Re: Secondary resonant frequency vs. Skin Effect

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

Original poster: dhmccauley-at-spacecatlighting-dot-com 




 > Hi All,
 >
 > I am new to the list, but jus  twanted to warn of the low frequency which
 > can be typical of a large coil. I built my first coil 28 years ago. It was
 > 20" diameter x 48" and easily produced a 50-60" discharge. The resonant
 > frequency was 278Khz. This is actually a very low frequency and "skin
 > effect" does not apply at these low freqencies. In my opinion it is the Q
 > of the secondary that produces the high voltage rather than a turns ratio.


On the contrary.  Skin depth applies equally well at those frequencies.
However, the skin depth is related to
conductivity.  Whereas highly conductive materials have shallow skin depths
where skin effect is greater, less conductive materials
(such as human flesh) have much higher skin depths and less skin effect.

Skin Depth is approximately equal to:  1 / (sqrt(f*pi*mu*conductivity)
(where skin depth is defined as the point where current flow decreases to
about 37%)

For a round frequency of 100kHz, the skin depth of copper would be about
0.008 inches.  Based on the average resistance of human flesh,
the skin depth would be about 10.5 feet.  So unless you were enormous, skin
effect is basically non-existance for the human body.

Also, the gain of a standard tesla coil (magnetically coupled) is equal to
the SQRT (primary capacitance / secondary capacitance) - of course in
actual practice, the theoretical gain may never be reached due to losses,
premature break-out etc...

Dan