Neon Sign Transformer (NST) Characteristics

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Neon Sign Transformer (NST) Characteristics

Post by swarm » Sun Feb 09, 2003 5:37 pm

Neon Sign Transformer (NST) Characteristics
from Sam's Laser FAQ, Samuel M. Goldwasser
http://www.repairfaq.org/sam/laserclp.htm

[This site had a lot of good info on HV PS issues. I'm just going to copy the NST section since there has been quite a bit of discussion. Go check it out.]

As noted above, neon sign or luminous tube transformers do not behave like ideal power transformers - or even real world power transformers. They are designed specifically for the needs of gas discharge tubes, which want to see a current based on their diameter and gas fill, but independent of tube length (and thus voltage drop). Therefore, a constant current source is required. The magnetic design of the core and windings very cleverly provides a decent regulation characteristic and protection from momentary (at least) short circuits.

* Open circuit voltage - This is the kV rating on the nameplate. A 12 kV unit will output about 12 kV when driven with the nominal AC input voltage and frequency with no load connected. The high peak voltage (actually almost 17 kV for this example - 1.414 * 12 kV) provides the means of initiating the discharge on each half cycle since the negative resistance behavior of a long gas tube - like a helium-neon or argon ion laser - requires a high voltage to start and a lower voltage to run.

* Short circuit current - This is the mA rating on the nameplate. A 30 mA unit will output about 30 mA into a short circuit when driven with the nominal input voltage and frequency. The output current supplied by the transformer will be relatively close to this value for a good portion of its output voltage compliance range.

* Reactive power - This is the VA rating on the nameplate and for a typical transformer is very close to the kV * mA rating.

* Input current - Varies from approximately .1 (no load) to 1 (short circuit) of the 'A' from the VA rating. For the 12 kV, 30 mA unit I tested, this was about .4 A to 3.1 A respectively (at a line voltage of 115 VAC).

* Power Factor - Probably not specified but typically quite low (unless unit has built in correction). Partial correction using a parallel capacitor is possible but its uF value may need to be determined under expected operating conditions since PF depends on load.

* Regulation - Between an open and a short circuit, the core and winding construction results in a quasi-constant current characteristic over much of this range. I did a test on a 12 kV, 30 mA transformer at reduced voltage (I didn't have any way of providing a variable load at full output so I used a Variac to set the no load output voltage to 1,000 VAC):

Load Output Voltage Output Current
-------------------------------------------
Open 1,000 VAC 0.00 mA
R 560 VAC 1.43 mA
R/2 350 VAC 1.79 mA
R/3 250 VAC 1.91 mA
R/4 195 VAC 1.99 mA
R/5 160 VAC 2.04 mA
Short 0 VAC 2.10 mA

R was equal to 392K ohms (I have a bunch of them). So, for loads resulting in between about 1/2 and rated output voltage, the current changes by less than 30 percent - which isn't bad for something without any silicon! The Thevenin equivalent for this transformer over the range of 0 to 350 V or 2.1 to 1.8 mA would be 1.129M ohms fed from a 2.45 kV source (remember, this was done at reduced voltage. At nominal input this would be equivalent to almost 30 kV). These measurements were very approximate. I expect that behavior at full voltage (and its associated current) won't be quite the same (actually, it will probably be better) but this demonstrates the general idea.

Another thing to note is that when a NST is used with an unbalanced load such as only one half of the centertapped secondary being used or when feeding a half wave rectifier, its characteristics may not be what are expected based on experience with a normal power transformer. When half wave rectified, for example, the output current won't be just half of the full wave current but may even be greater (2.9 mA compared to 2.1 mA for the short circuit case, above). Presumably, this results from the fact that on half the cycle, the core is seeing the full output voltage. While such characteristics may not be inherently bad, it may not be easy or even possible to make predictions so testing would be needed for each particular situation.

You can estimate the voltage rating of an unlabeled NST by running it as above on a Variac at say, 5 percent of line voltage, and measuring its output voltage. Then, multiply by 20. To determine the current rating, connect the output directly to an AC current meter. To be cautious, start at low input voltage and go up to full line voltage (since the NST should be current limited).

WARNING: The current test assumes a current limited neon sign or oil burner ignition type transformer. Doing this on a normal power transformer will probably result in a blown fuse/popped circuit breaker, blown meter, or both!

Here is some additional information on the electrical characteristics of neon sign transformers (NSTs) including power factor issues and correction. A 15 kV, 60 mA unit is assumed - adjust the numbers for whatever size you have.

