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Re: Another grounding question
Tesla List wrote:
>
> Original Poster: Adam <psycho-at-tradewind-dot-net>
>
> Most people say to use "copper strap" to connect the secondary to the
> ground, and to connect the ground rods to each other. What exactly is
> copper strap. I always assumed it was a roll of thin copper, but I've
> seen it in other places referred to as copper roof flashing cut into
> strips and soldered together. Doesn't this get expensive? Would it be
> cheaper to just use welding cable? And easier too, because wire is
> easier to work with? What's the trade-off in quality?
>
> Thanks a lot,
> Adam
If (and only if) you are going to obssess about RF impedance, the
inductance of a flat strip is lower than a round conductor of the same
cross sectional area. (note that the L of a round conductor of the same
diameter as the width of the strip is comparable)
Even better, from the impedance standpoint, is Litz wire (short for
Litzendraht), which is a bunch of separate insulated strands braided
together.
In any case, go ahead and use a bunch of parallel strands of regular old
#14 house wire. Other losses in your system will probably dominate. At
the frequencies a typical TC operates at, you don't have to worry about
the ground wire being a significant fraction of a wavelength (at 100
kHz, the wavelength is 3000 meters).
Say you've got a 20 foot length of wire at 1 uH/meter inductance (a good
working number for wires in general). So, your ground connection is 6
uH, or, at 200 kHz, an impedance of 7.5 ohms. (X=2 *pi * f * L). If that
seems a bit high, then run two parallel wires: now it is 3 ohms, 4
parallel wires gets you 1.5 ohms, etc....
Then, figure out how much voltage there will be across that wire (i.e.
what is the voltage at the bottom of the secondary). To do this, you'll
need to know the Peak RF current in the secondary. If it was, say 50
amps (which is probably high), and you were running the single wire,
above, the bottom of the secondary would be 375 volts. I wouldn't touch
the bottom of the coil if I were separately grounded, but the insulation
on the ground wire would probably hold.
Calculation of secondary current.... Assume coil secondary L= 60 mH and
C = 40 pF (fres =100-200 kHz). Assume Vpk on secondary = 1 MV. Energy in
secondary is CV^2/2 = 20 J.. turning it around using Energy = LI^2/2
gives Isec = 26 Amps. This is assuming no losses, etc.etc.etc.