Re: NST resonance

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jan,
        A way of investigating the interaction between the NST and a 
capacitance hooked to its secondary is to short the primary (it is 
normally connected to a voltage source) and use a high impedance 
generator shunting the secondary and caps with a scope also hooked 
across it. You should see the different resonances occurring then. 
You are right about the NST not being a well behaved animal. It is 
designed to be a constant current source. Measuring transformers of 
any kind with a scope and sig gen is an exercise fraught with 
difficulties and traps and you have to be aware of these in order to 
apply the correct techniques for the transformer you wish to measure. 
The most insidious problem that will cause trouble is distributed 
capacitances in the windings giving resonances and false readings, 
particularly if the windings are unloaded. Even "ideal" transformers 
with no airgaps/shunts in the core will give false readings. To 
measure the turns ratio for such a transformer the most reliable 
technique I use is to connect a very low Z sig gen across the entire 
winding with the most turns and measure the voltage across those with 
the least turns. The generator effectively shunts the high 
capacitances that can be present in the high-turns windings. 

Regards,
malcolm

On 8 Feb 01, at 16:58, Tesla list wrote:

> 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
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