date 21 October 2000
Revision date 14 March 2001
General: The primary circuit capacitor (Cp) is the heart of the Tesla Coil. It must withstand 8-20kV ac and be relatively lossless at RF frequencies of 50-500kHz. Furthermore, it must be able to handle large RF currents of 50 to several hundreds of amperes. Such capacitors are not normally available commercially, so most Tesla Coil builders resort to building them to meet these specifications. The most popular types of construction have been sandwich and Swiss roll using heavy duty aluminium foil and polyethylene sheet. Such units are usually immersed in mineral oil to prevent corona discharge and to keep them cool.
Recently, coilers started putting together commercially available pulse current grade capacitors in series parallel arrangements to meet the voltage and capacitance requirements. Such capacitors work well at RF frequencies and individually have voltage ratings in excess of a 1000 Volts. They are connected in series strings to provide the necessary voltage rating, and the strings are then connected in parallel to provide the necessary capacity. The capacitors are of tubular construcion, use polypropolene dielectric, and their axial leads are heavy duty for pulse current operation.
Home Made Polyethylene Capacitors: A large amount of care and patience is needed to build these capacitors. Single units built to operate at around 10kV are vulnerable to corona. The most vulnerable point is the air pocket between the dielectric and foil where corona occurs at high voltages. It eventually destroys the dielectric and results in short circuits, a common failure amongst these capacitors. It is far better to build larger capacity units and place them in series to distribute the voltage across them. Destructive corona is virtually none existent at a voltages below 5kV.
Sandwich Type Construction Example: The following capacitor, 0.026uF, is good only for light duty tesla coil operation, It has been used with an 8kV 35mA Neon Sign Transformer with modest success. It did arc externally at the ends and resulted in some foil damage. I subsequently immersed the entire capacitor in high quality mineral oil to solved that particular problem. The mineral oil soaked immediately into the Conti board (chip board), but did not weaken it. A slight swelling of the polyethylene did occur.
a. Purchasing polyethylene sheet: Ask for clear heavy duty (5 to 6 mils thick). It is sold by the metre off the roll. It will actually be 3 metres wide folded along its length in four places. So buying about four metres will give you 12 square metres, enough to build the above capacitor.
b. Cutting the polyethylene sheet: Unroll the sheet on a non-abrasive surface and cut it along the folds. This will give you 4 half metre (20") wide strips 4 metres long and one strip in 1 metre wide (40") and 4 metres long. From here on you will need to measure accurately and and cut carefully to obtain the necessary 275 pieces (4 1/2" x 9") of dielectric. For this you will need a Stanley knife with fresh new blades, a large carpenter's metal square (Iused one 15" x 24"), and some plastic electrical tape. The tape is used for marking your measurement on the polyethylene. To make the long strips manageable, I cut each of them to 9" wide and about 40" long. The edges of the purchased material are not even, so beware. Make certain the width is within 1/32" of 4 1/2 inches and one end is properly squared up with the sides.
c. Eight 4 1/2" x 9" can now be cut from each strip with relative ease using a cutting block as shown below.
The nine inch wide polyethylene strip is slipped under the steel rule and pushed against the butt for cutting with a Stanley knife. Double adhesive wall fasteners are used to hold the steel rule in place.
d. The steel rule can be repositioned to 3 1/2 inches for cutting the aluminium foil tsheets. To obtain clean cuts, use fresh new Stanley knife replacement blades, and hold the knife at a low angle when cutting.
A relatively easy and inexpensive method for making a capacitor suitable for Tesla Coils is to use a quantity of commercially available capacitors connected in series parallel combinations. Most experienced Tesla Coilers are going this route now. Typical capacitors are those designed for pulse applications. Generally they are rated 0.056uF or 0.068 uF at 1600 volt although you may find other suitable values around. Their dielectric is polyproponal which has reasonably good RF characteristics at Tesla Coil frequencies.
When you connect capacitors of the same value in series, the dc voltage rating of the series string is the sum of the voltages of the individual capaitors. For example, the voltage rating of four 1600 volt 0.056uF capacitors is 6400 volts dc. However, the string capacity is 1/4th that of the individual capacitors or .014uF (14nF). When you put the strings in parallel, you increase the capacity. three strings of 0.014uF capacitors in parallel gives you 0.042uF at 6400 volts. See figure 1 at right..
The photo , Figure 2, is an MMC arrangement used by the author. The strings are stacked to give capacitances in approximately 5nF increments at 22,400V dc. Paxolin, a high dielectric strength material is used to mount capacitors. The capacitors are connected and soldered together in a zig zap path along the board.
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