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

Neon sign transformer power: a new look

Post by Rich Feldman »

This is to re-open the topic of power output from neon sign transformers.

The most detail I've seen previously is Richard's old paper, neons.doc, linked in a HV FAQ thread called "#8 FAQ - Neon transformers - the facts". viewtopic.php?f=29&t=10333
One rule of thumb, from there, is that one might get 2/3 of nameplate OC voltage and 2/3 of nameplate SC current at the same time, into a resistive load chosen for maximum power. A more conservative expectation, consistent with a linear model, is 1/2 voltage and 1/2 current. In the case of a nominal 15 kV, 60 mA transformer, the maximum sustained power would be 400 W or 225 W.

As a first step toward data collection in my own lab, I just made a little battery-powered AC kilovoltmeter.

It's based on a voltage divider board from the flea market, whose resistors are mounted on PTFE-insulated eyelet terminals. Total resistance is about 90 megohms. Original design voltage is unknown. If we run it up to 18 kV, the meter current would be 200 microamperes and power dissipation would be 0.36 W per resistor. Max voltage per resistor would be 1800 volts, which I bet is OK even though the datasheet is missing.
kvmeter.PNG
ac_kvm3.PNG
Things I would do differently next time: Power switch less susceptible to accidental pushing ON, mostly soldered connections instead of clunky screw terminal strip, and a battery test button.

Of course for AC input, it indicates the average rectified voltage. This cheap DVM reacted to 120 Hz ripple voltage by aliasing, instead of rejecting it, so I added a filter capacitor. Rectifier needs to handle the divided voltage, not the full voltage. Forward drop of two diodes is an error term subtracting from full voltage. Maximum reverse leakage current in rectifier datasheet is 10 uA at 25 °C, which could cause substantial errors here, but is a super-conservative spec. I measured leakage on several specimens with 24 volts DC applied to the AC terminals. Chose the best: less than 40 nA in both directions (4 mV across a 100k resistor).

The first step for calibration was to measure the DVM response with respect to a 6-digit Agilent benchtop voltmeter. Between 0 and 15 V, the little meter reads about 0.6% low. Then I decided to calibrate for direct indication of RMS AC kilovoltage. A low voltage NST was dialed down to 950 V, as indicated by 1000 VAC range a handheld Fluke meter. Then the trimpot in picture was adjusted to make the divided voltage 956 mV (indicated as 0.95, occasionally 0.94). The little NST on Variac goes up to 4.4 kV AC, as indicated by new meter, but I have no independent measure of that.

Today I tried a DC test with an industrial hipot tester, found "in calibration" and preset to 2.13 kV. New meter in picture indicated 2.44, apparently 14% or 15% higher than the true DC voltage. It ought to read 11.1% high, the ratio of sine wave RMS to rectified average voltage. Good enough for now!
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 »

Just found another 15-year-old thread about NST characteristics, on this very forum. It contains lots of information and some arguing. viewtopic.php?t=4315 It even includes John DeArmond, "Neon John", whom I remember from pre-Internet Usenet days (1993-1994).

I've still not yet seen (except in my own data from last weekend) a printed I-V curve for a NST output, showing the alleged convexity to the right of simple linear model. Such as one passing through Richard's (2/3, 2/3) point. If there were any in Richard's neons.doc, they seem to have been lost in translation.

If the core and shunt materials had linear magnetization curves, without saturation, I think any consequent electrical parameters (magnetizing inductance, coupling coefficient, leakage inductance) would be independent of the operating point.

The output I/V curve would be linear, as shown in my sketch below, since the leakage inductance would have a constant reactance at 60 Hz. Or not, because its voltage drop is in quadrature with that in the resistive load? I'll let somebody else take a shot at answering that, by handwaving or formulas or simulation.
nst_iv1.PNG


Last Saturday, I made plenty of measurements on a tiny NST/ballast that was convenient. Don't have time or photographs to present the results today. There _is_ time to get the experiment description out of the way:

1. Chose one small shunted HV transformer with no nameplate. Primary behaves like it's designed for 120 V, 60 Hz. Then secondary V_oc is about 4 kV, and secondary I_sc is about 7 mA.

2. Used it with a variac, some power resistors, one "neon" element, and three digital meters:
- Benchtop 6-digit multimeter with Agilent brand, for true RMS AC volt measurement at transformer primary.
- Handheld multimeter with Fluke brand, for true RMS AC mA measurement in series with transformer primary or secondary.
- Homemade AC kilovoltmeter presented in OP above, used exclusively for AC volt measurement at transformer secondary.

3. Primary voltage was adjusted, using variac, to measured values closely approximating whole multiples of 10 V. All charts are based on the nominal step voltages, not the actual measured values.

