Friday, December 1, 2017

Neon glow lamp centennial

On November 30, 1917, Pennsylvanian Daniel McFarlan Moore, while working for General Electric in New Jersey, filed the patent for the neon glow lamp. Here's the top of the patent filing:
The top of the first page of Moore's patent filing for the neon glow lamp (US Patent 1,316,967)
D-mac called his invention a "gaseous-conduction lamp". In my opinion an apter name would have been "negative-glow lamp". At the time, it was commonly referred to as a "Moore lamp" and today we mostly call it "neon glow lamp".
Early gas-discharge tubes of the Geissler type existed at the time. In a Geissler tube most of the light is emitted by the positive-column part of the discharge. The negative-glow region close to the cathode only occupies a small fraction of the length of the tube. Modern fluorescent lamps, including CFLs, and the gas-discharge tubes used in neon signs are direct descendants of the Geissler tube. Moore's invention was to tweak the gas pressure and electrode spacing to make the negative glow occupy most of the discharge. In plasma physics we call this a short, or restricted, glow discharge and some weirdos with too much time on their hands perform elaborate computer simulations of such lowly devices.
The value of Moore's invention is that a short glow discharge can be operated at much lower voltages. Moore used an 80-20 mixture of neon and helium and mentions in the patent that 220 volts is then sufficient to turn the lamp on. In the US at that time, 220 volts was a common line voltage for commercial lighting, so that was probably his target voltage. Moore also made his lamp mechanically compatible with incandescent light bulbs:
One version of Moore's lamp, compatible with an incandescent bulb. The electrodes (13 and 14) are mounted on glass rods attached to the base of the glass tube. Figure from the patent filing.

With an optimal Penning mixture (99-1 neon-argon mixture), mains voltages of 110 volts, or even 100 volts, are more than sufficient. Miniature neon glow lamps are a commodity and can be bought for 6 cents to 60 cents, depending on order size and if you have time to wait for the boat from Shenzhen. They can be operated at DC or AC (as discussed at length in the patent filing) at voltages from about 75 volts, to all common mains voltages globally, up to the point where the glow discharge transitions to an arc discharge and the inside of the tube becomes covered by cathode material, causing a short circuit.
So, well done, Daniel McFarlan Moore! A wonderful invention, with some properties that could enable some novel applications, but that's another post, or two...

Monday, November 20, 2017

Battery-powered boost converter for neon glow lamp

I'm working on a battery-powered DC-DC boost converter for neon glow lamps. It's a switched-mode booster with a 9-volt battery providing input power. The output needs to be at least 90 volts or so, with a current draw in the range of 10 μA to 1 mA. For the target application I also need it to be very quiet below 1 MHz. These constraints (relatively high voltage, very low current, high frequency) puts it in an unusual corner of design-parameter space where I haven't found any ICs (integrated circuits) for sale.

So I'm rolling my own that will ultimately use a CMOS 555 timer (LMC or TLC) as a switch. I'm working my way up in frequency and am currently using a standard NE555. I've been breadboarding square-wave oscillators to compare the recommended datasheet configuration to some alternatives. The other night I felt lucky and skipped ahead and quickly implemented the rest of the booster using an available MPSA42 "high voltage" NPN BJT, a 4.7 mH axial inductor with unknown stray capacitance and self-resonance frequency and a neon glow lamp as the load. Here's the result:
Neon glow lamp DC-DC boost converter MkI. It's beautiful to me!

Close up with NE555 DIP-8 IC on the left and TO-92 MPSA42 BJT to its east-southeast. To the left of the glow lamp is an oversized black diode, with the green inductor behind it and the brown filter capacitor behind its anode lead.

And zoomed out: no cheating, the 9V battery is the only power source
So I passed the first milestone: I am powering a neon glow lamp with a 9V battery! The open-circuit output voltage was 120V. The good performance of the MPSA42 transistor was a positive surprise. It probably wasn't intended for power supplies, but I'll keep it at least for the next few design iterations. However, getting the operating frequency above a megahertz will require some effort. While I'm waiting for promising inductors to arrive (hopefully with low enough stray capacitance), I'm doing circuit simulations with ngspice, but that's another post.

Friday, November 17, 2017

Calling computers names in Swedish

I wrote a correction to the very nice article "Carl-Gustaf Rossby: Theorist, institution builder, bon vivant". Rossby and Germund Dahlquist developed a numerical weather model for the BESK computer in 1953-54 and the article made the erroneous claim that "Rossby pursued numerical weather prediction in Sweden in an era in which there was no Swedish word for digital computer". I listed the five Swedish terms for digital computer that were in use at the time. My purpose was not to nitpick, but to make the point that Stockholm in 1953 might well have been uniquely fertile soil for developing novel applications for computational science because of BESK and Dahlquist.

