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ElectroMagnetic Compatibility and Tesla coils (fwd)



---------- Forwarded message ----------
Date: Fri, 24 Apr 1998 08:13:07 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: tesla-at-pupman-dot-com
Subject: ElectroMagnetic Compatibility and Tesla coils

I have just been engaged in an effort to design some equipment for EMC, and
it occurred to me that there has been a lot of discussion on the list about
TC's and their unfortunate effect on various electronic equipment: radios,
clocks, PC's, etc.  Some of this seems a bit apocryphal to me, although
there are significant effects.

Here is my concern: Virtually all modern electronic equipment (particularly
PC's) has to meet fairly stringent EMC requirements, both to keep
interference inside, but also to insure that it's behavior isn't corrupted
by outside signals (e.g. a TC output). At the simplest level this is
accomplished by putting it in a metal box, although in practice, this
doesn't even get close to meeting the fairly stringent CE and FCC
requirements.

I can see an electronic device being affected, operationally, by the RF
from TC, but as for actual damage, I find it more unlikely. Modern
Electrostatic Discharge (ESD) precautions on equipment make it pretty
immune to fairly high voltage (although low energy) zaps.

Of course, the cost of the item and whether it is perceived as "mission
critical" do have an effect on how much time the designer spend on EMC.
$2000 PC's would be better than a $10 clock radio for instance.

Has anyone made any *Calibrated* measurements of the EM fields at some
distance from a typical small/medium sized coil (less than 2 kVA)?

We all know that Greg's 130 kVA monster will destroy equipment, just from
ohmic heating on a direct strike. A simple ballpark far field calculation
(which is certainly not valid, given the long wavelength of the typical TC)
would show that a 500 VA (e.g. a NST) coil would produce an average RF
power density of around 10 Watts per square meter at a distance of 2
meters. This assumes a point source, isotropic radiator, which a TC that is
some very small fraction of a wavelength would be.  The peak RF power
would, of course, be higher, due to the pulsed nature of a TC.

Those of you that have actually destroyed equipment (first hand, not friend
of a friend of a friend who heard it from a post on the Tesla list), how
far away was it? what power TC were you running? was the cover off the
equipment? (I know you all never take the cover off your PC, right???) what
sorts of cables were hooked up, to where?

For reference, my little 100 watt coil (which does NOT have any line
filters) doesn't seem to have any effect on a properly assembled PC that is
sitting on the same bench, about 2 feet away. The "digital pbx" phones, on
the other hand, are screwed up at a distance of 20-30 feet, no doubt due to
the lousy installation of the phone lines which act as a huge antenna to
absorb the significant radiated RF power (a fluorescent tube lights up 10
feet away). ALthough, the phones are disrupted during operation, they are
not damaged.

Comments solicited...I'm going to do some better calculations on the fields
and the interference levels relative to FCC and CE specs for EMC.