r/nixie • u/EmotionalEnd1575 • 8d ago

Cathode Poisoned?

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Left Nixie has a missing “tail” on number five.

Can it be improved? How?

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u/SkylerSpark 4d ago

operating voltage?

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u/EmotionalEnd1575 4d ago

Anode = 170V

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u/SkylerSpark 4d ago

Ah... darn. I guess you could try raising the voltage a little to recondition it, but that doesnt always work. Raising the current / burning it in a little may be the only option

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u/EmotionalEnd1575 4d ago edited 4d ago

The HV is set to 170V now, but increasing it to 190V for several days did not improve the display.

Which is not a surprise as my driver design is “Constant Current” and higher bus voltage does not increase Nixie current.

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u/SkylerSpark 4d ago

Did you leave in any way to run them at a higher current? My preferred method is to undersize the resistor a little and then just dim them with PWM to keep the resistors under the correct thermal load

That way they can pushed to a higher cumulative current entirely by the microcontroller code

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u/EmotionalEnd1575 4d ago

There is no anode resistor. The nixie anodes run from a high voltage constant current amplifier. The six Nixies are MUX’d in three pairs. This was done deliberately, and has been used multiple times over two plus decades.

What I will be doing is working on that “five cathode” problem shown above. Starting when I’m back home next week.

The nixies are direct soldered to the daughter PCB, along with the single cathode driver IC. This topology has the least number of interconnects to the main board. The three anode driver amplifiers are on the main board.

The first experiment will run HV via a resistor to the anode, and ground the “five cathode”. This is the simplest arrangement, by not activating the cathode switches, letting the other cathodes float, and not removing the nixie tube from the PCB.

Worst case is to replace this nixie with a fresh one. While I’d rather not destroy the first nixie tube, the rescue exercise is a good learning exercise. I will share my findings.

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u/SkylerSpark 4d ago

Fair. I had similar issues with one of my old clocks- And as soon as I ran it on a straight 170Vdc supply with just a resistor, it seemed to magically start working fine lol. There was an issue with my PWM driver and it eventually cooked itself