Through luck and skill, Sweden managed to stay out of WWII and had a booming, technologically advanced, industrial economy in the post-war period. In 1947 the Swedish government initiated attempts to buy a computer from the US. A Swedish delegation of five young engineers got remarkable access to the principals behind the early computers in the US. However, ultimately the Swedish government was denied an export license for a US computer and BESK was rapidly designed and built in Stockholm (at my alma mater KTH) using know-how gained during the visit to the US. BESK became operational in September 1953 and was briefly the fastest computer in the world, capable of 16,000 additions per second. It was also the first computer to (partially) use semiconductor electronics (400 germanium diodes).

The Swedish government's primary need for a computer was to support their nuclear-weapon program, which could explain the denied US export license, and to decrypt intercepted Russian radio communication. The ability to decrypt all electronic communication between Berlin and the German embassy in Stockholm had been instrumental in keeping Sweden out of WWII. Continued access to encrypted Russian diplomatic and military radio communications to help navigate the cold-war political environment was a strong motivation.

In stark contrast to this political and military pragmatism was Dahlquist's scientific idealism. His goal was to disperse rigorous computation through all fields of science and technology. Together with co-author Åke Björck he wrote the world's first textbook on numerical methods, with an emphasis on numerical accuracy and stability. Dahlquist's collaboration with Rossby on numerical weather modeling was one of the main achievements of this quest. Their work enabled 24-hour national weather forecast in Sweden from September 1954, 4-5 years earlier than in any other part of the world. Rossby's decision to return to Sweden from the US in 1953 to pursue weather modeling on computers was a wise one, not the folly of trying to do state-of-the-art computing in a place that did not yet have a word for computer.

Thursday, August 17, 2017

NE-2 and IN-3 neon glow lamps

I've been working on a summer project with an intern. One of the tasks has been to characterize various neon glow lamps by measuring the voltage across them as a function of the current through them. I need detailed data and models for certain alternative applications. The only remaining main stream intended use for neon glow lamps is as indicator lights capable of operating on mains voltage (100 to 240 V AC). If the on switch on your power strip glows red, it's probably due to a neon glow lamp inside. Electric coffee makers, waffle irons, et cetera, also have them. But neon glow lamps are slowly, but inevitably, being phased out by LEDs.

A company called CEC Industries still makes what seems like a perfect clone of GEs classical NE-2 neon glow lamp. It's a Taiwanese company that moved production first to Korea and then gradually from there to mainland China. I'm not sure if their neon glow lamps are still made in Korea, or in China. On the usual Chinese web sites I couldn't find any NE-2s, or similar clones. I did find mini neon glow lamps that are shorter, and sometimes smaller diameter, than the NE-2. So maybe the NE-2s are still made in Korea. We tested a couple of the Chinese mini neon lamps and they worked pretty well, but that's the topic of a future post. The NE-2s can be bought from Amazon or a couple of specialized web sites.

The classical GE NE-2 is a 1/8" diameter glass tube with filled with low-pressure Penning mixture (neon with a bit of argon) and two parallel cylindrical electrodes. All the other neon lamps I've seen have that same basic design. Except the Soviet IN-3, which has a square-plate cathode and a wire-frame anode. I recently got some IN-3s. They can be found on Ebay and on a couple of Ukrainian web sites that sell old Soviet electronics.

Here's a photo of an NE-2 (left) and two IN-3s, one showing the back (middle) and the other one showing the front (right).

I don't know why the IN-3 glass tubes have the smoky appearance. Here's a close up to better show the internal structure.

And here they are fully lit up by a DC voltage source!

The IN-3 on the right was intentionally inserted with the wrong polarization to show the difference.

Neon glow lamps have an interesting relationship between the voltage across them and the current through them. The plot below compares the measured voltage versus current for two seemingly identical NE-2 lamps (NE-2 #1 and #2, respectively, in the legend). For low currents (up to about 10 microamps), the device is in a state called a dark (or Townsend) discharge. As current is increased, the breakdown voltage is reached and the lamp becomes a glow discharge, with a clearly visible orange glow. The range of current values where voltage is decreasing (negative differential resistance) is called the subnormal glow. The normal glow is characterized by approximately constant voltage and the abnormal regime again sees the voltage increase with current.

The data displayed is for eight different measurement series, four for each lamp. To resolve the low-current end of the subnormal glow, which is of particular interest to us, a large resistor was put in series with the lamp to limit the current. The voltage across the resistor and neon glow lamp was then gradually increased from zero to 400 V and the current through them measured, as well as the voltage drop over the lamp. The measurements where then repeated with the voltage gradually reduced from 400 V to zero. To reach currents in the abnormal regime, the resistor value was reduced and the procedure repeated.
The V-I (voltage-current) characteristic for the two NE-2s is very similar, with the exception of the hysteresis exhibited by NE-2 #1 where for the central range of current values the voltage was 2-3 V larger with increasing current (the four blue squares in the middle of the plot) than with decreasing. This is probably a thermal effect that should disappear if enough time passed between measurements for the cathode temperature to reach steady state.
Two IN-3s were then similarly characterized, with the result shown in the plot below.

