Nixie tube

  A nixie tube is an electronic device for displaying numerals or other information. The glass tube contains a wire-mesh anode, and multiple cathodes in the shape of Arabic numerals. Applying power to one cathode surrounds it with an orange glow discharge. The tube is filled with a gas at low pressure, usually mostly neon and often a little mercury and/or argon (a Penning mixture, (Weston 1968, p. 334),(Bylander 1979, p. 65)).

Although it resembles a vacuum tube in appearance, its operation does not depend on thermionic emission of electrons from a heated cathode. It is therefore called a cold-cathode tube (a form of gas filled tube), or a variant of neon lamp. Such tubes rarely exceed 40 °C (104 °F) even under the severest of operating conditions in a room at ambient temperature (Bylander 1979, p. 60).

The most common form of nixie tube has ten cathodes in the shapes of the numerals 0 to 9 (and occasionally a decimal point or two), but there are also types that show various letters, signs and symbols. A related device is the pixie tube, which uses a stencil mask with numeral-shaped holes instead of shaped cathodes. Some Russian nixies, e.g. the IN-14, used an upside-down digit 2 as the digit 5, presumably to save manufacturing costs as there is no obvious technical or aesthetic reason.

Each cathode can be made to glow in the characteristic neon red-orange color by applying about 170 volts DC at a few milliamps between a cathode and the anode. The current limiting is normally implemented as an anode resistor of a few tens of thousands of ohms. Nixies exhibit negative resistance and will maintain their glow at typically 20V to 30V below the strike voltage. Some color variation can be observed between types, caused by differences in the materials and gas mixtures used. Longer-life tubes that were manufactured later in the nixie timeline have mercury added to reduce sputtering (Bylander 1979, p. 60) resulting in a purple tinge to the emitted light.

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Applications and lifetime

  Nixies were used as numeric displays in early digital voltmeters, multimeters, frequency counters and many other types of technical equipment. They also appeared in costly digital time displays used in research and military establishments, and in many early electronic desktop calculators, including the first: the Sumlock-Comptometer ANITA Mk VII of 1961. Later alphanumeric versions in fourteen segment display format found use in airport arrival/departure signs and stock ticker displays. Some elevators also used nixies to display the floor numbers.

Average longevity of nixie tubes varied from about 5,000 hours for the earliest types, to as high as 200,000 hours or more for some of the last types to be introduced. It should be noted that there is no formal definition as to what constitutes "end of life" for nixies, mechanical failure excepted. Some sources (Weston 1968, p. 340) suggest that 50% reduction in emitted light would not be acceptable, however cathode poisoning resulting in incomplete digit display, whilst generally not preventing the tube from being used, may also be considered unacceptable. Nixie tubes are susceptible to multiple failure modes including:

• simple breakage
• cracks and hermetic seal leaks allowing the atmosphere to enter
• cathode poisoning preventing part or all of one or more characters from illuminating
• increased striking voltage causing flicker or failure to light
• sputtering of electrode metal onto the glass envelope blocking the cathodes from view
• internal open or short circuits which may be due to physical abuse or sputtering

Driving nixies outside of their specified electrical parameters will accelerate their demise, especially excess current, which increases sputtering of the electrodes. A few extreme examples of sputtering have even resulted in complete disintegration of nixie tube cathodes.

As testament to their longevity, and that of the equipment which used them, in 2006 several suppliers still provide common nixie tube types as service replacement parts, new in original packaging. Equipment with nixie tube displays in excellent working condition is still plentiful, though much of it has been in frequent use for 30-40 years or more. Such items can easily be found as surplus and obtained at very little expense. It is worth noting that in the former Soviet Union, nixies were still being manufactured in volume in the 1980s, so Russian and Eastern European nixies are still generally easily and cheaply available.

With the exception of a few specially-made mercury-free types, nixie tubes are not permitted aboard submarines of the U.S. Navy because of their mercury content.

One significant advantage of the ordinary Nixie tube is that its cathodes are typographically designed, shaped for legibility. In most types, they are not placed in numerical sequence from back to front, but arranged so that cathodes in front of the one that is lit obscure it minimally.

History

  The Nixie display was developed by a small vacuum tube manufacturer called Haydu Brothers Laboratories, and introduced in 1954 by Burroughs Corporation, who purchased Haydu and owned the name Nixie as a trademark. Similar devices that functioned in the same way were patented in the 1920s, and the first mass-produced display tubes were introduced in the late 1930s by National Union Co. and Telefunken. However, their construction was cruder, and they failed to find many applications until digital electronics reached a suitable level of development in the 1950s.

