Neon sign transformer power: a new look

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

So far so good. With nominal primary voltage, I got:
425 watts into a matched resistor.
222 watts with 10 R's in series (14.4 kV x 15.5 mA). Still thinking of using 2 of these NST's in series for fusion someday.
159 watts into a string of 25 Christmas lights, last year converted from parallel to series by snipping wire in 24 places.
DSCN0931.JPG
The LED meters were hard to read in daylight, until some shades were added. Green optical filters would probably also help.
Here are the IV curves for 70, 140, 210, 277, and 290 V on primary. Plus an iso-power line at 425 watts.
nst_iv_1118.JPG
All models are wrong; some models are useful. -- George Box
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

Last night I measured NST primary current. It's a key design requirement for high-voltage isolating transformers to go upstream of the NST's.
Same setup as before, except with a true-RMS ammeter in series with primary. Primary voltage was set to nominal and many different resistive loads were measured. Also I tried loads of 6, 10, and 20 fluorescent lamps in series.
nst_iv_1208.JPG
.
Primary current ranged from 0.305 A (no load except kV meter) to 3.43 A (short circuit except secondary mA meter). It was 2.3 A at the peak power point (11 kV, 426 W).
I expected it to behave as the sum of two components 90 degrees apart in phase: a fixed magnetizing current and N times the secondary current. Not bad, with N of about 56.
Got a better fit with a mixed model for primary current: (quadrature sum of Ia and N * Ipri) + Ib, where N=53.6, Ia=0.1, Ib=0.19. THis is more precision than necessary, and more than is justified by the metering accuracy.

Right before inserting the primary current meter, I had fired up a test fixture that connects 20 fluorescent lamps in series. They are T5's about 45 inches long (1.2 m).
DSCN0993.JPG
That was an opportunity to do some plasma wrangling, and get in the mood to make special holiday light displays.
nst_iv_1208_c.JPG
First run, in blue, began at full blast (290 V on NST primary). 287 watts, if we ignore errors from metering non-sinusoidal kV and mA. As variac was dialed down, HV current went down and voltage went up as expected. Eventually the lamp array got flickery and almost went out. Then I dialed the variac back up in small steps. Currents lower and voltages higher than before, I guess because the cathodes were now all pretty cold. At a couple of points, there was a progression to higher current and lower voltage with no change to variac setting, I expect as the cathodes warmed up to a new equilibrium. Eventually the rising-Vpri sweep curve merged with the falling-Vpri sweep curve.

Second run, in orange, started with variac at zero. Got to over 8 kV before any significant current or flickers of light. Eventually, tubes were all warm and bright. From 35 mA to max, the voltage followed familiar "close to normal operation" curve. I think operation below that point is hard on the cathodes. Good thing the lamps were all free. :-)

How about that apparently bistable lamp characteristic, hypothetically with different cathode temperatures? I think experiments like this could see stable operation with different subsets of the lamps (or individual cathodes) in hot or cold state. Novel data storage technology?

[edit]Updated the lamps IV chart. Added scaled copies of the "Resistors w/ Vp=nom." curve, to represent variac knob effect. Removed one point in blue curve that was missing in notebook, WAG'd in spreadsheet, and obviously wrong.[\edit]
Last edited by Rich Feldman on Mon Dec 10, 2018 12:33 pm, edited 1 time in total.
All models are wrong; some models are useful. -- George Box
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Neon sign transformer power: a new look

Post by Richard Hull »

The lamps are a non-linear gas load and only a bit like a fusor. To bad the tests weren't run on a real fusor which is what folks here might be interested in. Resistive loads are smooth and remain constant. The transformer is designed for a gas load. Neon gas loads are really just diacs, (voltage regulators), It takes only so much voltage to light a tube of highly variable length and keep it lit. The transformer is made to crumple to its knees on a very specific ionized gas "arc" load and supply only enough current to keep it lit based on it magnetic flux shunting ability under varying load situations.
The varying load for neon signage makers is the total tube length of the signage. Ne-2 lamps have been used as low current voltage regulators of many years.