(From: John De Armond (johngd@bellsouth.net).)

Let me answer several questions at once. First, a 15 kV, 60 mA transformer will produce 60 ma almost up to its rated voltage. The transformer is designed to be a constant current device, to supply whatever compliance voltage is needed to push the 60 ma through the load. The 60 ma is nominal short-circuit. All magnetic transformers made for use in the US are designed for continuous use at no more than 80% of the short-circuit current.

I never actually sat down and plotted it out but I do know this: With 1 foot of neon tubing on a transformer (about 500 volt drop), it drives 60 mA. With over 60 feet of tubing on the tranny (more than specified), it still outputs about 50 to 53 mA. That's fairly constant current.

That said, a NST will NOT survive long if asked to supply full voltage at full current. It is designed to drive a gas discharge tube. The characteristic of a gas discharge tube is that it takes a large amount of voltage to ignite the discharge and then the voltage falls to a fraction of the starting voltage to sustain the discharge. Thus the high dissipation occurs only for a short period of time in each half cycle. On a scope, this looks like a sharp spike followed by a level, square wave form for the rest of the half cycle. This sequence occurs 120 times per second.

Regarding volt-amps and watts. You left off the critical part of the equation. While for DC and 100% resistive AC loads, the formula is W = E * I, for typical loads that include some capacitive or inductive reactance, the equation for power is W = E * I * cos(theta) where theta is the phase angle between the voltage and the current waveform. Volt-amps is simply E * I and includes both the real component and the reactive or out-of-phase component. The term "power factor" is simply cos(theta). In a pure inductor or capacitor, the current is 90 degrees out of phase with the voltage, cos(theta) = 0 and so no real power is dissipated. This even though the cap or inductor is drawing amps that can be measured. For an inductor, the current lags the voltage by 90 degrees and for a cap, the current leads the voltage by 90 degrees. If one measures the current to a reactive device (cap or inductor), the measured current will be the quadratic sum of the real (in phase) and imaginary (out of phase) current.

An AC wattmeter measures real power. In other words, it compensates for cos(theta) Wattmeter test instruments are available in a form that uses a clamp-on current probe to measure the current and a physical connection to measure the voltage. These will typically display volts, amps, watts, VARs (volt-amps reactive) and PF. They are also expensive. For the experimenter, an ordinary utility power meter is an accurate, if less convenient alternative. Widely available surplus (C&H Sales and others), the meter is accurate typically to better than 2% over a 10:1 range. the numbers on the front register watt-hours while the RPM of the meter wheel measures watts. The Kh factor printed on the meter face is how many watt-hours each revolution represents. Typically 7.2 for residential meters. Simply count the turns over a measured period of time, multiply by Kh and divide by the measured interval in hours to get watts. I have a recording watt-hour meter that was equipped with a photo-interruptor to count revolutions. One can easily add one to any meter using a reflective photo-interruptor to look at the black flag on the meter wheel. (Do NOT attempt to drill a hole in the dial for counting - that will destroy the calibration.)

The PF of a standard neon transformer is very low, typically in the range of 0.2 to 0.4 lagging. This is why the VA ratting is much higher than the watts that can be supplied. That means that the transformer draws more than twice the current required to supply the output wattage. This reactive current, called "wattless current" in the slang, can be countered by supplying an equal amount of leading phase angle wattless current. A capacitor does that. A motor run capacitor is the proper type which can handle the continuous duty. To compensate a tranny, simply start adding capacitance while watching the amperage draw from the line. When the draw is at the minimum, the capacitive reactance is equal to the inductive reactance, the PF to the line is 1 and all is well in paradise! A 15 kV, 60 mA tranny will need about 160 uF of parallel capacitance. This varies with secondary load so one must measure but that's a starting point.

Note that the full current (wattless + real) is still flowing in the circuit between the cap and the tranny.

This technique is widely used in neon sign work. It will allow twice as many transformers to run on a given branch ampacity or else it will allow lighter wire to be run to a given load. For fully enclosed transformers (HV terminals are inside the box), there is enough room for the cap inside.

(Portions from: Mark Dinsmore (dinsmore@ma.ultranet.com).)

There is a very good analysis of the design of neon sign transformers in:

* Neon Techniques and Handling: Handbook of Neon Sign and Cold Cathode Lighting
Samuel C. Miller
ASIN: 0-9113-8041-8

Among the multiple pages of design information is a graph of the load line of a typical magnetic (iron) neon sign transformer. Neon Sign Transformer Electrical Characteristics is redrawn version of this diagram. http://www.repairfaq.org/sam/nstchar1.gif Note that for laser tubes as well as neon signs, factors other than tube length are important in determining discharge voltage. These include tube diameter, gas fill, and pressure.

