I thought this was worthy of a separate discussion. Small VFDs, and even NIMO tubes, use the filament as the cathode and in doing so the filament is the sole source of electrons to produce the glowing segments. So, some of the filament current is going towards heating of the filament and some of it is going to the segments. For NIMO tubes, this is negligible because it's on the order of 30uA; for small VFDs it's probably a few mA.
But with big VFDs, like the ILC1-1/8, the segment currents are fairly large like 30-40mA each and on top of that there is the grid which basically amplifies the electron stream for brighter operation. All of those will add several hundred mA of current to the filament in addition to what is required for heating. So I'm a little confused about what the overall specs are for filament current on these big VFDs. I'm fairly certain that driving the filament with an AC source results in a net cancellation of this "extra" current, and if you measured the filament current with an AC ammeter you would not detect it (only the current to heat the filament is measurable). However, when you measure the current from the center-tap of the AC source to GND, you will definitely detect it. In fact, if the filament was driven by a floating AC source, there would be no glowing segments. Another reason why you dont want to use DC on the filament is that it sets up a small voltage difference across the display and it would contribute to non-uniform illumination; with AC, this is averaged over time and not visible to your eyes. I recall this being discussed awhile back. Has anyone here built their own jumbo VFD clock and had it running for a few years ? The other thing to know is that driving the grid on these large tubes with AC vs DC will likely affect the lifetime. When the grid and filaments are DC, there is a static electric field in the tube, and you can see darker vs lighter bands on the segments due to the electric field as it gets warped between the grid and filament. Over time, this will likely cause uneven wearout on the phosphor. When the grid is driven by AC or varying DC, the electric field changes, and the segment glow looks uniform. I dont yet have any observations for AC-filament drive. The VFD on my kitchen stove is 13 years old, and it clearly has darker bands closest to the filaments where the phosphor has been bombarded-away by the electrons. -- 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 neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/95bf90b1-6754-4c12-ac07-a685dc7fc37b%40googlegroups.com.
