12kV, 30ma NST wiring

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Rich Feldman
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Re: 12kV, 30ma NST wiring

Post by Rich Feldman »

Good luck with your experiments, Joshua.
Are you preparing instruments to measure kV and mA, AC or DC?

Here's a picture from a long fusor.net thread about NST power.
Measured on a 15 kV 60 mA transformer, but you could scale both axes to match yours.
Dotted lines are resistor loads, each measured at 5 different primary voltages. (Your thin wire test was similar to dotted line next to vertical axis, but so steep that voltage was in the millivolt range. Get it?)
Green and red are HV incandescent and luminous tube loads.
nst_iv_1118.JPG
viewtopic.php?f=11&t=12407&hilit=variac&start=10
The thread includes a SPICE model of NST.
All models are wrong; some models are useful. -- George Box
Joshua Thomas
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Re: 12kV, 30ma NST wiring

Post by Joshua Thomas »

Nice graph, Rich. Thanks for that.

I'm not currently preparing any measuring equipment - the first objective is just to get a working tesla coil. Once I have that going it would indeed be nice to be able to measure both input and output voltage and currents. I'm not familiar with high-voltage (above 1kV) measuring equipment, that would be a new area for me.

The tank capacitor for the tesla coil will be 15x 0.15uF/2kV metal-foil caps for a total capacitance of 10nF at a 30kV limit. That's actually a lot of capacitve reactance at 60 Hz, about 265k ohm. At 12kV I'd get about 45mA, which is clearly more than the NST is designed to produce, but it is interesting to know that the capacitance isn't a limiting factor in this setup.
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Rich Feldman
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Re: 12kV, 30ma NST wiring

Post by Rich Feldman »

>>At 12kV I'd get about 45mA, which is clearly more than the NST is designed to produce

Your NST is designed to LIMIT THE CURRENT to 30 mA.
The mA number on nameplate is not like that on a power transformer; it's the built-in current limit value.

That's the essence of its nominal job: deliver 30 mA to a directly connected "lowish" resistance load (at hundreds of volts) for decades. It's intrinsic to the magnetically-shunted-core and coil structure, since the dawn of neon technology 100 years ago.
There is no regulator or protector to bypass or override.

You'd get that current with a short circuit.
As load resistance increases from zero, voltage goes up and current goes down (slowly at first).
Maximum power output would be about 160 watts, when load R is up to 400 KΩ (voltage up to 8 kV, current down to 20 mA).

Things could be different with a capacitive load.
I don't know about capacitive loads that are discharged once or twice each cycle. Listen to Tesla coilers like Richard, or read elsewhere on the Internet, or play with the SPICE model.
I hope that after the gratification of measuring Tesla coil arcs, you will measure NST output voltage and current.
Some instrumentation details and pictures can be found in the thread with that chart.
All models are wrong; some models are useful. -- George Box
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