From Orange Glow to Logic Flow – The World of Glow Lamps

I’ve always been fascinated by gas-discharge lamps. I don’t know why—perhaps simply because they glow in the dark. Or perhaps because they don’t always shine with a steady light, but sometimes flicker a little, as if they were alive, or burning like a flame. Or maybe because it seems as though the light is created out of nothing, inside an empty glass bulb. Of course, it’s the gas inside that glows, but the gas itself is invisible, completely transparent, so the lamp is a bit like having a tiny aurora dancing inside it.

The glow lamp (also called a neon indicator lamp or cold-cathode lamp) is a miniature gas-discharge lamp. In its tiny glass envelope are two electrodes, and the space around them is filled with a noble gas mixture. When a sufficiently high voltage is applied to the electrodes, a glowing light appears around them—this is known as a glow discharge. This phenomenon occurs not only with noble gases but also with other gases, such as rarefied air, provided the gas is thin enough—at a pressure of about 0.1–1% of atmospheric pressure.

A glow discharge occurs due to the ionization of the gas. Positive ions and free electrons are always present in gases. They are produced by the ionizing effects of sunlight, natural background radiation, and cosmic rays. When a voltage is applied to the electrodes, these ions begin to move toward the electrodes. The rarefied gas conducts electricity slightly. The higher the voltage, the greater the speed the charged particles acquire. As they race along, they collide with other gas molecules and with the electrodes. These collisions produce additional charge carriers. This process, known as collision ionization, causes the number of charge carriers in the gas to grow in an avalanche-like manner. Within a short time, a strongly ionized gas mixture forms—plasma—that conducts electricity well. In the plasma, the gas molecules are excited to higher energy states. When they return to a lower energy state, they emit photons, and as a result the plasma begins to glow.

What is a glow lamp good for?

Glow lamps used to be manufactured in a wide variety of sizes and shapes. The smallest were tiny glass beads about the size of a pea, with thin, solderable wire leads, typically rated for 50–100 V. The largest were made for connection to the 230 V mains, with a 60 mm diameter bulb and a standard E27 Edison screw base. I think the most common type is the 120 V General Electric NE-2H (measuring Ø 6.35 mm × 27 mm).

This is an old technology—the first such lamp was patented in 1919, but gas-discharge tubes operating on the same principle (Geissler tubes) were already known in the 1850s. Today, the glow lamp is considered obsolete. LEDs are much brighter, last longer, and are cheaper. However, glow lamps can be easily adapted to the mains voltage with a simple series resistor, and during operation they consume very little current, which is why they are often used as voltage indicators. A typical glow-lamp voltage indicator is the test screwdriver used by electricians. Glow lamps can also be found in the red-lit switches of power strips with switches, in illuminated staircase light switches, in night lights, and in many clothes irons.

Today they are a rarity, but here and there you can still find a glow-lamp variant designed to imitate a candle. In these, the electrode is shaped like a flame, and the glow discharge flickers around it like a candle flame. Lamps with similarly shaped electrodes were manufactured all over the world, including here in Hungary.

When a glow lamp is powered by alternating current, the glow appears on both electrodes as well as in the space between them. However, if it is connected to direct current, the glow forms only around the negative electrode (the cathode). If the cathode is shaped into a particular form—such as a numeral—the glow will take on that shape. This principle was used in Nixie tubes, which were the numeric displays of the vacuum tube era.

Colors

Each gas glows with a different color. Neon produces a more intense orange, argon a bluish-purple, helium a yellowish-white, krypton white, and xenon a bluish-white. Rarefied air emits violet and green light—these are also the colors of the aurora. In glow lamps, the fill gas is usually a special mixture called a Penning mixture, consisting of 99.5% neon and 0.5% argon. The small amount of argon is added because the breakdown voltage of the mixture is lower than that of pure neon. The gas is highly rarefied, with a pressure—depending on the design—between 1 and 25 hPa [1]. Due to the neon content, this gas glows with an orange hue.

