IN-18 Cathode Poisoning Problem

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5 years 2 months ago #6481 by Kpangon
Hello,

I recently purchased a the PV Electronics Spectrum 18 IN-18 Nixie clock kit. The problem is that after about two weeks I am beginning to see cathode poisoning, mostly on the 8's. I have had it set to shuffle the digits for 5 seconds and display the date every minute since I assembled the kit. I purchased the tubes from a reputable seller on ebay, and am reasonably sure they are new old stock. They are all date code matched (05-85), show no darkening of the glass and didn't show any cathode poisoning when they were first used. Could having too low of current be causing this cathode poisoning? When I measure the current with my multimeter I get 3.1 mA, but when I researched these tubes online I seem to get the impression they should run at around 6 mA. The anode resistor provided with the kit is a 10k ohm resistor. If the current does need to be increased, how many ohms anode resistor would you recommended installing?

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5 years 2 months ago - 5 years 2 months ago #6482 by Ty_Eeberfest
My personal experiences (and the IN-18 datasheet specs) suggest that anything less than 4mA is too low and poisoning will occur quickly. I went thru this when initially testing out my chiming IN-18 clock design. I began with 15K resistors on my IN-18 anodes and by the time I was finished testing I was down to 6.8K resistors.

IN-18 forward voltage drop (Vf) per data sheet = 150VDC

Assuming your HV power supply is 180VDC.

180 - 150 = 30 volts dropped across resistor

I = E / R
I = 30 / 10000
so...
I = 0.003A or 3mA which is about what your meter said.

but if I = 30 / 6800 (6.8K resistor)
then...
I = 0.0044A or 4.4mA

So give 6.8K or even 5K a try. I promise even with 5K the current is nowhere near enough to destroy your tubes, and it may well solve your problem. It did for me... I have zero cathode poisoning issues on my chiming IN-18 clock, I only "spin" for 30 seconds once an hour, the tubes do not run hot, and they look as good today as they did when I first lit them up 4-ish years ago.



Look into it later when the dust is clearing off the crater.
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Last edit: 5 years 2 months ago by Ty_Eeberfest. Reason: Fix typo: 150Vf not 105

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5 years 2 months ago #6483 by Ty_Eeberfest
Corrected typo:

IN-18 forward voltage drop (Vf) per data sheet = 150VDC

(not 105VDC! oops)

Look into it later when the dust is clearing off the crater.

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5 years 2 months ago #6484 by Kpangon
Thanks, I'll give 6.8k resistors a try, and see how they work. Hopefully the higher current will get rid of the cathode poisoning I was seeing since it was only minimal.

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5 years 1 month ago #6485 by Ty_Eeberfest
Should work. If you could come back and post the results it would be appreciated :)

Look into it later when the dust is clearing off the crater.

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5 years 1 month ago #6486 by Pacman223
yes, please post your experience.

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