Re: RF Ground, House Ground, Ground....

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


> The operating frequency is of minor concern when dealing with large
> peak currents in the ground path. The simusoidal, 100kHz voltages
> produced across the ground wire DO follow the inductive reactance
> formula and DC resistance (together which is impedance as noted).
> 
> But much more importantly, they follow Vl = L (di/dt).

> 
> If the base current in your secondary coil hits 100A in microseconds,
> you can bet that there will be significant voltages formed across the
> L and R of the ground wire.

Hmmm.. what really is the di/dt?  Take a fairly good sized coil, say 100 pF
in the top load, resonating at 100 kHz.  If we let the top load get as high
as 1MV (good reasons why it's probably lower), it would store 50 joules..
To store that energy in the inductor (i.e. the peak secondary current) the
peak current will be around 60-70 Amps. We're at 100 kHz, so the time to
get from zero to peak is on the order of 2.5 uSec.. the slope is steeper at
the zero crossing, but, a di/dt of 20-30 A/usec is probably reasonable...

Taking 10 uH as an inductance of a hypothetical ground wire, V = L di/dt
would be 10E-6*30E6 = 300Volts... which is what I mentioned in a previous
post (a few hundred volts)... and that's for a BIG coil... 


 Regardless of the frequency. There is a
> voltage transient on ground at 120Hz (give or take) if you have a
> static spark gap. The 100 or 500kHz ringdown is nice, smooth, and
> pretty compared to what happens every time the spark gap fires. Large
> currents switched in a short time === VERY large voltages. This is
> why you want a low Z (impedance) ground.


Here, though, you are talking about the primary circuit, which isn't
connected to the ground.


>From an EMI standpoint, the real thing is the secondary discharge current