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Re: Impedance

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Sam,

Your stated reasons have some truth to them. Problems using the mains ground to ground the secondary base depend on several factors:

1. The power of the coil. Often, small coils are grounded to the mains and cause no problems. RF current in small coils is much lower than high power coils.

2. The use of a counterpoise connected to the secondary base can help reduce the RF current in the mains ground.

3. Whether or not you are on an independent branch of your breaker box shared with nothing else, RF voltages on that branch's ground will be greatly isolated from other branches. The breaker box is often earth grounded at the box (where safety ground and neutral become common).

Inductance of a wire is affected by the gauge to some degree that may or may not be significant. How the ground impedance affects the TC operation depends on the coil design (the impedance of the coil as compared to the impedance of the grounding). Yes, if the ground impedance is large enough, flashover to the primary could be a problem and tuning could be affected.

Specific answers to your question below are:

1) In general, the RF current at the base of the secondary is larger than at the top. The voltage at the top is the highest and the voltage at the base depends on how well grounded your are.

2) The magnetic field created by a wire carrying a fixed current I will be larger at the surface of a thinner wire than a thicker one. H=I/(2*pi*r). When determining the flux generated, the higher H field will result in more flux and hence more inductance.

3) "Solid ground" definition sorta depends on the application, but in this case, I would think it would be earth ground.

Gerry R.




Original poster: d a <btoc3000@xxxxxxxxxxxx>


I read this from: <http://users.tkk.fi/~jwagner/tesla/tc-plans.htm>http://users.tkk.fi/~jwagner/tesla/tc-plans.htm and I have some queries...

The Tesla coil secondary RF ground must be an own ground separate from mains ground. Reasons:

1) this separate ground will sink RF current and voltage, which - if you used mains ground - would fry all equipment in your house, even the surge protectors.

2) also, the mains ground wire is way too thin, and would have a considerable impedance at the high frequencies present. High impedance is not nice, as the TC base wouldn't be properly grounded then, and the wire would have a voltage drop from some 10s of kV on the base to 0V somewhere along the wire - i.e. the thin wire could still have a few kV some meters away from the coil base (corona, electrocution, damaged equipment etc).

3) the other thing that is bad about a high impedance ground is that the zero voltage node will shift down along the wire to the place where the solid ground is. This will cause a phase shift also in the TC secondary, meaning you could get breakouts from any part along the coil, not just the top.

for 1) Am I right to say that RF current and voltage is different from the HV current and voltage that we are getting at the secondary topload?

for 2) Why would thin wire have a considerable impedance at the high frequencies? Is there a formula for this?

for 3) In layman terms, does it mean that having a poor ground will cause the zero voltage node to "shift" to the solid ground. What is the "solid ground" here?

Thank you so much in advance

Sam