IN-18 Cathode Poisoning Problem

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5 years 4 months ago #6498 by Kpangon
Okay, Ill let the clock run for a while, and get back to you with the results.

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2 years 4 months ago #8523 by Moriniboy
HI, I'm suffering the same problem, can anyone provide some feedback on this subject?

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2 years 4 months ago #8524 by Ty_Eeberfest
Moriniboy, see my post earlier in this thread. I don't know if it worked for the other guy or not since he never came back but it definitely works for me.

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

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2 years 4 months ago #8525 by Moriniboy
Thanks Ty my feedback so far on this subject.
The PV Spectrum kit now uses 8k2 resistors and l have found that CP still occurs over an extended period of time i.e. 6+ months.
I am currently running with 6k8's as 5k6's caused blue spotting when the 1 was displyed (might be a tube fault?)
Will report back in a few months.

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2 years 4 months ago #8533 by judge

Moriniboy wrote: Thanks Ty my feedback so far on this subject.
The PV Spectrum kit now uses 8k2 resistors and l have found that CP still occurs over an extended period of time i.e. 6+ months.
I am currently running with 6k8's as 5k6's caused blue spotting when the 1 was displyed (might be a tube fault?)
Will report back in a few months.


The blue spot is caused by the unlit cathodes conducting current and acting as a beam-formers. This happens even if they aren't glowing - i.e. they are at some voltage that is somewhere between the anode voltage and zero. It happens especially with the IN-18 but can be made to happen with other tubes. The cut-off voltage can vary, but pulling the unlit cathodes up to 90V or more seems to fix the issue.

The spectrum uses an HV5622, which (I think), floats the off cathodes, so it would be pretty random which tubes might be prone to it. I've seen voltages on floating cathodes with other chips vary by around 40V!

I see that the build instructions for the Spectrum say to set the voltage to 170V. Nixie tubes have two levels of interest for the anode: The striking voltage and the maintenance voltage. The striking voltage for the IN-18 is 170V, though it will often strike at less. A lot of datasheets for other tubes recommend using a significantly higher voltage and a correspondingly higher current-limiting resitor to keep the current the same. The reason for this is that this lessens the impact on the current caused by variations in the maintenance voltage for different digits and different tubes. The current is determined by (V-Vmaint)/R, where V is the input voltage. So you can see that if V and R are made larger, the impact of varitions in Vmaint is reduced.

In other words, I wonder what the actual current flowing through those tubes is? Plus raising V and R would mean that you could be more sure of the current without having to measure it.

Vmaint would be around 140-150V, but I don't think the datasheets specify it, so it would have to be measured to be sure. Again, a higher V would make this less of an issue.

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2 years 3 months ago - 2 years 3 months ago #8547 by Ian
Replied by Ian on topic IN-18 Cathode Poisoning Problem
H Paul, I've been thinking about adding some high value (>=220k) pull-ups to the unlit cathodes. Do you have any opinon on that? It's a bit of a sweat and a lot of extra components, and propbably not at all necessary, but this is a recurring problem, and always causes consternation.

Another solution would be push-pull drivers, I suppose... The HV5622 is open-drain, nad there does not seem to be an equivalent part in push-pull (80V seems to be the limit).
Last edit: 2 years 3 months ago by Ian. Reason: make my English sound like English

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