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Re: secondaries and caps



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 01:09 PM 3/6/2006, you wrote:
Original poster: ben eells <squeels2171@xxxxxxxxx>

I'm sure this subject has probably been discussed here several times, but I haven't been able to find the answers I'm looking for. When designing a coil, what dictates the length, diameter, and wire gauge of your secondary, and how do you go about finding a capacitor that will work best with your transformer?

To a certain extent, it depends on the materials you have available. Generally, you start with the transformer (because that's what's hardest to come by).. As you say, height is 4 or 5 times the diameter, and you want diameters ranging from 3-4" for the low end (a 30mA neon sign transformer) to bigger than a foot for the pole transformer. However, the height might be different if you happen upon a piece of scrap that's useful (e.g. if you find a 4 ft long piece of pipe 1 ft in diameter, you might go with the secondary being 3.5 ft long.

Then, you figure out what size wire to wind the secondary with, typically shooting for a diameter that will give you 700-1200 turns on your secondary.

Figure out a toroid size that complements the secondary: inner diameter is usuallybigger than the diameter of the secondary so it will fit on top, the "tube" size of the toroid is usually driven by what you've got available to make it with (e.g. 4" dryer vent hose). There are a lot of toroids on small coils made from piepans and vent hose.

Use one of the programs to determine the self resonant frequency of the secondary.

Then, design a primary to match. There are a few primary capacitances that are "bad", especially with a neon sign transformer. You don't want what's called a resonant combination (that's where the inductance of the transformer resonates with the primary C at line frequency (60Hz)) Go either higher or lower. Most of the time, you'll find that practical capacitors only are available in discrete values (i.e. if you string up enough 0.15 uF caps to get the voltage rating, you have a string of say, 0.015 uF. So, you can get 0.015, 0.030, 0.045, etc.)

From the value of the C, figure out what the primary L needs to be to resonate with the secondary.

Build it, fool with it, be dis-satisfied, and rebuild it, fool with it, repeat, over and over and over again until your friends and family leave you in disgust, as your hobby has become all consuming, and you wind up babbling to yourself walking down the street as an old man dragging a shopping cart full of scrap wire, PVC sewer pipe, and chicken wire.



I'm sure many people here have varying opinions on what works best so I would like to hear them. I've found length to width ratios of 4/1 or 3/1 but nothing saying "x diameter works best with y setup." The best answer I've found for the cap question is to consult a tesla coil design program, which is good advice, but I'd like to know the science behind it. Thanks.