According to an article in the June 1973 issue of Scientific American magazine (p.66), the name Nixie was derived by Burroughs from "NIX I", an abbreviation of "Numeric Indicator eXperimental No. 1".

Burroughs even had another Haydu tube that could operate as a digital counter and directly drive a Nixie tube for display. This was called a "Trochotron", in later form known as the "Beam-X Switch" counter tube; another name was "magnetron beam-switching tube", referring to their similarity to a cavity magnetron. Trochotrons were used in the UNIVAC 1101 computer, as well as in clocks and frequency counters.

The first trochotrons were surrounded by a hollow cylindrical magnet, with poles at the ends. The field inside the magnet had essentially-parallel lines of force, parallel to the axis of the tube. It was a vacuum tube; inside were a center cathode, ten anodes, and ten "spade" electrodes. The magnetic field and voltages applied to the electrodes made the electrons form a thick sheet (as in a cavity magnetron) that went to only one anode. Applying a pulse with specfied width and voltages to the spades made the sheet advance to the next anode, where it stayed until the next advance pulse. Count direction was not reversible.

Later tubes, Beam-X Switches, had ten metal-alloy rod magnets internally; the magnets also served as electrodes.

Neon-glow counter tubes, usually base-10, similar in essential function, had a glow on one of ten cathodes, visible through the top of the glass envelope. Two sets of cathodes, ten per set, between the indicating cathodes moved the glow in two jumps to the next cathode. A well-known trade name for these in the U.K. was Dekatron.

Some Nixie-like displays made by other firms were called by various trademarked names including Numicator and Digitron. A proper generic term is "cold cathode neon readout tube", though the phrase "nixie tube" quickly entered the vernacular as a generic name. Hundreds of variations of this design were manufactured by many firms, from the 1950s until the 1990s.

Other numeric display technologies concurrently in use included backlit columnar transparencies (a.k.a. "thermometer displays"), light pipe, rear-projection and edge-lit lightguide displays (all using individual incandescent or neon light bulbs for illumination); Numitron incandescent filament readouts^[1]; and vacuum fluorescent display tubes. Prior to nixie tubes' coming into prominence, most numeric displays were mechanical in nature, using stepping switches either directly by use of cylinders bearing printed numerals attached to their rotors, or indirectly by wiring the switches' outputs to indicator bulbs. Later, a few vintage clocks even used a form of stepping switch to drive nixie tubes!

Nixie tubes were superseded in the 1970s by light-emitting diodes (LEDs) and Vacuum fluorescent displays (VFDs), often in the form of seven-segment displays. The direct descendant was the VFD, a cross between a nixie tube and a cathode ray tube. VFDs were simpler than nixie tubes, brighter, and did not require the complex cathode structure, resulting in high resolution graphics.

LEDs were better suited to the low voltages that integrated circuits used, which was a definite (sometimes essential) advantage for portable devices such as the emerging pocket calculators, digital watches and handheld digital measurement instruments. Also, LEDs were much smaller and sturdier without needing a fragile sealed glass tube and had lower power consumption than both VFDs and Nixie tubes, though they could not match the VFD in brightness until more than a decade later.

Revival

  Citing dissatisfaction with the aesthetics of modern digital displays and a nostalgic fondness for the styling of obsolete technology, significant numbers of electronics enthusiasts in recent years have shown interest in reviving nixies. Unsold tubes that have been sitting in warehouses for decades are being brought out and used, the most common application being in homemade digital clocks. This is somewhat ironic, since during their heyday, nixies were generally considered too expensive for use in mass-market consumer goods such as clocks. This recent surge in demand has caused prices to increase significantly, particularly for large tubes. The largest type known to be in the hands of collectors, the Rodan CD-47/GR-414 (220 mm [8.7 in.] tall)[1], have been sold for hundreds of dollars apiece, but these are extremely rare and only found in a few areas of the world by persistent and fortunate seekers. Prices for other large types displaying digits over 25 mm (1 inch) tall have increased by double, triple or more between 1998 and 2005.

In addition to the nixie tube itself, another important consideration is the circuitry to drive the tube. One of the more popular ways to do this is to use the Texas Instruments' SN74141 BCD Decoder Driver IC (or its Russian equivalents, the K155ID1 and KM155ID1, with plastic and ceramic packages, respectively). These have long since been out of production, much like the nixie tubes they were designed for.

See also

• Genericized trademark
• Sixteen-segment display

References

• Bylander, E.G. (1979), written at New York, , McGraw Hill, ISBN 0-07-009510-8, LCCN 78-31849.
• Dance, J.B. (1967), written at London, , ILIFFE Books Ltd, LCCN 67-13048.
• Weston, G.F. (1968), written at London, , ILIFFE Books Ltd