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

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

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi All,

(snip from Steve)
> I have been following this grounding thread and I think that some 
> are missing the point with the RF ground. What you want is a low 
> impedance RF ground. A low DC resistance is not sufficient. The DC 
> resistance of the house ground is typically very low but it not an 
> adequate RF ground because it has a relatively high impedance at 
> typical coil operating frequencies. Remember that the operating 
> frequency of the coil is about 100 KHz. This relatively high 
> frequency needs a low impedance ground otherwise very 
> large voltage drops will develop along the ground conductor. 

> A low DC resistance is not sufficient. The DC 
> resistance of the house ground is typically very low but it not an 
> adequate RF ground because it has a relatively high impedance at 
> typical coil operating frequencies.

....8 feet of copper ground rod isn't bad, and neither is the 12
gauge (typical) bare copper ground wire going to the breaker box, and
then to the ground. It's not bad at all for our purposes.

The reason we try to stay away from it, is that the fast changes in
current couple energy onto the other conductors...live and neutral,
and also the main V drop on the ground can be in the path too. It all
equates to not only annoying RFI/EMI/TVI, but damaging interference
as well. This doesn't mean that the house ground isn't a good ground.
It just means that it's a Tesla Coil, and should have its own.

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. 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.

Along those lines, some switched mode power supplies operate at
frequencies as low as 20kHz. But they switch tens of amps in
nanoseconds....and if you ignore proper grounding techniques,
shielding, and circuit layout in general because of the low operating
frequency, you're hosed.

Anyhow, what do all these big voltage spikes on a Tesla Coil ground
mean?

Not much unless you're worried about interference.

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com