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Re: What is going on??

Hi Reinhard,

> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> 
> 
> Hello All,
> 
> After reading the posts from Alex, Jon and Gary about
> resonance recharging, I tried something. However, I got
> more questions than answers.... :o(.
> 
> Please use courier font to view the experimental setup:
> 
> O--------p||s------|
> |        p||s      |
> |        p||s      |
> sig.     p||s     ---
> gen.     p||s     --- cap
> |        p||s      |
> |        p||s      |
> O--------p||s------|
> 
> 
> My xformer is a 7.5kV 75mA NST. The turns ratio is 1:32. I
> applied a variable signal generator to the input of my NST.
> The p-p voltage of the sine wave was 5V. The capacitorless
> output of my NST was 160V (no surprise here).
> 
> The impedance of this xformer is 99999.9 ohms. For a XL=XC
> cap (at 50Hz) I would need a 31.83nF cap. On the secondary
> side, the coil to coil resistance was 1414 ohms, so the Q of the
> xformer would be ~70. Core losses should be non-exsistant
> in my case (because of the low current flowing).
> 
> Now comes the surprise:
> With 31.83nF across the NST, I get a maximum output voltage
> of 190V, but at 25Hz !?!?. For a maximum voltage (193V in this
> case) at 50Hz, I found (experimentally) that I needed a 6nF cap.
> What the heck is going on? The voltage of my signal generator
> does NOT drop during any stage of the experiment. Interestingly
> enough at resonace, the input voltage (sig gen) actually rises.
> As the input is "dampend" by a factor of 1:32 (winding ratio), I find
> the voltage rise of about 2V (that|s a 40% increase) absolutely
> incredible. Does anyone have an explaination of the discrepancy
> between the calc|d and the experimentally found reso cap in this
> case???? The voltage increase 160V vs. 193V was not as dramatic
> as I had hoped.

If the output voltage of your signal generator is able to change in 
that fashion I would say that its internal impedance is influencing 
your results. The mains presents a near zero Ohm impedance to the 
transformer primary. I have done exactly the test you describe using 
a sig gen with a Zout of 7 Ohms and found the results to be those 
expected from the calcs.

 > To make it even more strange, I would 
like to report the results > of a similar experiment that followed:
> 
> The hookup is as above, except this time, I used a 2V/50Hz
> AC source (step down transformer). The output of the NST
> transformer alone was around 68V. I connected my 31nF
> cap across the output and the voltage jumped to 280Vac.

Now the source impedance is making the results more closely emulate 
what you would expect.
 
> Now for the next surprise:
> I used 18nF caps in a series/parallel arrangement and
> found the highest voltage of 654Vac achieved with a 50.2nF
> cap More ?!?!?. This, too, does NOT match the calculated
> value of a mains reso cap (31.8nF). However, the
> increase in voltage 68V vs 654V is what I would expect
> from a resonance circuit.
> 
> Now I am TOTALLY confused.
> 
> Of course, in a true TC tank circuit, one would have to take
> the series inductance of the primary coil into account.
> However, this is very small, when compared to the
> inductance of the HV transformer.
> 
> So how DOES one go about finding a true mains reso
> cap? What difference does the spark gap make?
> RSG vs. static (for equal break rates, of course).
> 
> Scratchin| my head for sure!!
> 
> Coiler questions from germany,
> Reinhard

Some of the answers come about because the circulating currents in 
the secondary are coupled via the core back to the primary. I think 
it is always best to use the lowest source impedance you can for all 
tests including measuring resonator Q by base feeding. I built a 
generator especially with this in mind.

Malcolm