VFD heater question

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3 years 10 months ago #9032 by Ian
VFD heater question was created by Ian
Hi All, I'm making a VFD clock at the moment, and have 3.2V across three IV-6 heaters in series. I know that we are at the top of the range for the voltage (0.85V - 1.25V), but I notice that the heater wires are glowing very dimly cherry red. You can only see it at night when the tubes are blanked.

Is this normal? Does it affect the life of the tubes?

Thanks for any thoughts...

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3 years 10 months ago #9033 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic VFD heater question
I've never designed anything using VFDs but have built one VFD clock kit and have several commercially built devices (including my car) that use VFDs. None of them glow visibly under any conditions. If I look at them through a Sony NightShot camera, or a little CCD camera with the hot mirror removed, I can see them glow at IR wavelengths, but nothing can be seen by naked eye.

Unfortunately the kit I have uses an IV-18 which has 1 really long heater spanning the entire display, so looking at how they did it probably isn't real useful. But FWIW they feed the heater from +5V rail through a 22 0hm 1/4 watt resistor.

Look into it later when the dust is clearing off the crater.

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3 years 10 months ago - 3 years 10 months ago #9034 by Torsten Lang
Replied by Torsten Lang on topic VFD heater question
Hi Ian,
a dim glow is absolutely normal. I have various VFD displays in use (video recorder, equalizer, amplifier, and other HiFi equipment, Linux VDR, and last but not least some VFD clocks). In every of these displays I can see the filaments glowing - I do not possess a single device with VFD where the glowing of the filament is not visible. Normally modern VFDs are very bright and you have some dark grey glass in front of the display so that the glow would be only visible when the ambient light is quite dark, so this is normal.

But to my experience a high temperature of the filaments will accelerate the aging of the tubes. So for my clock I reduced the heating voltage below the nominal voltage given in the datasheet.

But another point: Did I understand you right that you connected the three filaments in series? That will not work. The biasing of the filaments is quite delicate, many tubes must be driven with AC because the small voltage drop across the filament will cause uneven lighting. If the bias voltage is too low the segments may be turned off completely while if it is too high neighboured unlit segments may cause fading/blanking effects of lightened segments.

One example: The colon tubes in my clock, the IV-15, need 50V anode voltage and 0.6V heating voltage. As the DT1704 use 1.2V..1.6V, I reduced the voltage by a series resistor. The position of this resistor (GND or VCC side) already has a clearly visible effect, and we talk about 0.6V..1V change in the bias voltage.

With best regards,
Torsten
Last edit: 3 years 10 months ago by Torsten Lang.

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3 years 10 months ago #9035 by Ian
Replied by Ian on topic VFD heater question
Hi Torsten, thanks for the feedback! I'll try dropping the voltage and see what happens: long life is important for me.

I started by using a constant current source, but it's actually quite a difficult task to design a reliable, cheap constant current source for such a thing, so that idea went in the bin. I'm using 3 cheap forward biased diodes to drop the voltage from 5V down to 3.2V for three heaters in series.

About the heater bias: I am multiplexing the tubes and therefore using 60-70V to drive them (specified maximum), and the difference in the heater voltage doesn't appear to matter at all. 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. ;-)

I'm almost certainly moving over to the Wemos for everything after the experience of doing this. I'm not to the end of the software yet, there are still quite a few improvements to do, but right now, it's a fully featured VFD clock with a web interface and NTP time control including DST for about $10 parts cost. The Nixie clocks will follow in due course. The next major update will go to Wemos drive.

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3 years 10 months ago - 3 years 10 months ago #9036 by Ian
Replied by Ian on topic VFD heater question
Hey Ty: are you sure it's 22 Ohm @ 1/4W? That would be way out of spec. I guess that's a typo and you mean 220 Ohm.

For 22 Ohm:
Filament resistance is say 6 Ohm. Current = 5/28 = 178 mA. I guess you'd see the heater glowing red for a short while in that case! The resistor would have to stand 0.178 * 22 = ~4W. Hmmmmm.

For 220 Ohm:
Let's try with 220 Ohm. Current = 5/228 = 22 mA, which would be reasonable. Drop across the filament = 6 * 0.022 = 1.32V (Seems a bit high). Dissipation over the resistor = (5-1.32)^2 / 220 = > 4W. Hmmmmm.

I can't make sense of the figures, even with my 220 Ohm guess. Could you have a look to make sure you don't have some space alien technology in those resistors, there?
Last edit: 3 years 10 months ago by Ian.

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3 years 10 months ago #9037 by Ty_Eeberfest
Replied by Ty_Eeberfest on topic VFD heater question
Ian, I took that 22 ohms unthinkingly off the kit BOM. :blush: I see what you mean - it makes no sense. I suppose the BOM is wrong, not surprising since it's an Ate-A-Poot (Adafruit) kit! I'll open up the clock and see what was actually used there and get back to you in a bit.

Look into it later when the dust is clearing off the crater.

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