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How to drive nixie tubes

If you want to build a nixie display for your project, you'll have to know how use and drive nixie tubes. Discussed here are basic and multiplexed use. There are a number of schematics to illustrate it all.

How it works

A nixie tube is a gas discharge device. Their colour and working voltage are determined by the properties of the neon gas within. They ignite at 140-170 V, slightly varying by type. Once ignited, their resistance is very low so a series resistor is necessary to limit the current, typically to 1-5 mA. The working voltage is 90-130 V depending on type. If the voltage drops below the turn-off voltage, the nixie will go out. For a ZM1000 the turn-off voltage is specified at 118 V at room temperature, but my small japanese nixies still work at 100 V.

Because supply voltages of 170-300 V are used, some people think you need switching devices that can handle these large voltages. In practice, however, there are two circumstances that reduce the voltage you have to switch.


Basic cathode driver circuits

The 7441 and its successor, the 74141 were very common nixie driver IC's from the TTL era. If you can find a few 7441's or 74141's, you can use these vintage IC's in a project. But they are hard to get and being TTL devices, they use more power and have larger input currents than modern IC's. Many older MOS IC's are not able to drive TTL inputs. In those cases, you will need a buffer to drive your 74141! So what are your other options?

So your realistic options all take up more board space than the good old nixie driver decoders. Although Mike Harrison has shown in his nixie clock , that you can put a large crowd of discrete transistors on a small PCB area.

Multiplexing nixie tubes

In a multiplexed display, all the corresponding cathodes of the nixies are connected together in a bus structure. The anodes of the nixies are switched on one by one and the right cathode is activated for every digit. If you do this fast enough, you get the illusion that all digits are on simultaneously.

The advantage of multiplexing is that you need fewer decoders and less wiring.

Multiplexing is nothing new. Multiplexed LED displays are used on alarm clocks, digital room thermostats, digital voltmeters, etc. A lot of voltmeter chips, calculator chips, alarm clock chips etc. already have multiplexed outputs to save IC pins. If you use a microcontroller such as a PIC for your project, you'll have to program a display multiplexing routine.

So how do you multiplex nixie tubes? Isn't that complicated and expensive?

To multiplex nixies, you need to includer anode driver circuits in your design. These turn on and off the anodes of the nixies. The anode drivers "float" at the supply voltage of 170-250 V, and they are driven by the main circuit (your clock, meter, calculator) that is at ground potential. In order to bridge this voltage gap, either a high-voltage transistor or a capacitor is used. Depending on the technology used in the main circuit, the anode drivers are driven with a 2-5 voltage swing from TTL or CMOS powered by 5 V, or maybe 24 V for older PMOS or NMOS circuits.

I have found several examples of multiplexed circuits for nixies or 7-segment neon displays. One of them, an anode-scanning display by Philips Elcoma which even has a dimming control, was in a bulk post on the sci.electronics.schematics news group. Another was the driving circuitry of an old Japanese calculator that I was able to analyse and repair. In the Game Archive web site, which contains a lot of information for pinball machine and video game colectors I found a schematic of a Bally 7-digit pinball machine display that uses 7-segment flat-panel neon displays. National used to have some anode driver circuits for Panaplex® displays, like the DM8880, but they are in short supply today.


Sample schematics

In february 2000, someone sent a bulk post on sci.electronics.schematics of scans of old high-voltage circuit examples. Some of these were nixie driving circuits, such as

Getting nixie manuals and data

There are a lot of sites on the Net where you can find nixie manuals and data. See the links page.

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