At 10:53 AM 9/1/2006, you wrote:
....
Now we get to my question. Someone, I think maybe Terry, told me
once that doing this (as opposed to leaving the NST that coil) can
cause resonance problems that would kill the NST secondary winding -
something about stray capacitance in the HV leads going to the coil?
I have some questions about this, and a certain amount of skepticism:
1) wouldn't the primary capacitance swamp any stray lead capacitance,
making it rather moot?
I don't think it is normally a problem. A 15/30 NST has a resonant
capacitor size at 5.3nF. For neon sign installation they have to
be a bit careful of the wiring so the capacitance from the leads to
ground does not get near that. But that is really only if the
leads get very long inside a metal cased sign. For say a 9/120 the
37.5nF resonant capacitance is so high one should never reach it no
matter what the wiring is.
OBTW, I'm using a 15/120 if it matters.
2) wouldn't the resistors in the NST "Terry filter" tend to kill any
resonance?
No. The impedance of a 15/30 is 500,000 ohms. The 1K resistors
don't matter there. They are too small to damp anything.
Reactive vs. pure resistive?
3) Has anyone else used their NST remotely away from the coil and had
either problems you were able to trace to this issue, or no problems?
(either data point would be useful)
I think the only way to run into trouble is to maybe use something
like a very long run of RG-8 coax where the cable capacitance
becomes a big factor with a high impedance NST with a low resonant
capacitance size.
Funny you should mention RG8, that's exactly what I was going to use
for the HV leads.
Primary cap on this machine is going to be .02uF