The moment you take it out of a neon gas load, a transformer's performance can vary based on what is demanded of it in a total different load scenario than it was designed for. Fluorescent tubes are a mercury metal vapor load. Clear glass blue signage is often nothing but mercury vapor with neon electrodes which are quite different from fluorescent tube filaments which can heat to incandescence if over driven. Expect interesting and often surprising results.

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
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

Dusting off this thread because of recent work with luminous octahedra, and thinking about Joshua's cap charger.

Just made my first SPICE model of a NST, and am reasonably confident in its quantitative accuracy for first-order effects.
I'm sure there are plenty of better NST models to be found on Tesla coiler websites.
nst_sch.JPG
.
L1, L2, and L3 are perfectly coupled, and represent a 1:125 transformer with center tapped secondary.
Primary is assigned inductance of 1 henry, which I guess is in the right ballpark, and determines the no-load input current.
Each half-secondary has inductance greater by the square of voltage ratio, 62.5.

This circuit models the NST current limiting (ballasting) behavior with series inductance (LLP and LLN) instead of reduced coupling. I also put in RSP and RSN to represent secondary wire resistance.

With resistive loads we get the convex I-V characteristic seen elsewhere in this thread and Richard's old neons document.
30 mA RMS when shorted. 15 kV RMS when open. About 20 mA and 10 kV with 500 kΩ load (maximum power point).

Rectified to charge a capacitor, as shown in snip, it gets darn close to 10 kV DC in 100 ms. Most of the way in first couple of cycles.
All models are wrong; some models are useful. -- George Box
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Neon sign transformer power: a new look

Post by Richard Hull »

Nice ideal modeling of the average neon xfrmr.

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
Joshua Guertler
Posts: 198
Joined: Fri Apr 21, 2017 10:59 am
Real name: Joshua Guertler

Re: Neon sign transformer power: a new look

Post by Joshua Guertler »

Greetings,

Thank you very much for the model, it will certainly be very useful in the near future. Out of curiosity, what is R1 and RL for and if they represent a part built into the transformer.

Also, if one were to build the circuit, would it be better to go with a full wave rectifier design? Thank you.

Sincerely,
Joshua Guertler
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Neon sign transformer power: a new look

Post by Richard Hull »

Electronics engineers, demand an equivalent circuit for all alternating circuits. This allows for critical design and implementation with all reactive and passive components shown that represent a specific component. There is a resistance and inductance in the transformer. There is also some capacitance, but at line frequency it will not significantly enter into the equivalent circuit shown.

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
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

Joshua,
1. The R1 component is just a relic of LTSPICE example circuit that I started with. Not part of NST model. RL is an end-to-end load resistor that was set to various values for testing the model.
Beware that in SPICE, if you want a value to be mega something you need to say Meg or meg or 000000. The suffix M or m by itself means milli.

2. The schematic I posted shows a full wave rectifier, with 2 diodes and a center tapped transformer. Suggested for your application because one side of the capacitor can be grounded. You can get twice as much voltage with a 4-diode bridge rectifier, but then both terminals of the capacitor would be electrically "hot". I have charged capacitors using the 2 diode circuit, and found it to not work with NST's that have the SGFP feature. Expect that charging a cap through bridge rectifier won't bother SGFP, but haven't yet tried it.
All models are wrong; some models are useful. -- George Box
Joshua Guertler
Posts: 198
Joined: Fri Apr 21, 2017 10:59 am
Real name: Joshua Guertler

Re: Neon sign transformer power: a new look

Post by Joshua Guertler »

Greetings,

Thank you very much for your advice and contribution, Mr. Feldman.

Out of curiosity, would the following set up work safely? It's a slight modification to better fit my system, where the capacitor is connected to a grounded load. Thank you.

https://docs.google.com/drawings/d/1UQy ... sp=sharing

Sincerely,
Joshua Guertler
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

Joshua, we should take this back to your "10kV power supply wanted" thread. Or to Private Messages or email.

