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Neon sign transformer power output

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radiotech

Tue Sept 25 2018, 07:23AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

If you passed a constant DC current through the secondary
in the range of the rated RMS current, the voltage across
the winding would increase as the winding heated.

This would give R/t curve. At any point in the curve
you would know wattage dissipated in the winding.

The core loss would not enter into this value.

The winding temperature also would be known with the
copper wire constants.

With AC current, equated to temperature rise and
resistance , the actual out put power would be know,
by maximum power transfer theorem, from resistance of winding
to resistance of resistor load. Reactance could not affect this
because heat is the determinant.

But now comes the realization that the output power of
any transformer is temperature dependent.

Sulaiman

Tue Sept 25 2018, 03:32PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

kludgesmith has either faked his graph (I do not believe that)
or done some careful measurements and kindly shared his data with us.

Look at his graph then give answers or theories that fit the data.

Or supply better data.

radiotech

Tue Sept 25 2018, 09:04PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The original question was how much power
can a neon sign transformer deliver ?

I fetched a neon transformer on hand.

5000 volts 0.018 Amps secondary.

The DC resistance of the room temperature was 16,500 ohms.

I set my Clarostat power to 16,000 ohms.

With a milliamp meter in series, I adjusted the primary
voltage to 110 volts 60 Hz.

The secondary current was 0.018 amps, trim set with variac.

The AC voltage across the resistor was 280 volts.

The output power was about 5 watts.

I believe that these transformers do not operate at
high voltage, but furnish it to strike the load.

Transformer makers rate in volt amps on the primary,
and feet of tubing of a certain diameter they light.

So in fine detail, the stable (temperature) resistance of the secondary
would determine the power output.

klugesmith

Sun Nov 18 2018, 07:00AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Back at last, with two different topics. In two back-to-back posts.

First: respect for Radiotech's suggestions. I measured the DC resistances of both windings in that little 4000 volt transformer. 26.5K secondary. 14.4 ohms primary (equivalent to 16K in secondary circuit, if this were a regular, tightly coupled power transformer). Total 42.5K, for theoretical short-circuit current of 94 mA, and maximum power 94 watts into 42.5K load. (2000 V x 47 mA).

We saw that the actual short-circuit current is only 6.6 mA, implying that internal impedance is about 606K. This chart shows output I vs V if the 606K were real (resistive), imaginary (as from 1607 henries at 60 Hz), or a mixture that closely fits actual observations: 601K from inductance and 76K from resistance.

It came as a surprise that the resistance needed to be that high (in spreadsheet calculator simulation) to get 2/3 of open circuit voltage and 2/3 of short circuit current. Max power 11.73 watts, with 606K load. The voltage and current factor would be 0.707 if internal impedance were purely inductive.
Makes me want to re-measure the device with new instruments (see following post). Will include loads that match DC winding resistance, as Radiotech did. Expect their current to be about the same as into a short circuit, say 6.5 mA, for power of a watt or two.

External capacitors could, I bet, allow more short circuit current and/or more open circuit voltage. Maybe even more real powah, at the cost of stresses that the transformer isn't designed for. Plenty of videos on youtube, by people who like using high voltage power transformers to make arcs.

klugesmith

Sun Nov 18 2018, 07:32AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Came to present new work, finally putting the Altoids-box mA meter into service.

Tonight I measured the first of my 15-60 NST's, with primary voltages of 60, 120, and 140, and loads of short, luminous tube, 1R, 2R, 4R, 8R, and open circuit.
277 V on primary will come tomorrow, if I have time.

The I-V chart is unremarkable, except for showing an apparent blunder in transcribing a reading at 60V input and 1R load. And the I values don't include the voltmeter current.

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.

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.

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.

klugesmith

Sun Nov 18 2018, 10:38PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

So far so good. With nominal primary voltage, I got:
425 watts into a matched resistance.
222 watts with 10 R's in series (14.4 kV x 15.5 mA). This is around where I want to work.
159 watts into a string of 25 Christmas lights, last year converted from parallel to series by snipping wire in 24 places.

The LED meters were hard to read in daylight, until some shades were added. Green optical filters would probably also help.

Here are today's IV curves for 70, 140, 210, 277, and 290 V on primary. Plus an iso-power line at 425 watts.

klugesmith

Tue Dec 11 2018, 10:48PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Once again I forgot to measure the DC resistances of big NST. Sorry, Radiotech, I'll catch up sooner or later.

Last Saturday 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.

Hey administrators, the pictures attachment problem here is even worse than last week!
It seems silly to continue this report when I can't upload charts.
Dialog says
"Please note
Allowed file types:
Any other file types uploaded will be instantly deleted."

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.

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