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What is going on??



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.

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 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