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Subject: Discharge impedance of a CW Tesla coil
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Original Poster: Terry Fritz
Hi All,
I have been playing with my patched together RF generator and measurement
equipment. Today I took off work early to do something important. ;-)) I
was able to measure the impedance of the brush discharge of my coil run
from the generator in CW (continuous wave) mode.
The secondary is 1180 turns of #24 wire on a 4.25 diameter PVC form 26.125
inches long with an inductance of 22.1mH (from my little 9kV/30mA coil).
The primary is 10 turns of 0.092 inch Litz that is 6 inches in diameter.
That is about 2uH. I used the generator to drive the coil at 100, 200,
300, and 400 watts delivered power compensating for the small reflected
power. All the coil components stayed cool so I assume almost all the real
power was going to the discharge. I can adjust the frequency to get the
lowest reflected power so it is very well in tune at each level. I ran out
of tuning range at 400 watts with the fiber optic probe transducer on the
top of the coil (added top capacitance). I will need to wind a new
slightly higher frequency secondary to get the generator's full 900 watts
forward power in tune with the probe in place. Hopefully, nothing will
melt down at that level...
It appears that the discharge is composed of a resistive region a few
inches in diameter (the part you see) that provides a relatively low
resistance path to a capacitive sphere around this discharge. Thus the
discharge is a real resistor in series with a capacitance around the discharge.
Obviously, I need to write one of my papers on this to fully explain all
the details, but for a point discharge at around 330kHz, I got a discharge
impedance of:
Z = 20K + 3K ohms / 100watts delivered (real resistance) in series with
0.58pF + 0.070 pF / 100 watts delivered (capacitance).
The actual numbers are as follows:
Delivered power Resistance Capacitance
100 22887 0.665pF
200 26122 0.754
300 29409 0.827
400* 30779 0.892
*The 400 watt measurement is messed up because the frequency dropped too
low for the generator to tune properly.
I know very little about CW coils (but am learning fast ;-)).
I noticed that this coil produces almost no ozone or other noxious fumes in
operation. I was very surprised by this! Perhaps all CW coils are ozone
free and I just missed that bit of info. Not gassing out the basement
during this testing is VERY pleasant as is the quiet discharge!! It is
very cool but does not strike the "fear" those big disruptive coil
streamers, gaps, and all do...
The current at the top and bottom of the coil are almost perfectly in phase
but there is about a 60nS shift. 20nS of this should be caused by the load
impedance having a slight angle (87.7Deg. at 640K) and perhaps the other
40nS is due to the very slight transmission line effects. Apparently, the
top and bottom currents in the secondary inductor of CW coils are just as
phase locked as disruptive coils. The last coffin nail for those old 1/4
wave wire length theories ;-))
Much more to come on all this but I wanted to let everyone know about this
fun project to this point. Obviously impedance matching, computer models
and all that will be greatly aided by such information. In many ways, CW
coils are far simpler than disruptive coils. I was very surprised that at
300 watts I was able to tune the coil with ZERO watts reflected (the
generator has a 50 ohm output through about 10 feet of RG-8). My primitive
computer models suggested such things but I didn't believe them...
Apparently, primary inductance is not at all critical but coupling is...
Of course, the RF generator is much more predictable than tubes (at least
for me)...
Much much fun was had today! ;-)))
Cheers,
Terry