Your modified picture shows C1 disconnected from ground & connected "to plasma load".
Other side of plasma load is not shown, but cover letter mentions "grounded load".
Average current in the load will be zero, of course, because it's in series with a capacitor.

Is the latest picture unrelated to the 10-per-second pulse discharges described in that other thread?

p.s. Sharing essential images by outside link, e.g. under docs.google.com, is discouraged here. Outside links are, justifiably, regarded as ephemeral. Even more so than images uploaded to fusor.net. :-)
All models are wrong; some models are useful. -- George Box
Joshua Guertler
Posts: 198
Joined: Fri Apr 21, 2017 10:59 am
Real name: Joshua Guertler

Re: Neon sign transformer power: a new look

Post by Joshua Guertler »

Greetings,

If I find an NST that is secondary midpoint grounded, would I be able to cut the ground on the secondary to make it operate in a way more desirable for this set up? Thank you.
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

We don't know enough about "this set up" to know. What do you think would be gained by disconnecting center tap, if that were possible?
You could learn a lot about NST's by searching and reading for a few hours. This forum site is a good place to start.
All models are wrong; some models are useful. -- George Box
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Neon sign transformer power: a new look

Post by Richard Hull »

To remove the center tap, you must tear the case apart, but first, short the two knobs together and turn the transformer on for about 1 hour. This will heat the transformer tar enough so that you can rip the metal case apart completely. you will wind up with a gooey tar lump that can, with a lot more effort than it takes to say, be removed from the transformer. Dip the black, filthy, tarless transformer into a 5 gallon pail of kerosene over night. Wipe it off carefully and once you examine the frame and windings, you will see two braided wires off of the two HV coils going to a riveted lug on the core's frame. (these ground each winding) take them loose and insulate them well and join them together, well insulated from the core. They want to arc to the core as they are near the core, (outside HV windings, near the core), because they are at half the potential of the total voltage. The entire job should take about two days with a number of cuts from sharp jagged metal tearing the steel case off the tar block and just might work without arcing. Do this outside and buy a lot of hand cleaner, bandaids and bactracin..... it is a filthy business. Finally the whole transformer must be placed in oil in a suitable plastic tub prior to powering up.

I have done this to about 6 large 60 ma transformers in my Tesla coiling days. I was younger and eager then, and this entire effort is a fools errand. It can be done, but only by the bold and stupid. See how easy it is to lift the grounded center tap from the core? it's a snap!

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
Joshua Guertler
Posts: 198
Joined: Fri Apr 21, 2017 10:59 am
Real name: Joshua Guertler

Re: Neon sign transformer power: a new look

Post by Joshua Guertler »

Greetings,

I recently got a 9 kV 30 mA NST with an internal resistance of 9.07k Ohms (not running). If I use a similar set up, what voltage output should I be expected, assuming that the current is roughly 20 mA? Thank you.

Sincerely,
Joshua Guertler
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Neon sign transformer power: a new look

Post by Rich Feldman »

Read Richard Hull's "neons" document, in a recent FAQ.
Or read this thread and scale the numbers for your NST nameplate. Too bad if pictures can't be seen without opening them. Check images with IV in name, near top of second page (?).

Or make a kV meter and 20 mA dummy load, which you need anyway, and measure it yourself. Get hands dirty, the sooner the better.

Plain old NST's are an exception to general rule: "avoid intentional arcing with HV power supplies, unless arcs are your main objective".
Arc at will, starting with a wire well attached to NST case, brought to touch one HV terminal and then backed off.
Next exercise: add a sense resistor in series, and measure the AC current under short circuit and arc conditions. You will need to do that sort of thing anyway.
All models are wrong; some models are useful. -- George Box
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Neon sign transformer power: a new look

Post by Richard Hull »

With the full wave circuit Rich shows, you might expect about 5.6kv or less, at 20ma. Just a guess of course. Doing the math....1.414 X 4.5 = 6.36kv ideal, no load and filtered. This ideal value will drop under load.

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
Post Reply

Return to “High Voltage - Fusor Input Power (& FAQs)”