There are also colored lamps, but in these the color comes not from the gas, but from a layer of phosphor applied to the inner wall of the glass envelope. Such lamps are filled with more argon, which also emits ultraviolet light; the UV light excites the phosphor, causing it to glow green, blue, and so on. The electrical properties of these lamps are similar to those of neon-filled ones, but their operating voltage is usually slightly higher.

Electrical properties

Glow lamps are not efficient. For example, the luminous efficacy of the General Electric NE-2H lamp is only 2.2 lm/W, which falls far short of the roughly 100 lm/W achievable with an LED or a fluorescent lamp.

From an electrical point of view, a glow lamp can be characterized by three different voltages. The first is the ionization voltage (or striking voltage), which—depending on the lamp—ranges between 100 and 120 V. In order for the lamp to start glowing, this voltage must be reached across the electrodes. The maintaining voltage (10–20 V) is the minimum voltage required to keep the plasma continuously sustained. When the lamp is lit, the voltage measured across the electrodes is called the operating voltage. This is fairly constant—usually around 70–90 V—and is practically independent of the lamp’s current.

Megjegyzés: Véleményem szerint a Wikipédián ( https://en.wikipedia.org/wiki/Neon_lamp ) feltüntetett „tipikus lámpakarakterisztika” helytelen. Valójában egy Geissler-cső jelleggörbéjének alsó szakasza, meglehetősen durván megrajzolva. A kritikus 0,1…3,0 mA-es működési tartomány nincs megfelelően feltüntetve rajta, ezért magam mértem meg egy GE NE–2H lámpát. Ez lett az eredmény:

Működés közben körülbelül 0,1–3,0 mA folyik át a lámpán. A glóbuszlámpát nem lehet közvetlenül a hálózatra csatlakoztatni, mivel túl nagy áram folyna rajta keresztül, és a lámpa károsodna. Sorba kell kötni egy előtét ellenállással, amelynek értéke könnyen kiszámítható:

Resistance [kΩ] = ( Power voltage [V] – Operating voltage [V] ) / Operating current [mA]

That is, for example, if we want to operate a GE NE–2H lamp from a 230 V AC mains supply, we calculate as follows: the peak value of the mains voltage is 230 V × √2 = 325 V. The operating voltage of the lamp is 156 V, and let’s set its maximum current to, say, 250 μA. In that case: Re = (325 − 156) / 250 = 0.677 MΩ ≈ 680 kΩ.

At this current, the lamp already produces a good level of brightness, but it will also glow at a current of around 50 μA, which would require a 3.3 MΩ ballast resistor.

The effect of darkness

A glow discharge occurs due to the ionization of the gas. Positive ions and free electrons are naturally present in gases. They are generated by the ionizing effects of ambient light, natural background radiation, and cosmic rays. It is an observable phenomenon that glow lamps ignite more reluctantly in darkness: ignition takes more time, and the striking voltage will be higher. This is because, when the gas is not exposed to light, fewer ions are produced in it (and the ions previously created eventually recombine and disappear).

Several solutions have been developed to ensure continuous, weak pre-ionization. One method is to add a small amount of radioactive thorium to the glass of the lamp, which emits ionizing radiation. Such lamps emit radioactivity at a harmless level. The problem with them is not this, but that thorium-containing glass gradually turns yellow and loses mechanical strength. If the envelope breaks, it can cause a short circuit and, in hazardous environments, even start a fire.

Another method is to apply a coating containing barium or strontium to the surface of the electrodes, which are usually made of nickel or molybdenum. However, such a coating wears off over time due to constant ion bombardment, and the lamp’s electrical properties deteriorate.

A third common solution is to add a small amount of krypton-85 to the gas mixture inside the envelope. Krypton-85 is a radioactive noble gas that emits beta particles, which ionize the neon. Since its half-life is 10.8 years, its effect does not last forever. The general experience is that older lamps of this type still work, but are slower and less reliable than when they were new.