4. Measured primary current, with secondary open and secondary shorted, from 10 V to 140 V. Both show incipient saturation at 140 V.

5. For the same set of primary voltages, measured secondary voltage loaded only by voltmeter (90-megohm), and secondary current burdened only by milliammeter (about 1 ohm).

6. For about six different resistive loads, and one luminous tube, primary voltage was set to 30, 60, 90, 120, and 140 V as measured. At each setting I recorded the voltage and current at secondary. That's more tedious than computing one from the other and the known resistance. It offers some protection against transcription blunders, because the computed V/I ratios should be constant in each sweep.

Pictures, charts, and conclusions to follow.
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 »

Just got clear on something that should've been obvious, and on which I speculated in prev. post.
The simple linear model of "AC voltage source with series inductance" yields an I-V curve that's one quadrant of a circle.
That's because the voltage drop due to leakage inductance is in quadrature with the voltage across a resistive load.

If we then include the in-phase voltage drop due to winding resistance, the curve from model is an intermediate shape
much like those from measurements.
nst_iv4.PNG
nst_iv4.PNG (17.32 KiB) Viewed 15821 times
Source impedances in this picture each have a magnitude of 500,000 ohms:
all real (500k resistor)
all imaginary (reactance of 1326 henries at 60 Hz)
a hypothetical mixture (100k + j*490k).

Minor effects from nonlinear magnetization are plainly evident in the real data.
But I bet linear LR models will be plenty useful for most predictions, such as what would happen if we put capacitors in the circuit.
All models are wrong; some models are useful. -- George Box
John Futter
Posts: 1848
Joined: Wed Apr 21, 2004 10:29 pm
Real name: John Futter
Contact:

Re: Neon sign transformer power: a new look

Post by John Futter »

so just to clear up the graphs
Blue = Real R load
Black dashes = immaginary reactance and grey line - mixture of both
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 »

Yes, what John said. Sorry about posting a chart without labeling the curves.

This little transformer is responsible for first set of data from my lab. The device, and the method, are described a couple of posts ago.
DSCN0803.JPG
Core laminations have no joints on the outside perimeter. Looks like the center leg is pressed into the outer section, a thing I've seen before in small motors but not previously in a transformer. Shunts have well-defined air gaps without need of paper spacers. Here sketched from a photograph.
nst_core.JPG
nst_core.JPG (12.44 KiB) Viewed 15216 times
Here are the measured I-V curves for five different primary voltages and six different resistive loads, plus a luminous tube.
nst_iv_meas.JPG
Black dashed lines connect the measured V,I points for each resistor configuration. Their straightness adds confidence that the measurements were transcribed properly. A red dashed line connects the measured V,I points for the luminous tube, which lit up fine even as primary voltage was variac'd up from 0 to 30 V.

In the next chart, the numbers are normalized to the measured V_OC and I_SC for each primary voltage. Colors are the same. On close inspection, the curve shapes are slightly sensitive to primary voltage. I bet that's from magnetic nonlinearity in the steel.
nst_iv_norm.JPG
I added an iso-power line passing through V = 2/3, I = 2/3 point that Richard presented as a rule of thumb, long ago. So close to my "nominal primary voltage" measured curve, I suspect the experiment was somehow rigged. :-)
My intended application is on the lightly loaded, higher voltage side of the chart. With _this_ specimen, we can get 50% current at 80% voltage, or 35% current at 90% voltage.

Next report will include curves for simple linear models. That is, models with internal L and R values that don't depend on voltages or currents. Sorry about one hasty earlier post, where I said "linear" to mean straight-line I-V source characteristic.
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 »

Here's a preview of a 100 mA AC/DC digital panel meter, put together much more quickly than the kilovoltmeter in OP. I hope it can serve on the high side of multi-kV circuits, like NST's and color TV sets, when put in a compact enclosure with an anti-corona shape. Other high side metering options include current transformers and optoisolators, I think neither trivial nor inexpensive for tens of mA and tens of kV.
DSCN0810.JPG
.
A different digital panel meter of the same size, made for 0-900 mA DC, was considered and dismissed. Its built-in shunt resistor is less than 0.1 ohm. When fed with rectified AC milliamps, the reading wandered too much. it would need a very high-value capacitor to keep the ripple amplitude down.

The meter in picture is identical to that in OP. 0 to +30 volts DC, with automatic decimal point shift between 9.99 and 10.0.
AC or DC current goes through full-wave bridge rectifier to a 100 ohm burden resistor. 99.9 mA develops 9.99 V and is displayed as 9.99, with 1 watt dissipated in the resistor. A 220 uF capacitor is enough to take care of the ripple voltage. Meter indicates the average rectified current. For sinusoidal AC, we can get the RMS value by multiplying by 1.111.

