VFD heater question

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3 years 10 months ago - 3 years 10 months ago #9038 by Torsten Lang
Replied by Torsten Lang on topic VFD heater question
Hello Ty,
I assume you mean the Adafruit Ice Tube Clock, don't you? I posses this kit myself but received it through an eBay seller who flashed an alternative firmware. The design has a lot of flaws, one of them is that the filaments are driven with DC voltage which causes a visible brightness drop over the length of the tube. According to Dieter Wächter's IV-18 datasheet translation the filaments need 5V/11mA. The 22R resistor causes a voltage drop of ~0,24V (in the Ice Tube Clock the tube's filaments are connected to the regulated 5V supply through a transistor and this resistor). I assume they used the resistor to limit the peak current when the cold filament is powered up.

Ian,
the resistance of the hot IV-18 filaments should be ~450R. And regarding your second post: Not only the positive voltage does matter - you're right, due to the high voltage there most likely will be no visible effect. But you also will drive the segments slightly below the cathode (=filament) potential, and here every slight deviation could matter.

If you turn off the tubes completely off in some power save mode a constant current source may be helpful, but I drive my tube filaments simply with constant voltage. A simple current source can be built up of two transistors and two resistors. I used such a circuitry in the first incarnation of my VFD clock as the "fresh" tubes had extreme variations in segments brightness and segment currents. I had to learn that this was unnecessary, but I can send you the schematics page of this simple driver.

There are some nice articles about the VFD operation principles, one is at the Noritake page. I remember some other article with the urgent recommendation to to implement some hardware protection in case of multiplexing (like a retriggerable monoflop that will turn off the drivers when multiplexing stops) as when your multiplexer stops the tube will die very quick due to the high anode currents. This can hurt you when debugging your code. I remember darkly that I found that article at Futaba but I can't find it any more.

Regards,
Torsten
Last edit: 3 years 10 months ago by Torsten Lang.

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3 years 10 months ago #9040 by Ian
Replied by Ian on topic VFD heater question
Ah, there's the thing. I'm using IV-6 with a measured resistance of 6 Ohms. Also the heater is top to bottom.

I'm currently not reverse-biassing the segments wrt the heater, but it is on my list of things to try.

Thanks!

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3 years 10 months ago #9041 by Torsten Lang
Replied by Torsten Lang on topic VFD heater question
Hi Ian,
I just found the application note: www.noritake-itron.jp/eng/cs/appnote/apf140_bdvfd/03-2.html.

Regards,
Torsten

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3 years 10 months ago - 3 years 10 months ago #9042 by Ian
Replied by Ian on topic VFD heater question
This one is also a good one:

www.noritake-elec.com/technology/general...mation/vfd-operation

Section 4.2 was very helpful for me: I implemented pull down resistors on all driven lines, but in fact it says here that it is only needed for the grids. I'll have my new boards on Monday, I'm going to try installing only the grid pull downs. I may also split the diode droppers in such a way to drive all filaments in parallel, with 1 diode drop positive bias.
Last edit: 3 years 10 months ago by Ian.

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3 years 10 months ago - 3 years 10 months ago #9043 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic VFD heater question
Hello Torsten,
Yes, I have the Ice Tube clock. Thanks for clearing up the question of the 22R resistor, you just saved me the pain of taking apart that ridiculous jigsaw puzzle case to look at the resistor. You are fortunate to have received your Ice Tube with alternative firmware. The original Adafruit firmware I received was basically unfit for purpose and the source looked like it was some student's first attempt at programming in C. I spent many hours debugging and patching it and probably should have just started over from scratch. I threw away the (out of spec) crystal provided in the kit and put in a proper TCXO. The display looks fine in spite of the heater being powered by DC - guess I just got lucky there. Eventually I ended up with a decently working clock but it took much effort. Like everything I've ever bought from the Kiddie Clique (Adafruit / Sparkfun / Makershed) it was low quality at a high price. Live & learn I guess. Anyway, I don't mena to turn this thread into an anti-Adafruit rant so I will stop now. :whistle:

I think I'll go read those app notes you linked and see if I can learn something...

Look into it later when the dust is clearing off the crater.
Last edit: 3 years 10 months ago by Ty_Eeberfest. Reason: 22R not 222R - duh.

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3 years 10 months ago #9044 by judge
Replied by judge on topic VFD heater question

Ian wrote: ... Getting it to work well is tricky, but I'm taking care of that in software by doing all of the non-display processing (e.g. handling the web server requests in the period between digits to allow the capactive residual voltage on the segment bus to decay sufficiently so that there is no bleed over. I'm using only high side drivers, so this is a real problem. Luckily there are lots of other things going on, so I have enough useful work to do between digits. ;-)
...


Hi Ian, if you aren't already, you should take a look at github.com/me-no-dev/ESPAsyncTCP. I wrote an async HTTP client, also at github.com/judge2005/ESPAsyncHttpClient and an async WiFi manager: github.com/judge2005/ESPAsyncWiFiManager. Going with an async stack will give you way more time to do other things.

To keep the realtime stuff ticking along, I user a timer0 interrupt handler, then you don't have to try manually interleaving stuff. The downside is that everything called from the interrupt handler has to tagged with the ICACHE_RAM_ATTR attribute, or you will get resets. You can see my drivers at github.com/judge2005/NixieDriver. You can see everything pulled together in github.com/judge2005/SingleTubeClock

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