How to drive nixie tubes

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.

• First, the voltage across your switch will only rise to the supply voltage minus the turn-off voltage of the tube. When the voltage drops below this, the tube will become an isolator.
• Second, when there is one of the cathodes switched on, the anode voltage will be significantly lower than the supply voltage. So if your driver circuit always turns the next cathode on before switching off the previous one, your cathode voltages will stay below 65 volts no matter what your supply voltage is. You could connect a zener diode to one of the cathodes to prevent a situation where all cathodes are off. The once-popular SN74141 decoder - nixie driver had built-in zener diodes of 55 volts on all outputs that could handle 1 mA. It's predecessor, the 7441, did not have these, so you would have to take care to keep it's output voltage under 70 V under all cicumstances.

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?

• Use high-voltage transistors after a CMOS BCD-to-decimal decoder. As mentioned above, an Uceo rating of 70V is sufficient if you keep the supply voltage below 200V or clamp all cathodes to some clamping voltage of 60V. You could try and connect only one cathode to a zener diode of 45V, so this cathode will light if no other is switched on, but you would probably need to use a 120 V transistor. Anyway, 250V transistors like BF422 or MPSA42 are quite affordable (€ 0,45), so you could decide to just use these.
• Use high-voltage driving ICs. I tried the SN75468 and it worked fine. It contains 7 darlington transistors intended to switch inductive loads up to 500 mA and has a maximum output voltage of 100 V. Unfortunately, it has been declared obsolete by TI and it is becoming rare. The ULN2003 is pin-compatible with the 75468 but it can only handle 50V. You will have to protect it by connecting its pin 9 to a clamping voltage of 45 V. I have had good results with a zener diode of 44 V connected to pin 9.
• Use other more "modern" gas discharge tube driver circuits, intended for flat-panel plasma displays such as Burroughs Panaplex displays. TI and National sold a number of these in the 70'ies, but they are even more hard to find than the 74141. Plasma display driver circuits intended for flat-screen computer displays tend to have a large number of outputs and very compact packages. Seems to be overdoing things a bit.
• Finally, according to my old TI TTL Data Book, once upon a time there was the SN74142. It is said to contain a decimal counter, a 4-bit latch and a decimal decoder and nixie driver. In 1976, this must have been a great IC, saving 2 packages per digit in a TTL frequency counter or voltmeter. Unfortunately, I've never met anyone who has seen one.

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

• A display bank with memory, using SCR's. A lot of discrete components, though, for a single digit
• An SCR-based nixie-driver, nice but SCR's are expensive today.
• An SCR-based neon lamp driver, usable as a cathode driver in a multiplexed circuit if there is a small interdigit pause to turn off the SCR's.
• An anode-scanning display, which even has a dimming control.
• An SCR-based ring counter. Very interesting historical type of circuit.
• A discrete decoder and driver circuit, in case you don't want a decoder IC.

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.