Here the the V-I curves for the two lamps differ significantly! Notably, the subnormal-glow regime extends to very high currents, at least a milliamp. This should make an IN-3 an excellent negative differential resistor for the oscillator we used as our entry for the Flashing Light Prize 2017, but that's a separate post! It's also striking that the IN-3 #1 lamp has an extinction voltage below 40 V.
So characterizing neon glow lamps well enough to develop realistic models for their V-I curves takes a lot of diligence and patience! Or you could automate things by putting together a little circuit controlled by an Arduino sketch. I've opted to do the latter and am waiting for some ICs to arrive. Stay tuned...

Thursday, July 27, 2017

Flashing Light Prize 2017: "Discharge-tube oscillator with transformer"

I'm working on a project this summer with an excellent student name Barbara. We are getting some real results that I expect to post about soon. However, this first post on the project is just a silly spin off: our entry for the Flashing Light Prize 2017:

We had characterized a number of different neon glow lamps by measuring the voltage V across them as a function of the current I pushed through them. All glow discharges, including the ubiquitous neon glow lamps, have an interesting V(I) graph with a region known as the subnormal regime where the differential resistance is negative: dV/dI < 0. With the operating point in the subnormal regime a glow discharge can be used in a Pearson-Anson oscillator (I show a print out of that Wikipedia page in the video). We had a breadboard with a working oscillator circuit when I heard about the geeky fun known as the Flashing Light Prize and I thought we could spend an afternoon to modify the oscillator circuit to make an incandescent bulb blink. Here's what I came up with:

The circuit does work and is the one used for our entry. However, we first had to go through a frustrating number of different glow discharges, transformers and incandescent bulbs. In the video I mention the severe current limitation imposed by keeping the operating point in the subnormal glow regime and how poor a fit that is for an incandescent bulb. I forgot to mention how hard it is to make a transformer perform acceptably at a frequency of one Hertz, which makes the inductive reactance awfully small.

The resistor R1 is a 720 kOhm one, rated at 2 W, in series with a smaller high-power resistor (inside the glass jar in the video). We think the latter is a couple of hundred kOhms. The capacitor is one microFarad, rated at 1500 V. The glow discharge is a discharge tube with adjustable pressure and distance between the electrodes. The working gas is air and we pumped the pressure down to about 400 mTorr. The final adjustment we made was to increase the electrode distance to move out on the right side of the Paschen minimum to increase the breakdown voltage from about 400 V to 700 V. The transformer is just a power transformer with about 30:1 turn ratio. Finding a small enough incandescent bulb was also hard until we found a parts number in a comment on one of the other entries. It's supposed to operate at 1.5 V and 15 mA, but we could not get the secondary current that high, but apparently we got close enough! Search for "grain of wheat light bulb" on AliExpress or similar site. I found a bag of 50 for $20.

More work than expected to finally get steady and reliable blinking, but it was a sight to behold! The video does not do the colors justice. The discharge tube glow was a beautiful blueish purple and the incandescent bulb a yellowish orange.

Saturday, May 27, 2017

Validation and benchmarking of two particle-in-cell codes for a glow discharge

Here's a PDF with the slides for my ICOPS 2017 talk entitled Validation and benchmarking of two particle-in-cell codes for a glow discharge (presentation "TH 1.1-3" in the conference program).

A follow-up comment to the talk was that "a code with known flaws is much better than a code with unknown flaws". I think that summarizes the topic of validation nicely. The purpose of validation isn't necessarily to pass with flying colors, but to learn for what cases to trust a code, and for which cases NOT to trust a code. If you find issues that are easy to fix, great! But even if you find issues that are not possible to fix immediately (or ever), validation can still be worthwhile.

The full paper can be downloaded from here.

Thursday, May 25, 2017

Particle-in-cell simulation of anomalous transport in a Penning discharge

I gave two talks at the IEEE International Conference on Plasma Science (ICOPS) this morning in Atlantic City. The first one was called Particle-in-cell simulation of anomalous transport in a Penning discharge (Carlsson, Kaganovich, Raitses, Powis, Smolyakov and Romadanov). It was the first time I used Google Slides for a talk. Worked like a charm and I'm pretty happy with it, even if I'm uncomfortable with having the source files for my slides in the Google cloud and not in a git repo of my choosing. I think ICOPS will make all talks publicly available online, but I think they expect PPT or PDF slides, hence the above link to the original version of the talk, if something gets lost in translation.