Hi Greg, Thanks!
I've made a number of direct drive designs before, and when trying to pass a current of a couple of mA through one of the small tubes, I've used an anode voltage of 170V, an anode current limiting resistor as calculated, and had no problems achieving the current flow I want. The problem has occurred here when I've wanted a very much larger current (13mA in this case), and found that I cannot achieve this on 170V even without a limiting resistor! I've concluded from this, and from the graph you've kindly shared that if I want to achieve 13mA anode current I will have to have a higher voltage across the tube, especially if I want 'room' for want of a better word, to include a current limiting resistor. I say that because at 170V i am already failing to achieve my desired current without one, so hopefully at 190V (which is the voltage the datasheet specifies for multiplexing) I'll be able to include one. This means I can, for example, get a 20V drop across the limiting resistor, have a 170V drop across the tube and have a 'limited current' of around 8mA, which I'd be happy enough with here. The thing that is surprising me here is that I expected the voltage across the tube to fall to Vsustain once conduction started, but when trying to pass a larger current, it isn't doing that. For example, this tube is specified to have a Vsustain at around 135V. However, when I had a 2k current limiting resistor, I was seeing a 10V drop across it, and a 160V drop across the tube. Presumably, once you hare above the 'unstable region' you describe, it demonstrates positive resistance again, and current flow is proportional to voltage across it? I'm assuming this behaviour is why it specifies 170V as anode voltage for direct drive, and 190V for multiplexing (presumably to achieve the higher pulse currents)? David On Mon, 21 Jan 2019 at 22:22, gregebert <[email protected]> wrote: > Welcome to the group; I assume no responsibility if you get addicted to > nixie tubes, and all sorts of vintage display devices. > > Be very careful about how much current you pass thru a nixie; too much > will cause premature wearout and too-little can result in cathode poisoning. > Also, be aware that nixie tubes have non-linear characteristics, > especially in the region where ionization occurs. > Initially, the current in the nixie is zero as you increase the voltage to > just below the point where ionization occurs. As the voltage is slightly > increased you can see very faint glowing, and the current is usually below > 100uA. When the voltage is bumped-up slightly, the voltage drop across the > tube reduces and the current increases. This region is unstable and > exhibits negative resistance. If you limit the current with a resistor, you > will see a steady glow and the voltage across the tube will be > significantly less than the ionization voltage. The graph below is data I > collected for a b7971 tube. Visible glowing occurs above 135V; ionization > started as high as 180V in some cases . Once the cathode has ionized, > though, it has reasonably linear behavior: More voltage across the tube > results in more current, hence higher brightness. > > Now, getting to your question, you definitely want an anode resistor to > limit the current; without it your design will be unstable and the nixie > current will be determined by the tube's raw characteristics, which vary > from tube-to-tube, as well as over time. > > I suggest you learn first with a direct-drive design, then work on > multiplexing. > > > > [image: ScreenHunter_15 Jan. 21 14.08.jpg] > > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/513eb93d-3672-42c6-a6b9-ded554048f7c%40googlegroups.com > <https://groups.google.com/d/msgid/neonixie-l/513eb93d-3672-42c6-a6b9-ded554048f7c%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/CAOQ6x0EpMQ0daOAe1zinb7yHf4y%3DdOJinLB1kgHsZKLNJbT0Dw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
