IN-18 Series Resistors

[Richard Scales]

Very sorry for asking such a fundamental question.

I thought I had this all sorted but I'm having a problem in correlating my understanding with what I see others doing.

When checking specifications for IN-18's over at Tube-tester.com - he suggests:

Ignition Voltage 170V

Voltage drop 150V

Nominal current 6mA

So I deduce, with a 170V HV supply, the series resistor should be (for nominal current) (170-150)/0.006 = 3K3

Similarly for something like ZM1040, supply volts 170V, maintaining volts 140V, nominal current 4.5mA, series resistor (170-140)/0.0045 = 6K6

I note that in kits such as PV Spectrum and others - for this kind of tube they specify 10K series resistors.

Am I mis-interpreting the specifications, in particular those for the IN-18 as the voltage references are worded slightly differently?

Are the tubes in question being driven at a lower current on purpose?

I'm ready to learn!

Richard

[newxito]

Maybe the kit providers don't want to generate too much heat with their DC-DC boosters, 36 mA is a lot.

[Richard Scales]

An interesting point which I had not considered - you could be right.

[David Pye]

I thought the ignition voltage is a *minimum*, so people are probably running with a higher voltage and thus a higher series resistor to limit the current?

David

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[Richard Scales]

I could understand that but having built several of PV's spectrums I know that the HV power supply is set to 170v during construction.

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[gregebert]

The other things to consider are tube-to-tube variations, and aging.

You dont want to exceed the maximum-rated current, because the wearout becomes exponentially faster.

And you dont want to be too low, because you will get cathode poisoning, though it's reversible.

You basically have 2 options for an optimum design

#1 - Use a high anode voltage. My first nixie clock is mains-operated (no transformer) and doubles the voltage to +340 VDC. I use a 75K anode resistor and get 2.4mA of current, which is between the datasheet limits of 1.5 to 3.0mA. This keeps the tube current safe if the tube and/or line-voltage varies by +/- 15 VRMS, and tube-aging down to 145 volts. The downside is that this wastes energy as heat, roughly an extra 1/2 watt per tube. However, with no transformer I believe that is a zero-sum tradeoff. I have 3 clocks with this design that have been running 24/7 since 2012, with zero failures across 18 tubes. These are Burroughs 5092's, which are abnormally durable.

#2 - Use a current regulator. As long as the voltage doesn't go too low, you will always have the same current. As above, the downside is wasted energy (heat), but that is probably acceptable if you are protecting a valuable tube. This is what I've been doing on all of my subsequent designs.

If you stick with a +170 VDC supply, be sure to check how much margin you have for variations in line-voltage and tube-voltage (aging). You're better off boosting it to around 200V.

[Richard Scales]

This is all very helpful. I had kind of thought that 170v was the goto voltage for driving nixies. Clearly I'm mistaken!

In order to prolong tube life, most of my clocks only come on when someone is close by so a clock with IN-18, Z566 or B-7971 probably actually has the tubes active for not much more than 10 or 20 minutes each day depending on the activity in the room where the clock is.

Richard

Sent from my Huawei phone

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[gregebert]

IN-18's are a good choice; best size vs cost. None of mine has failed.

I have a large clock with fourteen IN-18 tubes (MM.DD.YYYY  HH:MM:SS format) , and I run them around 200V with a current-limiter. My supply is a bit wimpy, so I dont get the full 4mA. As a result, 2 tubes that are static get slight poisoning even though I run 1 hour of depoisoning each night. None of the other tubes show signs of poisoning even if they are static during the day. So I conclude that some tube lots might be more vulnerable to poisoning than others. Most of mine were manufactured in 1989 or 1991; bless their hearts for keeping those nixie factories open for so many years !!

I didn't collect much I-V data on these tubes; buried in my lab notebook from several years ago.

[Paul Andrews]

10k for 170V and IN-18 does sound very high. 2mA instead of 6mA? To quote a thread on tubeclockdb.com (about cathode poisoning with IN-18s), someone was measuring 3.1mA with those specs, so the maintenance voltage would be more like 140V. Also, I thought more recent kits recommended 8k2 resistors? Presumably this would give a current of around 3.6mA. It did not affect his experience with cathode poisoning.

Apart from the power supply issue, I think one reason people run IN-18 with a low current is to avoid the 'infamous blue dot'. This happens more as you crank up the current.

As Greg has pointed out (and the thread on tubeclockdb.com corroborates), running tubes at too low a current will lead to premature cathode poisoning.

[David Pye]

Is that definitely the case for the IN18 though?

It might be a different voltage for them perhaps?

David

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[Paul Andrews]

So the datasheet here https://tubehobby.com/datasheets/in18.pdf is a little confusing. It says the normal current is 4mA, but it also says the maximum current is not less than 4mA, not more than 8mA. So maybe 4mA is OK? Anyone care to chime in? That would put a whole different slant on things.

[gregebert]

My interpretation is that 4mA (DC) is the ideal current; less than that risks cathode poisoning.

If you operate in multiplexed mode, you can pulse no higher than 8mA, regardless of duty-cycle.

I know from multiplexing LEDs, you have to boost the current to maintain readability; I'm forced to multiplex in my next nixie project, because it's a panaplex display so we will see where things go with that. At least the datasheet provides a max-current spec.

[Richard Scales]

I can see that the PV Remote System kit does indeed specify 8k2 anode resistors.

[Dekatron42]

Not directly related to IN-18 but fundamental in explanation of Nixie driving you can read this article if you haven't done so already (one among many similar articles): http://www.decodesystems.com/re-how-nixies-work.html , the original magazine can be found here: https://www.americanradiohistory.com/Archive-Radio-Electronics/70s/1971/Radio-Electronics-1971-09.pdf , you can also read the Burroughs Application Notes N101 & N102 on Nixie driving that the article is most probably based on (some figures are identical between them), just Google "burroughs N101 (or N102) application note" and download them from for instance zapro.dk .

/Martin