Wednesday, July 1, 2009

Mjöllnir I: the beginnings of a tesla coil

Mjöllnir was Thor's hammer. According to Norse mythology, Thor's hammer had the power to throw lightning bolts. As Thor was the god of thunder, this seems only appropriate. The most identifiable parts of a Tesla coil (the tall secondary coil and large top load) are also roughly hammer shaped. For this reason (and the fact that it will release bolts of lightning) I felt Mjöllnir was a suitable name for my Tesla coil. When further research revealed Mjöllnir meant 'crusher', I pretty much had to go with it.

My goal for this project is to successfully design and build a Tesla coil at relatively low cost. All in all I hope to spend less than $300 total (right now I'm just over $100 for the NST, toroid components and cords). I hope to achieve a spark length of about 30in. with the largest theoretical length being 36.6 inches for a 15000volt, 30ma transformer. After completion of the basic coil, I expect my tinkering and alterations/improvements will increase the cost, but $300 will give me a good goal for the initial project.

To test my neon sign transformer (and to have a little fun in the process), I threw together a simple Jacob's ladder using an old coat hanger I stole from my roommate (thanks Ian ^_^). Note the plasma created between the two electrodes as the spark travels up the ladder.
After staring at a coat hanger and being mesmerized for far too long, I decided that my transformer was working. On to the top load.
Here is the toroid I created using 4" aluminum dryer ducting and an 8" pie pan I bought from ACE hardware. The size and shape of this top load gives a theoretical capacitance of 17.4 pF. My goal (to tune the coil to it's 1/4 wavelength frequency) was 17.2pF. I'm guessing the theoretical capacitance will be a bit high anyway so this should do just fine.

The next major step in the design of my coil will be to determine the appropriate tank capacitor size given the power output of the transformer. This will require a few calculations and some time spent online hunting down suitable high voltage capacitors. More to come later!


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