Lifetime

In theory, the lifetime of glow lamps (and Nixie tubes) is unlimited. In practice, experience shows that they can operate flawlessly for 30–40 years, provided the glass envelope does not crack. The most common problem is cathode corrosion. During operation, the electrodes are bombarded by high-speed ions, which cause the electrode material to start disintegrating. There have even been Nixie tubes whose cathodes completely fell apart. The disintegrated metal condenses on the inside surface of the envelope, at first forming a dark, and later a completely black coating. Interestingly, such lamps remain electrically sound, with only a slight increase in striking voltage. However, since the light can no longer pass through the envelope, they become unusable.

A fekete metál bevonat csak működés közben képződik, míg a diffúzió miatti gázkárosodás az eredeti csomagolásukban használatlanul tárolt lámpákat is érintheti. A katódkorrózió csökkenthető a lámpa alacsonyabb áramerősséggel történő működtetésével. Manapság, mivel a Nixie csövek reneszánszukat élik a hobbi felhasználók körében, gyakran látott megoldás egy folyamatosan világító kékes UV LED felszerelése a cső alá. Ez nemcsak esztétikailag vonzó tulajdonság – az UV-sugárzás segít ionizálni a gázt, így a cső valamivel alacsonyabb feszültségen és áramerősséggel működik, ami növeli az élettartamát.

The composition of the gas mixture inside the lamp can deteriorate not only if the glass cracks and air enters the envelope, but also over time on its own. The reason for this is diffusion: noble gases—albeit with great difficulty and very slowly—are capable of passing even through solid glass and escaping. This same phenomenon is responsible for the failure of those fashionable decorative plasma globes. As the gas escapes, the striking voltage increases, the lamp ignites more slowly and with greater difficulty, and instead of glowing steadily, it begins to flicker.

Interesting circuits

If we connect a glow lamp in series with a resistor and apply the supply voltage, it will light up. So far, nothing unusual—but now let’s connect a photoresistor in parallel with the glow lamp! A photoresistor becomes conductive when exposed to light, diverting current away from the glow lamp and causing it to go out. If we cover the photoresistor with a black cap, it receives no light, its resistance increases, and it conducts poorly—effectively as if it weren’t there—so the glow lamp lights up.

Light present → lamp dark; no light → lamp lights.

This works well as a night guide light, but note that it is essentially an inverter, i.e., a NOT gate. If we place two photoresistors in parallel, either one will trigger this behavior regardless of the other. We then have two inputs, and what we have built is a NOR gate.

From this, it is possible to build logic networks if the photoresistors are illuminated by the glow lamps of other gates. Flip-flops, memory elements, and counters can also be constructed. In principle, one could even make registers, adders, and arithmetic-logic units, but describing these would go beyond the scope of this article. Those interested should look up the GE Glow Lamp Manual (General Electric, 1965) online—it contains extensive descriptions.

In addition, I recommend the website of Dutch radio amateur Pieter-Tjerk de Boer (PA3FWM). He built Johnson counters from glow lamps, and from these, along with Nixie display tubes, assembled a digital clock (http://www.pa3fwm.nl/projects/neonclock/).

In the September 1964 issue of the Australian magazine Radio Television & Hobbies, Keith Jeffcoat presented an interesting idea: a voltmeter operating with a glow lamp. Who knows—one day we might even find it useful…

Conclusion

Glow lamps may no longer be the shining stars of modern electronics, but their charm is hard to ignore. They are simple, robust, and capable of far more than just telling you whether the mains is live. From humble voltage indicators to quirky logic gates, from candle imitations to intricate digital clocks, these tiny glass bulbs have served in countless roles over the decades.

Today, they mostly survive as curiosities—flickering away in an old power strip switch, hidden in a phase tester, or glowing proudly in a hobbyist’s hand-built Nixie clock. But behind their warm orange glow lies over a century of electrical ingenuity and a reminder that even “obsolete” technology can still teach, inspire, and delight.

So if you ever come across a dusty box of NE-2Hs at a flea market, don’t dismiss them as relics. They might just spark your next project—or at least light up your day.

References

William G. Miller. Using and understanding miniature neon lamps. Howard W. Sams & Co., Inc., Indianapolis, IN, 1969.