I'm looking into driving nixies and i came across this multiplexing circuit

anode scanning circuit

I am a little bit confused about the chosen voltages. The 200v line I would assume is to meet the required strike voltage, if this is the case could it be reduced to the average 170v strike voltage?

My assumption is that the 110v line is to maintain the voltage across the tube. But that is considerably lower than the maintenance voltage for the zm1000. So what is the purpose of this rail?

Finally on the cathode side. It's connected to 75v then pulled down to the 0v potential, is 75v an arbitrary voltage, perhaps what was available from the supply? Why was this chosen?

  • \$\begingroup\$ My guess clamp collector potentials below Vceo max and inductive kickback clamping \$\endgroup\$ – sstobbe Oct 24 '18 at 3:13
  • \$\begingroup\$ I'm with @sstobbe, although why the whole thing couldn't have been done working against the 100V rail is beyond me, unless there was a time when high-voltage PNP transistors were easier to come by than HV NPN ones. \$\endgroup\$ – TimWescott Oct 24 '18 at 3:31
  • \$\begingroup\$ HV pnp in the days of germanium perhaps. I wonder if NXP still has the data-sheet/app-note that article cites \$\endgroup\$ – Jasen Oct 24 '18 at 4:43
  • \$\begingroup\$ In saying that, I could just remove the 75v rail and connect high-voltage NPN transistors to the 100v rail \$\endgroup\$ – Will D Oct 24 '18 at 4:46
  • \$\begingroup\$ what could be the benefit of having 35V across an idle nixie tube. \$\endgroup\$ – Jasen Oct 24 '18 at 5:01

The 75v rail is a function of the limitations of the BSX21 transistor, which has a max 80v VCEO specification. When that circuit was produced, low cost high voltage NPNs were not available. These days, you could dispense with the 75v line and use a transistor rated for >200v.

Even with 170v as the maximum strike voltage, then it would foolish to provide exactly 170v. You need to provide a voltage higher than the maximum strike voltage, and that's after voltage drops in transistors and tolerances. 200v sounds not unreasonable.

Apart from R1 and R2, which seem to have values way too small, I don't see any current limiting in this circuit, individual cathode resistors would be the norm.

  • \$\begingroup\$ My experience with 300+ ZM1080 and ИН-12s is that 160V is more than enough - they reliably strike. Higher voltage shortens the life of the nixies, so going for 200V "just to be sure" may not be justified depending on the application. \$\endgroup\$ – anrieff Oct 24 '18 at 6:22

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