Greg -- regarding the resistor size and power dissipation, if there are 6 digits (7 in this case with the neons), can't you figure the resistor duty cycle is 1/6th, and therefore smaller wattage resistors can be used? The average dissipation is more like .043 watts, by my back-of-the-napkin math.
I realize the instantaneous dissipation is .25 watts, but resistors do have some thermal mass.... If I'm wrong here please correct me. I know I've worked on a similar problem before and I recall the answer was not so obvious. Terry On Tuesday, February 24, 2015 at 11:12:19 AM UTC-6, gregebert wrote: > I think this is the problem: The base-emitter of the PNP is not shunted > with a resistor (see note from Terry). The purpose of the base-emitter > resistor is to provide a path for the leakage-current of the NPN predriver. > > With the schematic as shown, roughly half of the NPN's leakage current > goes thru the base of the PNP. That current is them multiplied by beta, > which I think was around 130 from the datasheet. It's not a lot of current, > but it could certainly contribute to unwanted glowing. > > Before you start hacking-up your PCB, can you try adjusting the timing of > your anode drivers so there is some 'dead-time', say 50-100usec, after 1 > anode is turned off, and the next one is turned on ? Also, if you can turn > off all cathode drivers during the dead-time, that might help. This will > ensure the tube is no longer ionized, and that should either reduce the > unwanted glow or make it more difficult to re-ionize from leakage. And to > be safe, dont turn off the anode & cathodes at exactly the same time > (stagger by ~1usec); otherwise you could create a di/dt problem from the > stray inductance of the wiring. > > Now, if that doesn't work, next thing to try is a true base-emitter > resistor, for example, moving the connection of R22 to the other side of > R21 (see Terry's note). I dont think the resistor values are critical as > long as they are at least 100K and they are 1/4 watt (or larger). You dont > want them to overheat P=Vcc^2/R, so with Vcc=160V, R=100K, a 1/4W resistor > is at the limit. If your local Radio Shack store hasn't shut the doors yet, > grab some resistors. (I just got some stuff last night at a 95% discount) > > -- 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/5ec88575-ab0c-4a83-9970-e5d7e10a90ae%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
