Re: NST resonance

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

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Jan,

At 08:06 PM 2/8/2001 +0100, you wrote:
>I am a newcomer to the list, so please forgive me if this is covered before! 

Welcome to the list!

>
>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.

This is a fascinating idea you have!  I know of no one ever doing this
before.  

A normal signal generator will not have enough current drive to magnetize
the big iron core of an NST but if you used a power amplifier to drive the
signal it should work fine.

>
>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.

An NST use current shunts to limit voltage.  This is a rather odd mechanism
and I don't think it has even been "proven" that an NST's impedance
matching is as simple as matching the resistance found from the nameplate
numbers to a capacitor value.  However, it does seem to work...


>
>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. 

WOW!!

>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.

Very interesting!  I have no idea what is going on...

>
>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.

I have the signal generator, probe toys, and I have all the parts for the
wide band low Z amplifier but it will not be assembled till later this
weekend (hope, hope...).  I should then be able to try your experiments
here too...

>
>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!

I just worry that you need enough current to magnetize the big iron core.
Other than that, this sounds like a brand new idea that should be super
cool to study!  Pumping high frequencies backward into an NST is also the
subject of John's and myself's post tonight.  Perhaps this method could
shed light on that too...

Cheers,

	Terry


>
>BTW, are there other coilers in my homeland Sweden?
>
>Jan
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