On 16-02-20 04:03 PM, ZY wrote:
Hello. I'm pretty new to nixies/vfds, and I have some questions on how to drive a VFD tube before I pick some up on ebay: ? I've seen some circuits where they just bias the filament voltage a bit higher with a diode or a resistor before the ground. Does that work? Is this what the max6931 filament bias does? As in I supply -7v to the IC, 4) Why do some circuits bias the filament at a negative voltage like -30v?
8) If I drive one segment more than the other segments, would it "wear out"? 9) What contributes more to tube life? Less filament current, or lower grid/anode voltages? Is it both? 10) What is the mechanism in which VFDs fail?
VFD displays are little triode Vacuum tubes. with some phosphor on the anodes which glow when their is electron current. (*)
The anode needs to be typicaly 30V more positive than the Filament/Cathode. SO you can make the cathode negative and keep the anode near ground or make the anode positive and have the cathode near ground. Or even a mixture! as long as the DIFFERENCE is whatever the display is expecting - Ie 30V.
If their is a grid, it is generaly there to allow things like multiplexing. The data sheet should show the range of voltage it is expecting. It can be used to turn on or turn off the entire digit, or even one way of dimming the display.
the filament is the source of electrons so you generally want it to have an AC voltage at the rated voltage and current. If it has DC and goes across a multi-digit display the DC levels between the filament and the grid will vary and so with the light output.
The Phosphor will gradually loose output over use, and that loss tends to be more pronounced during the first part of the Tubes life. Not using a segment for a long time then lighting it up MAY result in the "fresh Segment" being brighter.
The other burn-out mechanism is the filament burning out like a light bulb, or losing emission over time. In some cases leving the filament lit up but the tubes biased off may lead to cathode contamination, and reduced emission.
(*) some enterprising folks have made Phonograph pre-amplifiers using a VFD. it is not a great triode, but the glow does change with the current.
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