IN-18 Nominal cathode current - Confused -

[Eddy VB]

Looking at different websites, translated IN-18 datasheets, I get more and more confused what the correct direct drive cathode current is.

Here at tube-tester: http://www.tube-tester.com/sites/nixie/data/in18.htm the nominal current is stated 6mA.

On the same site a translated datasheet http://www.tube-tester.com/sites/nixie/dat_arch/IN-18_03.pdf here we see 'Basic electrical parameters', Bad translation of 'Nominal'?. In this table the 'no more than' (=max?) 4mA. The next table 'Maximum operating parameters' is confusing. Anode current at rectified voltage: Is this 50Hz rectified or smoothed DC? And how can a absolute max rating table have 2 values (min 4 and max 8) without any other parameter? And a absolute minimum 4mA?? Minimum drive current = 4mA?

A not translated datasheet : http://www.tubeclockdb.com/nixie-tubes/5-in-18-18.html here I see 4-7mA and 2-4 (50Hz/DC, nominal, absolute max ???) My Russian is non existing..

But if I look at most schematics they use a 10K anode resistor, that's about 2mA.

Some on the web are stating to keep the current as low as possible, others warn about excessive cathode poisoning with low currents, and recommending 6mA.. 

Can anyone shed some 'light' on this?

Thanks!!

[gregebert]

I run my IN-18's at 5mA, with pure DC. Each tube has it's own current-regulator so it's a constant 5mA regardless of tube-aging, power-supply variations, etc. Since my clock has only been in operation a few months, I have no reliability information. But it has 14 tubes, so it wont take as long to get that info as a 6-tube clock.

Going over 6mA would likely cause lifetime reduction. Using less then 4mA could risk poisoning according to some posts on this group. I've seen min & max current values on other datasheets, notably Burroughs, so I use the typical value (if specified), otherwise the average of min & max.

[Eddy]

Thanks, Indeed, a current source seems to be a good idea. Did you put a transistor in the anode line? I'm wondering what the anode current does with a normal current limiting resistor when lighting e.g. the '1' and the '4'. The '1' character has only about the half of surface area of the '4'. So current density is the double of the '4' (or '8' etc.). Maybe introduce a SW controlled current limiter. :)

[gregebert]

Yes, I typically use a PMOS device to regulate the current at the anode. You can also use a PNP, but the non-zero base-current needs to be accounted-for. I drive the gate with an isolated DCDC converter at roughly -12V; to make the current adjustable you can use a pot to drive the gate. Using a source-resistance of 1.6K ohms, the current is around 5mA. Let me know if you want me to post a schematic; I have additional protection for ESD and driver-failure.

And, yes, it costs more money to use a current-limiter rather than a simple series resistor, but I spend SO much $$ for nixie tubes, not to mention they are getting rarer, that it's OK for me. I'm sure it's overkill. 

I've also done current-regulation at the cathode side for segmented displays; you cant regulate the anode current because it varies based on the number of illuminated segments.

One subtle note about the '1' numeral in a nixie tube: It's often constructed with 2 adjacent wires and located at the very rear of the tube. Though you would expect more brightness from a '1', it's obscured by all of the other cathodes in front so it doesn't appear to be much brighter compared to the other numerals.

[Jon D.]

Greg,

I'd like to see the schematic of your current limiting.

Jon