Current limiting a nixie tube

Discussion in 'The Projects Forum' started by magnet18, May 22, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, my nixie tubes are rated for 2.5 mA max, and 170V max, and I have a 180V supply.
    When I tried to current limit them nothing works.
    I tried a 100K pot, but by the time I got it down to 6ma, I had lost too much voltage, and when I tried the calculator in here http://www.mcamafia.de/nixie/ncp_en/ncp.htm
    I get 10K (160V, 2mA) , but I try this, and the whole thing just lights up in a ball of plasma, so theres obviously something wrong.
    If it's worth anything, I'm pulling the cathodes to ground through a transistor.

    So, help?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    My Nixie data gives the combo of 170 VDC and 15K. That works to a current of 11 ma. That sounds like a lot of current. For 180 volts, I would use 20K for the anode limiter, and adjust according to the result.

    That is hard to follow. How have you got the Nixie hooked up?
     
  3. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    The current limiting resistor is on the anode, and when I had the pot at 100k ohm, it was still at 6ma, but the voltage drop of the resistor had become quite large (I'm guessing, since the digits were only glowing faintly at the center)
    [​IMG]

    I'm confused, because if the resistor limits the current enough, it drops too much voltage...
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Got another Nixie? I wonder if yours might be a tad gassy.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    It's a Nixie clock that he's had working before.

    I thought we went over this a few months back? Can't remember the thread.

    Nixies are sort of like neon tubes and CCFL's; they need a high voltage to start them glowing, but low current to maintain the glow. I don't know why your tube needs more current to light, unless you have more than one number trying to turn on at a time.

    If you run them with current over their specification, they will have a short life.

    Don't "guess" about the voltage across the resistor; measure it. Use alligator clips on the meter leads so that you don't have to hold them while power is applied.
     
  6. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I know we went over it, that's why I'm a bit confused too...
    I'll check it out again tonight and see if there's a short causing multiple numbers to light up.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    OK. I don't know how much voltage your Nixies need across them to keep 'em glowing once lit.

    You can figure that out by measuring your B+ and the voltage drop across your resistor. You might start out using a 20k or so fixed resistor, and measure the voltage drop across it when powered up - that way you can easily calculate the current. V(resistor)/20k Ohms = current. If there was a 20v drop across a 20k resistor, you'd know that there was 1mA current flowing through the resistor, as 20v/20k Ohms = 1mA. If your B+ still measures 180v, then you know your tube has 160v across it when there is a 1mA current flow.

    Don't just use a single pot without another fixed resistor in series. If you accidentally turn the pot to too low of a resistance, you'll either fry the pot or burn the tube up.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    I would think that if more than one element is going to be on at any time each cathode should have it's own limiting resistor and the anode resistor removed.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Well, that should never happen - the cathodes are driven by NPN transistors whos' bases are driven by the outputs from a 4017 Johnson counter via resistors. If (one/some of) the transistors happened to short from emitter to collector, it's possible that more than one cathode could be grounded, but not terribly likely.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Thanks Sgt. I was unaware of that.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Magnet, can you post a link to the datasheet for your Nixie tube?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    I had to look back a ways to find it; it was in Magnet's 4th thread on the forum.
    Thread:
    http://forum.allaboutcircuits.com/showthread.php?t=48600
    Nixie tube IN-14 datasheet: http://download.elektronicastynus.be/57/in-14_datasheet.pdf
    Unfortunately, the original datasheet is in Russian, although some comments have been added to the end of it in English.

    Here's a kit project that is directly related:
    http://blog.makezine.com/archive/2006/05/in14-nixie-clock-kit.html
    Build instructions: http://www.ledsales.com.au/pdf/microcontroller_in-14_clock_kit.pdf

    They're using a 180v supply for the Nixies and a 22k Ohm current limiting resistor. I'm thinking they're getting around a 55v drop across the current limiting resistor; that would leave 125 across the Nixies and have 2.5mA current flowing through it.

    But, Magnet might start out using 24k or 27k resistors just to make relatively certain that current won't be excessive.
    24k Ohms * 2.5mA = 60v dropped across the resistor, 27k is 67.5v.

    The schematics for Magnets' project are on page 2 of this thread:
    http://forum.allaboutcircuits.com/showthread.php?t=49013

    The prior discussion about how to select a current limiting resistor started on the bottom (reply #40) of this page:
    http://forum.allaboutcircuits.com/showthread.php?t=48459&page=4
     
    Last edited: May 27, 2011
    magnet18 likes this.
  13. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Thanks for digging those up for them wookie
    and for the link to the make project, didn't know about that one.
    I tried it with a 22K resistor and it does look like multiple digits are trying to light up. A lot of them. I'll look into it.
    Unfortunately I don't have a multimeter, my sister took it with her to their competition in Tennessee...
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Ahhh - you shouldn't have powered it up without having a meter available. After all, you don't know if the current will be correct. The ledzone kit instructions didn't mention the specifications of the Nixies that they used, so we're doing the "wild guess" thing.

    If your 4017's are not in the circuit (or the base current limiting resistors are missing), the bases of the transistors might be floating, which could be causing the multiple digits to light up.

    Anyway, leave it off until you have your meter handy - it would be a shame to ruin the Nixies.
     
  15. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Well, I'm off to a movie at the moment, but when I get back I might go old school with a couple analog meters and see what damage I can do.
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    Magnet, sorry about the earlier post suggesting limiting resistors on each cathode. What was I thinking? It's been so long since I've seen a Nixie I forgot that it makes no sense at all to light more than one character at any one time.... Duh!!!
     
  17. SgtWookie

    Expert

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    Don't feel bad Cdrive - I think I did the same thing a few months back. :rolleyes: ;)
     
  18. bertus

    Administrator

    Apr 5, 2008
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    Last edited: May 28, 2011
  19. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Interesting application note on nixie's. www.tube-tester.com/sites/nixie
    click Nixie Tubes-Nixie Rohren-Nixie World, down to Dieter's Nixie Tube Archives, and again down to: Doc# 062.
     
    Last edited: May 28, 2011
  20. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, got my ohmmeter back, verified 0 ohms (.7, meter calibration) between the emitter of the transistors, the ground for the HV side of the buck boost converter, and pin 8 of the 4017, so I don't know why multiple numerals are trying to light :(
    I'm about to scope the 4017 outputs to see if that reveals anything.

    @bernard, I don't follow where I'm supposed to click, did you mean Nixie News for the first link?
    [EDIT]
    Nevermind, found it :rolleyes:
     
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