NST resonance

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

Original poster: "Jan Ohlsson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jan.ohlsson-at-mbox319.swipnet.se>

I am a newcomer to the list, so please forgive me if this is covered before! 

To investigate the resonance of a 8/60 NST I parallelled the secondary with
MMC:s from 1.6 nF to 6.8 nF and made impedance sweeps with an audio sweep
generator of the primary impedance. The impedance curve of the primary
showed as expected a sharp impedance peak of high value and then a gentle
rising caracteristic towards the upper end of the audio spectrum. The last
probably due to high frequency losses in the NST core. 

What surprised me was that the center frequency of the resonance peak did
not change as expected if I, for example, quadrupled the value of the MMC.
That should theoretically give half  the resonance frequency, but the
actual value was higher than that.

I can only come to the conclusion that there are other sources of
capacitance at work, probably within the NST, of the order of a couple of
nanofarads. That would make simple calculations of NST/MMC resonances
unpredictable.

I also made some sine sweeps with a constant current source to the primary
and registered the secondary voltage through a very high impedance voltage
divider, the MMC still in parallell. What took me by total surprise was
that the curves showed several resonances without a simple arithmetic
connection. The lower part of the audio spectrum was more or less filled
with resonances of different magnitudes. To be able to exclude
instabilities in the audio amplifier that was used to feed the NST primary
I tired the same thing with the audio generators 300 ohms output directly
to the primary, but that gave me the same kind of curves. I also changed
the NST to a 4/45 and tried different values of the MMC, but the resonances
just moved a bit, the general character of the frequency response was the
same with multiple resonances. These resonances donīt seem to couple to the
primary, as there is just one resonance evidenced by the primary resonance
sweep.

Have others on the list made similar tests with a sweep generator on NST:s
and caps? My result seems to point out that the NST do not behave as the
lumped component models commonly used for transformers suggest. If that is
the case many values of primary caps could give a resonant condition with a
specific NST. One more argument for a good RCR filter between NST and
primary, in that case.

Please feel free to critisize my methods and conclusions, I am after all a
newcomer to coiling. And to those of you who wonder: Yes, I have fried a
NST before I started thinking and set up the sweep generator!

BTW, are there other coilers in my homeland Sweden?

Jan