I think you can compare it very much to the result you see when one tube is broken. If the second tube doesn't ionize, it is basically the same as if it is not there. In your case, suppose you take out 1 tube, you will most likely find that the anode voltage of the other tube will not rise above 170V even though your power supply can go up to 200V. This is because the leakage currents from all the other cathodes to ground through the zener diode, consume all the current that the HV power supply can deliver. All voltages from anode to each cathode will be around 120V, together with the 50V zener, you will find a 170V anode voltage rather than 200V. Of course this depends on how much current your HV power supply can deliver, but I assume it is not more than 3mA otherwise the battery currents will be enormous. Suppose 3mA over 10 cathodes, would be 300uA per cathode, enough to just light them up.
My HV power supply is quite a bit different from your one, I vary the supplied energy as well as the "on" time during multiplexing, they are 2 individual PWM signals. So there is not necessary a long delay between switching left-tube and right-tube. In fact, the PWM signal for multiplexing only varies between 40 and 50% I think. Michel On Mar 12, 2:06 pm, David Forbes <[email protected]> wrote: > On 3/11/12 4:44 PM, Cobra007 wrote: > > > > > > > > > > > > >> Then don't turn on the other tube at that time! I turn on only one tube > >> at a time, which removes that problem. > > > That is not completely true, it's not that I turn on both tubes at the > > same time, it is because the current starts to flow through the wrong > > tube because IT CAN. With a 170V anode voltage and a 50V clamping > > zener, the minimum voltage on either of the tubes (on or off) is 120V. > > A tube that has 120V, is switched off but was switched on just a > > fraction earlier (due to multiplexing) will leak current from anode to > > ground through the zener clamping diode. You can't stop that unless > > you disconnect the anode from the HV power supply, through an extra > > transistor or something else. > > > I agree that adding complexity will in most cases reduce the > > reliability. In my case, it is not so much that the circuit becomes > > more complex (replacing 8 transistors in 1 package by 8 discrete > > transistors) it is more an increase in PCB complexity. So I am not > > worried that it will decrease the reliability. > > > Michel > > I am confused. If you are turning off one tube then turning on the other > tube, and operating at a low duty cycle (I assume you're still > discussing the dimmed case), then there will be a long delay between > turning off the first tube and turning on the second tube. So the first > tube should have time to deionize. Is the leakage strong enough to make > a glow? For how long? > > My Nixie watch displays an interesting result when one tube is broken - > the other tube will light all cathodes faintly when the broken tube is > selected. That's because all the cathodes are clamped to 50V, but the > power supply makes about 200V to an open circuit. (I connect both anodes > directly to HV without resistors.) In that case, the tube is lighting > when it shouldn't. But there is a broken tube, so there's no reason to > make the display function properly in that case. > > I provide display blanking in my watch, which is accomplished by feeding > an MPU output to the HV supply feedback node through a carefully-chosen > resistor, which makes the HV drop to 110V when the bit is set to 1. This > allows me to turn off the display and not excite the case described > above in normal operation. It's a very low cost solution to blanking, as > I share the MPU pin with the accelerometer enable. > > The long and short of it is that you ought to be able to use a 50V > driver chip in a two-tube design with a bit of effort. > > -- > David Forbes, Tucson AZ -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
