RE: Primary dummy-loading

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Chuck:

For my coil, the 164uH primary resonates with the .02uF cap at 88 KHz.
The primary reactance at 88KHz is 2*pi*F*L or about 90 Ohms.  The
120V/500W lamps each have a 29 Ohm hot-resistance, or 58 Ohms for two.
So, at least they're in the ballpark.  I could add a 3rd series lamp for
a very close impadance match, but I don't think a close match is really
important for what I'm trying to accomplish.  I just want to dump the
cap's charge after each charging cycle and ensure that the peak
discharge current isn't damaging to the cap.  A different impedance only
means that my cap sees a different peak discharge current compared to
what it would see with a normal primary.  In the case of an impedance
"mismatch" there won't be any signal reflections as in RF impedance
matching.  The peak voltage is certainly no mystery - it is whatever the
peak cap charging voltage is, and with a resistive load, the waveforms
are simple exponential decays, not RF oscillations.

While a "Cantenna" dummy RF load may work, I'm not sure that it can
sustain 25KV+ peak voltages.

Yesterday I did manage to replace my primary with the two lamps and they
did light VERY brightly, and I only had my variac turned up to about
60%.  I would have cranked it up further but my safety gap across the
SRSG was firing at a separation of ~0.75" and I wanted to measure the
actual charging voltage with a scope before going any further.  But the
lamps did show a very clear indication of optimal SRSG phase, as I'm
able to mechanically adjust the motor case rotation as it's running.

Regards, Gary Lau
MA, USA

> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

>Hello all, This is an intriguing problem, but have hesitated jumping in
on
>this thread until the notion of a "ham" dummy load was mentioned. Most
>antenna dummy loads have a resistive impedance of 50 ohms. It appears
that
>you would need to substitute a secondary coil with another coil to
match the
>primary coil impedances to the dummy load and use the same coefficient
of
>coupling. However, by doing this, (no mean task)  there would be  high
peak
>RF currents and voltages to deal with. . For example, consider 4 joules
per
>bang, and the estimated duration of energy dissappation being 50usec (a
>conservative estimate), the peak power would be 80kW. With a 50 ohm
load,
>this works out to 40 amps rms during this 50usec interval. The RF
voltage
>during this same interval would be 2000V rms. Based on this, I think a
dummy
>load rated at  a kW will probably handle this. The interconnecting
coaxial
>cable needs to be considered also for these kinds of currents. If there
was
>a mismatch between the Tesla Coil primary and the simulated load, the
80kW
>of peak power would be ringing in and out and back and forth until it
is all
>dissipated.  I hope this helps.
>
>Chuck