I've been following this discussion for a couple of days now, and thought I knew what was going on, but I just went to my bench a physically put my hands on an IV-6 VFD tube. The filaments for these tubes are specified at 0.85-1.15 Vrms (nominal 1V), with a current draw of 50 mA. Note that the spec sheet says Vrms, implying an AC filament drive. I know this matters for the long, skinny tubes like the IV-18 8-digit tubes, but for single digits DC is just fine. Fortunately, ACV rms is *exactly* equal to DCV when going into a pure resistive heater; in fact, that's one commonly used simplified definition of RMS, and in the olden days the physical method used to make a true-RMS meter.
I agree with other posters that the reactive components of this tube should be nearly nil, but I went ahead and measured it anyway. It turned out to be 1.7 uH - higher than I expected, but still an insignificantly low value. So I measured the resistance of the filament cold with a four-wire ohmmeter, so that lead resistance would not be significant. This turned out to be 5.1 ohms, which at 1V filament supply would cause a current of 196 mA - about four times the specified draw. I then set my bench supply to 1.00 VDC, and connected it directly to the filament. The power supply's current meter read 0.05 A, or 50 mA - right at spec, but not with very good resolution, so I then put a milliamp meter in series. I've previously measured the internal resistance of this meter, and know it to be 0.1 ohm, so it wouldn't significantly affect the reading. Sure enough, it read 51.2 mA, which is *plenty* close enough for any practical use. Therefore, the hot filament resistance is just about 200 ohms. This means that there's a surge when power is first applied, which might (does, actually) shorten the life of the tube. As one poster mentioned, light bulbs and other filament-based devices fail most frequently when power is first applied; this is mostly due to the filament rapidly changing tension and shape as it heats up. (If you fire up a Numitron tube, you can actually see the filament sag as it begins to glow.) I've found that a very easy way to give a soft start, and simultaneously solving the problem of getting a 1V supply for the filament, is to simply place an 80 ohm (I use the standard 82 ohm value) resistor in series with the filament, and run it off the 5V supply. The 82 ohm resistor is by far the largest part of the current path, so even if the tube filament started out with zero resistance, the maximum current is limited to 5/82 = 60mA - only 20% over spec, instead of 400%. (Older stepper-motor drives used to use this technique, as well.) Of course, there's a penalty, as always: this is quite inefficient, since 4/5 of the filament power is being dissipated - wasted - in the dropper resistor, but if you've got plenty of 5V power around, who cares. (For this tube, there's still only 200 mW dissipated in the resistor, so a standard quarter-watt resistor is fine. Those old stepper drives had big honking power resistors, though...) Sorry for the dissertation. I accidentally bought a large batch of these a couple of years ago, and to make back my money (except for time, of course...) broke it into small batches for eBay. I sold them with a little data sheet I printed up that included the original Russian data sheet, plus an English-language text sheet and a quick-test jig using a 1.5V cell and a couple of 9-volt batteries. Since I had so many, I also spent a bit of time playing with them, and got to know them pretty well. They really are considerably easier to use than Nixies, just because of the lower voltage requirements, and I think they're longer-lasting as well. I don't find them nearly as attractive as a nixie's warm glow, though. Numitrons, on the other hand, I like a lot... ~~ Mark -- 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/1cb3b74c-8203-45ce-81a2-86be891739fb%40googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
