On 08/08/2011 02:19, Peter Johansson wrote:
> That's certainly the easiest and cheapest way to go, but I*am*
> curious what is involved from the mathematical and design point of
> view.

I think the main thing here is the voltage/distance needed to ionise=20
air, which according to Wiki is around 30kV/cm. Here is a paper that=20
goes into some detail:
http://iopscience.iop.org/0022-3727/25/2/012/pdf/jd920212.pdf
Once you know what voltage/current you need, and for how long, the only=20
thing that remains is to design a circuit to produce it. I expect a=20
coil/tx will be needed for all but the smallest spark gaps although I've=20
no expert, you can calculate the voltage produced when current changes=20
in a coil with V =3D L(DI/Dt)
so for a 1mm spark gap and 3kV needed, a 3mH inductor with 1A flowing,=20
you'd want to shut off the current in 1us to get your 3kV.
I would maybe tinker with an old spark plug (or some known spark gap=20
distance) a battery and coil and see what you come up with.
With a switch where you have little control over DI/Dt, a resistor or=20
small cap across the coil will "tune" the maximum voltage depending on=20
initial current, rather than it relying on the coils parasitic R and C=20
to regulate the voltage if you just shut the current off "instantly" and=20
assume "infinite" voltage (i.e. you have a known quantity rather than=20
unknown if you have your 1A flowing initially in the coil, a 3K resistor=20
will make sure the voltage does not rise above 3kV)
A spice simulation would be good if you don't have any electronics at=20
hand (or to see the ideal characteristics)
 From there you could go on to designing the switching circuit depending=20
on how often you need the spark, but for a spud gun I think a=20
switch/button, battery, coil, resistor/cap should do fine, though I'm no=20
expert so I reserve the right to be talking complete rubbish :-)

--=20
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