Anode-Scanning Display

[Paul Andrews]

I just came across this: http://www.glowbug.nl/neon/944125938_2356.gif (which seems to be an excerpt from https://archive.org/details/ElectronicCircuitsManual/page/n209) and would be interested in a critique of it from people who know more than I do about electronics - which is just about everyone here!

The cathode controls are pretty standard fare for multiplexed tubes - including clamping the off cathodes to 75V (it is low enough to stop them acting as anodes, but high enough that they won't glow). I'm more interested in the anode controls. In particular:

• Using capacitors to de-couple the 5V signal from the 200V rail (I assume)?
• Tying the collector of the transistors to 110V, presumably to keep the voltage across the transistor within its limits, though I'm not sure what actual voltage would be present across them, because:
• The anode resistor, which is shared by all anodes here, apparently varies from 180 to 300 Ohms. Is that a misprint? Can it be that low because of the multiplexing? Or is there something else going on here.

Naturally now that I've seen it, I want to build it. Like I don't have enough projects going on.

[gregebert]

The trick here is to size the coupling capacitor appropriately, because it affects the on-time of the tube. SPICE simulation would be the best thing to do if you want to use this self-timed circuit. Larger cap = longer on-time, but also more energy to get rid of (heat).

My preference is to have everything directly controllable by your logic/CPU+software; in that case an opto-isolator will provide the isolation you get from the capacitor, without the annoyance of self-timed circuits that age over time.

[Dekatron42]

I've seen similar circuits when the Nixies have been driven from digital clock ic's like the MM5309-series but then the Nixies have had separate anode resistors too.

/Martin

[David Forbes]

This is a current regulator circuit. It's pretty much the same as the one I use in my Nixie watches. The voltage developed across R1+R2 due to anode current serves to reduce the forward bias of the active anode transistor. 

It's tricky, but not that tricky.

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[Paul Andrews]

Greg, my brain must be getting slow. I just came back to find this reference (I hadn't bookmarked it), and now I understand what you mean by a self-timed circuit.