A filament is best run with AC - DC will cause a brightness slope across the display. Google for information on driving them as well as the theory of their operation - I know Futaba and Noritake have good white papers available.
Ok thanks, I've never made 1 with 4 transistors, but I got it working in LTSpice, and I'll use some variable resistor, besides some fixed ones, to see what looks like enough. In the sim I have around 20mApeak, centred around -10V, I should play with the circuit and try to get a lower VDC average.
I think the VFD came from a working unit, but I probably killed the motor IC a few years ago, it was reading near short, so I removed it so I can try the main PCB too.
Time to make the circuit and try it, but yeah then I need to try some basic commands or something, over I2C.
Ok that looks interesting too, I went ahead and made something and did some calculations for some of it and it about matches what I made on the breadboard.
So far I've had up to I think 56mA going through the VFD, centred around -17V or so, and with 5Vcc and -20Vee, and 3V3 for the 2 RAM chips. But so far I can't see anything light at all from the VFD. But actually I get some much higher frequency oscillation as the bridge current increases, like I did early somehow with more current and larger capacitance. In general I need more capacitance as the bridge current goes up, to keep the regular flip-flop rate reasonable. There's audio on the main PCB, so they probably run it over AF tho too.
Now looking at what happens when the PWR-ON button is pressed, the 5V current goes from 15mA to 25mA, and I see some activity on I think a grid pin of the VFD, but still no light.
The cold VFD filament measures 7R, and there's ~7R in series on the PCB, and then I'm adding another 30R or down to 10R in series with the filament. But with 10R and 1uF caps anyways, I had all that HF osc.
SO IDK what to try next, just a bit of DC analysis on this is tricky, I haven't tried any AC on it. I should try and get LTSpice to osc. like this too.
In those pictures with AC coupling, I probed directly across the 30R or 10R, etc
Hmmm, what's with the 33Vpp on the VFD pin 9 ??
Ok thank I did see some Vbe near 0.8V in LTSpice but never really looked at it hard, maybe it goes too negative too. I did make sure to use big enough resistors tho.
I still don't understand why you're using the -19.5V rail and not the 5V rail for the filament drive. You're dropping most of the voltage with a series resistor, so it makes sense to use the 5V rail and not drop as much.
I still don't understand why you're using the -19.5V rail and not the 5V rail for the filament drive. You're dropping most of the voltage with a series resistor, so it makes sense to use the 5V rail and not drop as much.
Probably because the filament has to be just above the low voltage to make the display work. The pixels and grid need to be at a higher voltage than the filament to make the pixels glow. The resistors are needed to lower the filament voltage to not burn it up, and lift the lower end slightly above the negative rail. The latter to make sure the pixels are fully off when not driven.
A couple of transistors driving a small transformer would be a more sensible option. The secondary is isolated and can be biased at any voltage.
And for my driver design inspired by the one you said, according to LTSpice the most negative voltage the lower NPN's see is only -2.4Vbe, so that's ok.