[edit] Fusors and other sparky HV circuits are notorious for destroying series milliammeters. To this circuit I'd add a Transient Voltage Suppressor diode (between 10V and 25V) in parallel with the R and C. I think the C itself affords some protection. [/edit]
All models are wrong; some models are useful. -- George Box
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Neon sign transformer power: a new look

Post by Richard Hull »

Note some of these little meters use 3.3v MPUs and will not indicate a voltage below that value. They are mostly created for monitoring 12 and 24 volt storage battery systems. The hamfests and catalogs are flooded with these little chicom items ranging in price from $1.95 on sale to over $7.00 for the same little item. These little two wire wonders have no need to read a voltage below 3- 5 volts.

I notice three wires to your device. You might have one that has a true zero to 30 volt range. Two wires for a 9v battery to power the item up and a third wire for the voltage to be measured input.

Of course the free Harbor Freight digital meters could be worked the same way, but don't have the bright glowing display of an LED.

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 »

Yes, my tiny LED panel meters are from ebay for $2 or $3 each. Could have got some about the same for $7 at the flea market.
The voltmeters are all 0 to +30 with three wires; negative power and negative V_in share a wire.
The ammeters (in various ranges) have built-in shunts and four wires (two thin and two thick). Haven't looked to see how the two pairs are connected internally.

I just got a new cheapie DMM, like the ones from Harbor Freight. Figured on using it with the voltage divider in OP, and no rectifier, to conveniently switch between correctly scaled AC and DC kilovolt ranges. Then discovered that the meter, and others like it, are poorly suited for that plan.

We all know that most "nice" DMM's have standard 10 megohm input impedance.
Some of us have noticed that the "cheap" DMM's are often only 1 megohm. Caveat emptor, if you plan on using one with a standard HV divider probe!
Those input impedance are only for DC, and I learned new things today about AC voltage mode on various meters.
The "nice" ones are AC coupled, and don't respond to DC voltage of either polarity. Input resistance is infinite (viewed with a DC ohmeter).
The "cheap" ones, in AC V mode, look like a 1/2 megohm resistance in one direction and infinite in the other direction. When we apply +10 volts DC, the meter reads about 21 volts AC. When we apply -10 volts DC, the meter reads zero.
I think the presence of a transistor socket and hFE mode generally goes with meters in the second category.

Here are some meters and some of their properties. Not shown in picture are fancy Agilent benchtop meter and one Micronta analog V-O-M.
DSCN0824.JPG
dvm_r_in.JPG
Turns out the fat red meter, from an estate sale, is UL listed and might be made in USA. Henceforth it will get more respect in my lab.
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 »

Haven't had much time for the next step: floatable digital milliammeter. As mentioned before, general purpose $3 digital multimeters are very limited for measuring AC. If we need an external rectifier, might as well opt for a whole package that's much smaller and more fun.

It took just a few minutes to build and test the whole circuit in breadboard form. Then hours, so far, putting it into a corona-resistant metal enclosure. The box has room for all the parts. So far, all that's installed is the DPM and a set of home-made receptacles for banana plugs.
DSCN0826.JPG
DSCN0827.JPG
Without banana plugs, the electrical subassembly can be lifted right out.
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 »

The Altoids-box mA meter has been assembled, calibrated, and put into service.
Tonight I measured the first of my 15060 NST's, with primary voltages of 60, 120, and 140, and loads of short, luminous tube, 1R, 2R, 4R, 8R, and open circuit. Next time out, will crank it up to 277 (the nominal primary voltage), or 290 or even 320 as discussed in another thread.
DSCN0924.JPG
DSCN0925.JPG
The I-V chart is unremarkable, except for showing an apparent blunder in transcribing a reading at 60V input and 8R load.
nst_iv_1117.JPG
nst_iv_1117.JPG (37.04 KiB) Viewed 14197 times
Here's a close-up of the new instrument. The power switch is in middle position, to read the battery voltage when loaded by our 100 ohm current sense resistor.
DSCN0926.JPG
A full scale reading on the digital panel meter would be 300 mA, with 9 watts in the resistor. In case that's ever wanted, briefly, the R is as big and as thermally isolated as I could easily manage, short of thermal attachment to the case and/or air holes. Next time, get a panel meter with a more appropriate native voltage range, like 2V or 0.2 V as Richard recommends in fusor FAQs. Then current sense R value and power dissipation are much smaller, but anti-ripple capacitor value (for AC use) needs to be much larger.

It would have been much easier to put a small analog mA meter in a box, with a similar bridge rectifier. But that wouldn't be so easy to read precisely from a safe distance.
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 »

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: 14991
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: 14991
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: 14991
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: 14991
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
Post Reply

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