors are based on PP copolymer foils used in laser copiers and printers. Each sheet should stand 14 kV and depending on how many sheets you use you can get different capacitance (two sheets 9 nF - six sheets 3.5 nF). The foil is thick kitchen aluminium-foil (for grilling).

Total number of transparency sheets are 15 per capacitor (2 for covering each aluminium-sheet, 5 between the covered aluminium-sheets, 5 on top of the second covered aluminium-sheet and one more for final rolling)

The size of the aluminium-sheets is 22x15.5 cm

The PP copolymer should be resistent against ozone. These capacitors have no failures after more than 100 shots in a Marx generator. They have also worked in a small Tesla coil without breakdown for quite a long time. But of course this is not professional product so don't expect the lifetime of a commercial capacitor.


The required tools and supplies.
Cut the foil to size using a sharp knife.
Use a smooth rolling pin on a smooth surface to get rid of as much air as possible. Air bubbles will cause corona and ozone to form above a certain voltage/frequency.
Interleave the plastic sheets (5 pcs) between and above the already made doublets with aluminium foils between.
Use the rolling pin again and prepare the cables by removing 5-6 cm of the isolation.
Find the upper aluminium foil and place the first cable on it. Let only the unisolated part of the cable in the contact with the aluminium foil.
Tricky part one: Roll the sheets slowly...
Find the uppermost sheet (the lowest one before rolling).
Tricky part two: Unroll the uppermost sheet until the second aluminium foil is exposed. Place the second cable and roll it back. Hold the capacitor properly during this process otherwise it can unroll very quickly in your hand and you have to start all over again!
Use one or two more sheets to roll over to protect unrolling and it also makes the upper aluminium foil more than one sheet isolated.
Use the glue tape to protect against unrolling.
Use the scissors and follow the upper cable with the tip and cut a track for it.
Put the cable through the track you have cut with the scissors. This is important for gluing the sides of the capacitor.
Cut the same track for the inner cable...
Place the inner cable inside the capacitor. You move it back after the ends are glued.
Use the glue gun to put some more easy-to-melt plastic at the ends and fix and isolate the cables with the glue properly!
Do the same on the other end.
Final appearance before the important heating stage.
Tricky part three: Push the end of the capacitor against preheated metal plate (use an electric cooker) to melt the PP foils and the hot-glue into a compact structure. This is very improtant part which needs a lot of focus. It will protect the cap before the side spark-overs. (Side spark-overs are the biggest risk for the final capacitor).
Remember to remove the inner cable. In case the whole is blinded after the previous step make a whole with a pencil while the plastic is still hot and soft.
Use more hot-glue to fix and isolate the cables. It is good idea to put additional PVC or silicon tubing around the cables and glue it to the main body of the capacitor.
Check the capacitance.
Test setting for the capacitor. Use the spark gap distance to set the final voltage to approx. 50-60 kV DC (5-6 cm distance).
Put on your earplugs and sunglasses and let the capacitor run for while. Remember to discharge the capacitor properly after the test (and any other) run. It can store the charge for a long time and touching both ends of a 3.5 nF capacitor charged to 50 kV is shocking experience!


Projects where these capacitors are being used

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