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On 9/27/15 6:51 PM, Matthew Sweeney wrote:
No that is exactly correct. Another user mentioned that I should be careful
using NST for VDG seemingly because there could be short down in the inside
of the VDG to the HV brush, allowing potentially lethal currents up to the
collector. I've not been able to confirm this exact reason - maybe someone
else can chime in on this.
I was mostly only concerned about the maximum wattage I'd need for
resistors, which I realized I can actually easily calculate by assuming a
short to ground. This gives me the maximum possible current flow through
the resistor network and thus I can calculate maximum wattage.
Your problem won't be the power handling of the resistors, but their
voltage handling.
In any case, a typical 15kV, 30mA NST essentially has an inductive
impedance of 15E3/30E-3 ohms or 500 kOhms. That is, with the output
shorted (0 volts), it will source 30mA: with a 500k load, it will source
15mA at 7.5kV (which is 7.5E3*15E-3 watts = 112.5 Watts) and with no
load, it sources 0mA at 30kV.
The 250k load is the maximum power dissipation point (Thevenin, and all
that).
In your case, you're worrying about limiting the current if there's a
spark or short? What's your actual current limit? Microamps is pretty
small. Milliamps is also small, but more practical.
If you're just interested in limiting current, you could put the
secondary of another NST in series (leaving the primary disconnected).
That would limit the current to half.