12kV, 30ma NST wiring

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Joshua Thomas
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12kV, 30ma NST wiring

Post by Joshua Thomas »

Hello,

I just received a used Tesco NST, model number 4B12N3-07.

The nst came without input or output wiring, so I wired it up as follows:

* internal white wire to neutral on 120v cord
* internal black wire to hot on 120v cord
* internal green to ground on 120v cord

Continuity is good for all the connections from the wall plug to the internal connections. The internal green wire connects to the case, and a continuity check shows that it is properly connected to the external parts of the case eg the screws.

In order to test the high-voltage side I wanted a “fail to safe” method and put a string of 32ga nichrome wire between the two HV terminals. 12kv/30ma should instantly vaporize the wire and and leave it as an open circuit.

On connecting to mains power, nothing happened. The mains outlet has power and the breaker was not tripped.

Any ideas here?
Joshua Thomas
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Re: 12kV, 30ma NST wiring

Post by Joshua Thomas »

Edit: nope, maybe one side is busted. I’ll have to use a grounding stick and test that way.
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Joe Gayo
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Re: 12kV, 30ma NST wiring

Post by Joe Gayo »

Just because they are referenced to ground doesn't mean there isn't a potential difference between them. Look at the FAQs and look up a magnetic neon sign transformer schematic. The secondary is a center-taped (connected to ground, case) and the two outputs are out of phase (+6/-6 kV, under no load).
Joshua Thomas
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Re: 12kV, 30ma NST wiring

Post by Joshua Thomas »

Good news, using a test stick connected to ground, both sides definitely work. Bad news, there still seems to be a ground-fault protection circuit - a very loud relay clicks over right after sparks are observed and the transformer goes dead; but works again on power-cycle.
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Richard Hull
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Re: 12kV, 30ma NST wiring

Post by Richard Hull »

This is normal operation. GFI's are crap in that they are there to save lives!! We want to die if we touch the HV leads. By this it means we want the transformer to do a transformer like thing and supply voltage no matter what! If it is shorted out, we want it to try and power the dead short. it is magnetically shunted therefore it will not explode or blowup any dead short wires. Shorted out, it will not blow the house fuse. If we want to see continuous HV flaming arcs it is fine with that and will kill you should your go afoul of the HV knobs or, if you are lucky, just get knocked across the room.

Throw in a GFI and if shorted the GFI will trip and end operation until reset or the short removed. The GFI can't tell a dead short from a flaming arc so it shuts the show down, too. If operating a demo fusor, you may or may not be able to use this transformer with GFI as when the plasma ignites it is just a flaming arc that looks real pretty as a nice flashed big glow in the demo fusor. The GFI might not like this and shut down your fusor every time you try and get a plasma.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Rich Feldman
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Re: 12kV, 30ma NST wiring

Post by Rich Feldman »

>>12kv/30ma should instantly vaporize the wire and and leave it as an open circuit.

Not so fast. You do realize that it means 12 kV _OR_ 30 mA, and can't do both at the same time even with a 400,000 ohm resistive load. RIght?

Your trial short circuit between HV terminals probably had 30 mA running through your wire, a thing the transformer is designed to do for decades. Normal operating point with a lighted sign tube is not much different: 30 mA and voltage much less than 1 kV.

I bet you can arc from one HV terminal to the other, emulating a normally wired secondary circuit, without tripping the Secondary Ground Fault Protection circuit. It's meant to prevent fires when one of the HV connections arcs to some grounded thing over or through a dry wooden wall surface or window frame.

Finally, please mind the hierarchy of subforums here. The name of one stage in the path to here is:
"Construction / Technical Do not post simple questions here. Use the New User Chat area."
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Joshua Thomas
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Re: 12kV, 30ma NST wiring

Post by Joshua Thomas »

32ga nichrome wire has a resistance of about 10 ohm/ft. I was using about a foot. It will quickly melt with a 12V supply - I use the same for rocket motor ignitors. I'm reasonably sure 360W of electrical power (or even a smaller fraction of that) will pretty much destroy it in the 15 seconds I left the wire connection on there. I think more likely that I just didn't make a good enough of a connection - the nichrome wire is very thin and probally slipped away from the connection.

In any case, I do understand that a NST is not designed to provide its rated voltage and amperage continuously in normal operation.

I see that removal of the ground fault protection circuit is both possible and has been documented before, so that's my next step. I take high voltage precautions such as having another person present who has CPR/FA training, keeping one hand in pocket when working near live high voltage, observing safe distances between metals so as to prevent arcing, etc. Lab work has given me a healthy respect for the power of electricity.

Information or schematics of ground fault circuits in Tesco transformers are appreciated, if anyone has them!
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Richard Hull
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Re: 12kV, 30ma NST wiring

Post by Richard Hull »

In short, you can have 30ma at virtually zero volts output or 12kv at 5 ua out. In the middle you might get 7kv A 20ma or such, but never both max volts and current together. The name plate gets its wattage from the normal neon run current of 30ma at 4kv across the lit neon signage when running. As most old neon signs were "joes" or "open" the short tubes could run off of a 5kv xfrmr. "Anthony's Diner" would need a 12kv, etc. The longer signage required higher voltage to break down the gas., but they all typically ran at 30ma, unless very large diameter tubing was used to be seen at greater distances and that was for the 60ma class of transformers. Magnetic shunting internal to the core automatically limited the current and dropped the voltage. An ingenious bit of early 1900's engineering.

GFI's are for electrical house circuits to protect people and keep all heavy start motors from big drills to circular saws from even starting to run. I tore all of my "code" GFI wall outlets from my lab, replacing them with common wall outlets.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Rich Feldman
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Re: 12kV, 30ma NST wiring

Post by Rich Feldman »

>>I do understand that a NST is not designed to provide its rated voltage and amperage continuously in normal operation.

That is equally true if you omit the word "continuously", and remove the secondary ground fault protection.

Current rating is for dead short, the weakest possible load you can put on a current source. Dead short will never hurt the transformer; it's less stressful than open circuit.
Transformer will never reach rated current for even 1 cycle if the load allows voltage to go above 1/4 of nominal.

Voltage rating is for open circuit, the most demanding load you can put on a current source. Transformer will never reach rated voltage for even 1 cycle if current goes above 1/4 of nominal -- due to voltage drop from internal impedance which needs to be there as ballast for luminous tubes.
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 »

I removed the ground fault protector box from this NST today. It was rather easier than I expected. Remove the front plate, drill out the rivets. The wires from the transformer core to the GFP can then be clipped and the entire box removed. This leaves three large wires (green, blue, yellow) and four small (18ga or so) in pairs of yellow and brown. The GFP circuit itself embedded in tar material like the core.

The ground and coil primary wires were easy to verify, and the small yellow/brown seem to be power for the GFP internals. I taped those off, soldered on some new 14ga THHN to the primary wires, and I'm good to go. Not sure how I want to terminate those wires - directly to a NEMA 5-15P, or to mounting points? The transformer didn't come with external mounts for the primary connections, so I'd have to figure out my own. Better to be safe and just wire it the cord, I think.

This transformer has seem some better days and most of the small hardware is in rough shape so I think I shall be replacing them with new pieces before proceeding. High-voltage side wiring as well. While I know I need to use GTO 15 on the HV side, any idea if the small hardware (washers, bolts, nuts) have any particular requirements?

Thanks again.
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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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