I don't know if the following is a newer edition of the same book, but it might be an alternative source of a lot of additional information on these topics:

* Neon Techniques: Handbook of Neon Sign and Cold-Cathode Lighting, 4th Edition
Wayne Strattman (Editor)
St, Pubns, 1997
ISBN: 0-9440-9427-9 (paperback)

Used Neon Sign Transformers - Cost, Recommended Brands, Testing, Availability
The discussions below relate to neon sign transformer availability and size, testing, use of larger (pole pig) transformers where more power is needed, and reasons NOT to consider microwave oven transformers in these applications. However, microwave oven transformers can be modified for other uses like powering the filaments of argon/krypton ion laser or thyratron tubes. See the section: Rewinding a Microwave Oven Transformer for use as a Low Voltage Filament Supply.

(From: Jason Freeburg (egraffiti@iname.com).)

A used neon sign transformer should not cost more than $20 or so. Find a neon shop in your area. They usually have the used ones stacked up somewhere and will sell cheap. The 60 mA models are usually somewhat cheaper than the 30 mA type if you buy them used from a neon shop because they are really too hot (e.g., provide too much current) for running neon and they cause staining and premature burnouts. It all depends on the particular shop you go to. I don't suggest buying new for something like this, the performance will be the same but the price much higher. A new 15 kV, 60 mA transformer lists for about $80.

BTW, the best name to look for in neon sign transformers is France. These things are ruggedly built and will take a lot of abuse without dying. The name to avoid is Actown - their transformers are wimpy and usually don't deliver the rated current.

(From: John De Armond (johngd@bellsouth.net).)

Testing of a used neon sign transformer is pretty easy even without test equipment. These normally fail with a secondary short and all that does is (slowly) cause them to overheat and let all the magic black goo run out.

Wire the transformer primary up with a 3 wire grounded cord (green to the case!), plug it in, and see if you can use a plastic-handled screwdriver to draw an arc from each insulator to the case. If that works and they don't make any funny noises, they're probably OK. The grounded cord will also weed out any trannys with a primary short to ground.

(From: Sam.)

While some of the discussion above might suggest that you should run right out and corner the market on old neon sign transformers because newer ones won't work properly for home-built lasers, you can relax. The nice simple iron current limited type aren't going to disappear overnight - there will be plenty of piles of used transformers in neon sign shops for years, if not decades, to come!

(From: John De Armond (johngd@bellsouth.net).)

The glory days of neon transformers for experimenting are coming to an end. UL and the NEC have conspired to rewrite the code to require secondary ground fault protection built into new transformers. These protectors trip the transformer if the secondary current is unbalanced or goes to ground. My testing of the units on the market show them to be very sensitive to spurious currents, particularly RF (as will exist in a gas discharge tube at higher pressure). It must be built in and be inaccessible to the installer. This means potted in tar. I've X-rayed a couple to try and figure out where that "special place" would be to drill a hole to disable the devices but since most of these are still at least partially hand-assembled, the parts placement isn't accurate enough to make a template. Used transformers will still be available from sign shops as they are replaced with SGFI (Secondary Ground Fault Interrupter) transformers but then the supply of un-protected ones will go away. Therefore basing plans on neon transformers could be shortsighted.

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Re: Neon Sign Transformer (NST) Characteristics

Post by r_c_edgar » Sun Feb 09, 2003 8:06 pm

Just a quick recommendation, in the future I'd suggest posting only a link and not the actual text of the article - some people are not comfortable with others reposting their material without permission (and it can easily become out of date compared to their current page).

Other than that, though, quite good info for those using NSTs - definitely check it out.

--Ryan

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Re: Neon Sign Transformer (NST) Characteristics

Post by swarm » Mon Feb 10, 2003 2:57 am

I posted an excerpt with exactly what I felt relevant so that a person could get a feel for the information at the site while also reading something relevant to Richard's NST thread.

The author's name and site were included.

I will not give up my fair use rights.

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Re: Neon Sign Transformer (NST) Characteristics

Post by Richard Hull » Mon Feb 10, 2003 9:23 pm

Of course one of the posters (John DeArmond) got it totally incorrect and gave some great misinformation that will forever mislead.

