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Repost of :Strange NSTs (was Re: Found it ! :o(( )
Hello All,
Here is a re-send of the mail that got scrambled somehow... (:o(?)
John Freau, Jim Lux et all who got this mail can hit delete.....
Hello Jon, Terry, Gary, Malcom, all,
I wrote:
> Now, while I know what I did wrong, I find Jon Rosenstiel´s
> problem (30mA vs 48mA NSTs, yet exact same resocap) a
> bit hard to understand, because I could actually adjust the
> NST (via it´s movable shunt) to give me the best resonant
> charging condition (i.e: I could match the NST TO THE CAP).
> I tried various cap sizes and could always adjust the NST for
> highest voltage rise (being ~650V in all cases, with 2Vac on
> the primary side. I find that you will have to actually MEASURE
> the output current (to find the proper resocap), rather than
> trust the faceplate ratings. Strangely enough Jon´s NST(s) didn´t
> seem to care what the real output current is. Any ideas ?
> BTW Jon: Did you measure the 48 vs. 30mA?
Jon wrote:
>Tonight I measured a stock Jefferson 15/30 and compared it to my
>modified Jefferson 15/48. To measure the current I put a 120 ohm
>resistor across the secondary, measured the voltage across it and
>did the math. Results are 30mA and 48mA. I also reran the tests
>with various values of capacitors across
>the secondary, 2V input. Everything came out the same as before.
>My gut instinct tells me that at 2V input and open circuit, (except for
>the meter), output the shunts wouldn't make any difference, but you
>found that they do? It would seem that in order to properly run this
>test the >transformer would have to be putting out it's maximum
>power. The output load in my test must be only a few microamps,
>(the meter draw). The resonant cap value is derived using the
> transformer impedance, and the trans impedance is E/I...
>Any thoughts?
The equation E/I would seem to tell us, the higher the delivered current
(for equal voltage), the lower the transformer impedance is. This means,
for resonant charging, the impedance of the cap has to be lower, too
(bigger cap for a given xmfr voltage). This is exactly what I have found
experimentally. Yet, in your case, this doesn´t seem to hold true. Very
strange indeed. However, I´m not sure if you can consider a NST and
a matched cap in parallel an open circuit, because as the XC comes
closer to XL, the current is slowly maximized. At the point where XL is
equal to XC, you have maximum current flowing. The impedances are
equal, but in "opposite direction", so they cancel each other out. This
means the only losses left in the circuit are the resistive or DC
losses. The problem (or something I have NEVER really understood)
with the NST faceplate rating is that V*A (E*I) gives you the
faceplate VA rating, BUT V is measured in no load condition and I(A)
is measured in a dead short.
Example:
------------
NST VA rating: 562W
NST Voltage 7.5kV rms
NST Current:75mA
Okay, 7.5kV times 0.75A IS 562W, but when the 7.5kV are present,
you have little or no current flowing (in whatever circuit it is hooked
up to). The 75mA flow when the NST is shorted or very little
resistance is present. However (as the faceplate value seems to
suggest) you will NEVER have 7.5kV AND 75mA present at the
SAME time.
My question to you is: Why did (do) you use a 120ohm resistor and
"measure" the current indirectly?. While it shouldn´t make a difference,
I measure my NSTs with a DMM set to the 20Aac scale. The internal
(DMM) measuring resistor on the 20A scale is very low (~0.1ohm or
less), so the NST is effectively shorted and it won´t fry your meter. If
I understood your setup correctly, you placed the NST, (cap), meter
and the 120 ohm resistor all in parallel, correct? Did you do this
(which is why I placed the word cap in parenthesis) during voltage
rise (meaning the resocap now being connected) measurements,
too?
While I DO BELIEVE what Terry is seeing (I have two left hands,
when it comes to PSpice. Just drawing the schematic takes me
about 3x as long as doing the same by hand), the only explanation
I have for it, is that it must have something to do with the breakrate.
Terry´s setup uses a 120bps SRSG. Impedance matched setups
(usually) use a static gap. While I have never actually measured my
bps, I think it is a lot higher than 100bps (2 * 50Hz). I have been
thinking about using a optoisolated (fiber cable) tachometer (like
those used to measure the rpms on model airplane engines) to
actually measure the true bps.
In Terry´s case (fixed at 120bps) the cap has a lot more time to
charge, meaning he can use a bigger cap. In (my) static gap case,
the bps varies with cap and gap size (smaller (g)cap = higher
bps, bigger (g)cap = lower bps). As the static gap is "self
triggered" it might even be possible that the bps rate changes
during a run. Terry´s setup ("forced" triggering) has no choice:
Either it fires when the electrodes line up (= 120bps) or it doesn´t
fire at all. Due to the bigger cap, Terry is using, the xformer needs
more time to fully charge it. This is my only explanation why he
gets "best" voltage far after the peak on sine wave. So, what we
need, is an equation that will let us determine (for RSG users) the
best cap size for a given breakrate. While I am familiar with the
"wattage" equation, I don´t like it, because it includes a variable
(the efficiency of the AC circuit), which is very hard or impossible
to determine. For example, just changing from a sync gap to a
async gap will (I think) change the efficiency of the setup.
Coiler greets from germany,
Reinhard