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| Acceleration voltage effect on VFD phosphor lifetime |
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| SeanB:
Put the resistor in, you either dissipate power in the resistor, or do it in the tube itself. Better to run the tube within ratings, that run it at around 4 times the voltage, where it is extremely likely to suffer from arcing over between the anode and the other electrodes, as they have been built to withstand 50V only, likely fine to 100V, but above that arcing, or just simple slow build up of ion deposits, is very likely. Resistor and a zener diode to clamp to 47V will work. Heater run at the specified, if you want longer life put a series resistor to drop the voltage to the lower end of the allowed filament voltage range, and thus also have a bonus of inrush limiting when cold. Running at lower heater current, and thus making the heater current limited, will strip it badly within a short time, and kill the tube. You want full emission with a VFD, it will otherwise band, as the limited current is going to flow to the closest portions o the anode. |
| f36:
--- Quote from: SeanB on February 28, 2024, 08:17:35 am ---Put the resistor in, you either dissipate power in the resistor, or do it in the tube itself. Better to run the tube within ratings, that run it at around 4 times the voltage, where it is extremely likely to suffer from arcing over between the anode and the other electrodes, as they have been built to withstand 50V only, likely fine to 100V, but above that arcing, or just simple slow build up of ion deposits, is very likely. Resistor and a zener diode to clamp to 47V will work. Heater run at the specified, if you want longer life put a series resistor to drop the voltage to the lower end of the allowed filament voltage range, and thus also have a bonus of inrush limiting when cold. Running at lower heater current, and thus making the heater current limited, will strip it badly within a short time, and kill the tube. You want full emission with a VFD, it will otherwise band, as the limited current is going to flow to the closest portions o the anode. --- End quote --- You say that when running the tube at a higher than designed voltage, other than arcing, there may be movement of ions between the electrodes that will cause damage. Although I did find reference to CRTs with the same phosphor being run at a much higher voltage in the Tektronix book, I understand that there are many other considerations that the engineers who designed the tube made when specifying the anode voltage. From what I'm getting, these ions will come from the cathode or the material coating it, and so this is the material that will be (directly) damaged by the higher voltage. From my experience, the exact composition of the cathode seems less readily apparent and is less commonly discussed. The main reason I started this thread was to understand the effects high voltage has on VFDs, and I assumed it was the phosphor itself that we should care about since burn-in is clearly an effect on the phosphor. So, perhaps my question should be reformulated to include the rest of the components of the tube. When you say that there will be ion movement across the tube that will damage it, what exactly moderates this process and causes it to become a problem beyond a certain voltage? Is there an E-field strength that, beyond which, said ions will have a much lower chance of returning to the cathode and instead will be launched towards the anode? People sometimes report successfully "repairing" VFDs by overdriving the heater temporarily. Is the aging mechanism that leads to this related? Obviously the ideal solution would be to run the tube at its rated voltage, and I can easily design a circuit to do so. I am wondering, even if it ends up being for purely educational purposes due to the impracticality of it, what exactly is the damage mechanism associated with overvolting a VFD. Perhaps if it is a quantifiable "risk," some engineering tradeoff decisions can be made regarding it, instead of blindly declaring the datasheet values absolute maximums (though, admittedly, that's what we EEs should do 99.9% of the time...) |
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