No neon transformer will ever supply rated current at even a tiny fraction of its rated voltage....Nor are they designed to do this. They are NOT constant current systems. So, the discussion posted is misleading and erroneous.

The first poster sort of got it right, but never went the full distance like I did in the full empirical experiments posted in the files forum on NST.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Neon Sign Transformer (NST) Characteristics

Post by JohnCuthbert » Mon Feb 10, 2003 11:47 pm

First off, sorry for making this thread longer.
Second, I am a chemist by training so please remember that when you point out why this wouln't work, but this is the thought that struck me, perhaps someone will be kind enough to tell me why it won't work.
The trouble with NSTs is they are current limited. They have to be in order to offset the negative resistance of the neon tubes they run. The makers could do that by just putting a very big resistor in series with a high voltage supply, but the efficiency would be poor. They could use a capacitor to act as the current limiter; but they are transformer manufacturers, they like coils of wire, so they use an inductor.
I have a nst sitting arround upstairs, it is rated for 10 KV centre tapped and 35 mA (Not at the same time, of coure). That means that it could be represented by a 10 KV transformer with a big enough inductance to limit the current to 35 mA. that's about 286 K Ohms.
What happens if I put a capacitor (of a high enough voltage rating ) in the circuit too? Can I "resonate out" the inductance?
If I find a capacitor with the same reactance as the "built in" inductance, will the current be limited by the internal resistance of the secondary (about 13 KOhms) and whatever the load is?
If I have the maths right (at 50Hz) I need 11 nF at 14120V , OK that's not exactly a "junk box" component, but I could make one.
If the rest of the circuit limits the current to 35 mA then the I^2 R losses in the transformer are the same, and it isn't running any more volts than usual, so it should be OK.
The $6,000,000 question is could I then drive 35 mA through a 286 -13 ie 273 KOhm load?
Any thoughts?

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Re: Neon Sign Transformer (NST) Characteristics

Post by Richard Hull » Tue Feb 11, 2003 3:56 pm

You can resonate the secondary. However the real limitation in current is the iron core and how it is made to limit current. The system is not just current limited, it is energy limited. Something a lot of folks lose sight of. The iron magnetic shunt WILL bypass flux under load and this is power. Magnetizing energy that is shunted out to do no useful work. Even at resonance there would be a limit back at the core which doesn't care if the secondry resistance/impedance is 6 ohms or 60,000 ohms.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Neon Sign Transformer (NST) Characteristics

Post by grrr6 » Tue Feb 11, 2003 7:50 pm

Has anyone ever tried completely taking the shunt laminations off the core rebuilding a NST? I suppose that wouldn't be too smart, as you would easily burn your secondary if you didnt current limit the secondary on your own, but that would make the supply more efficient and powerful. Either that or take a few layers off the shunt laminations, however much you need, is however much you saw off.

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Re: Neon Sign Transformer (NST) Characteristics

Post by swarm » Tue Feb 11, 2003 9:15 pm

Richard, I understand that you find the NST has definate limits in fusor design but since that is what I'm limited to at the moment for my "demo" fusor (I'm saving the hydrogen feed and worries about hard xrays and fast neutrons for mark II), I'd be interested in hearing your thoughts on how to ring the most from a nst for a demo, short of depoting it and attacking the shunts with a hack saw.

At various sites and places I've seen speculation about power factor correction, various resonance balencing schemes and voltage multipliers. All in isolation and with a heavy emphasis on the speculation aspect.

What would you recommend for a 9kV 30A nst?

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Re: Neon Sign Transformer (NST) Characteristics

Post by Richard Hull » Tue Feb 11, 2003 10:18 pm

Tesla coil builders remove the magnetic shunts all the time. Is it worth it? I doubt it for you are still limited by the impedance of the secondary a bit. However, you WILL get more energy out of the device. How much will depend on the individual manufacturers core design and secondary impedance. If it is all you have, and there is no other solution, and you know what you are about, you could remove the shunts after the messy job of depotting the tar out of the transformer and maybe double the power output.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Neon Sign Transformer (NST) Characteristics

Post by swarm » Tue Feb 11, 2003 10:40 pm

Unpotting and modifying NST's from:

http://www.hills2.u-net.com/tesla/neon.htm

"While the core is apart, the current capacbility of the transformer secondary may be increased by removing a few of the shunt laminations. Never take out more than 2 or 3 of these laminations per side, as the additional power output will burn out the secondaries. 70-75 ma can be achieved from a 60 ma unit using this method."

Personally I think my family would rebel during the depotting process, which is also described at the site.

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