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Vacuum Fluorescent Display Driver

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floobydust:
1kΩ is the lowest I would go to work with the tube. Operated as a triode, heater to screen-grid current limit is that I find.

I don't see you checking for shorts between "G-lines" (as you call the screen grid)? You've checked between segments I think.
Another check for shorted elements is to energize one segment and see if any others will light. Same for the grids, I tie a bunch of segments together high and energize each grid one at a time, looking for drama. I was staying out of the thread so you can work with pqass, haven't followed all the tests.

Lexicon reverb common problem is the DC-DC converter loses regulation and goes too high in output voltage and blows the VFD driver IC.
Here, the HV518PJ is good to 90V so not the problem but I mention it as another VFD-related issue I have seen.

pqass:

--- Quote from: bostonman on May 28, 2024, 03:59:00 am ---Also note that all the filament currents are the same in my spreadsheet. They varied very little and I gave up taking notes for each measurement. I noticed it would start at maybe 117mA while switching G lines (i.e. nothing lit) and then drop to about 116.5mA when the G line was connected (i.e. a segment was lit).

I know and understand the trick is to sneak up on 25mA, but is it worth trying to jump from 10k to 100ohms (or 100R as it seems to be used on here) to save steps or is it too risky?

--- End quote ---

117mA filament current is good to see given that it's just under/at what the DC-DC filament output can provide.

Given how low the current readings are in your tables, it'll probably be fine to skip from 10K to 100R.  I was just being overly cautious by trying with 1K too.  Just only take a few seconds per measurement at most.   If you're way beyond 25mA it may be pointless to continue to record all measurements.   

If the round of measurements are below 25mA using 100R, then try the final round of measurements with NO resistor (ie. directly attaching GRIDn and SEG-m to +27V).  Again, If you're way beyond 25mA it may be pointless to continue to record all measurements.   

pqass:

--- Quote from: floobydust on May 28, 2024, 04:22:27 am ---1kΩ is the lowest I would go to work with the tube. Operated as a triode, heater to screen-grid current limit is that I find.

I don't see you checking for shorts between "G-lines" (as you call the screen grid)? You've checked between segments I think.
Another check for shorted elements is to energize one segment and see if any others will light. Same for the grids, I tie a bunch of segments together high and energize each grid one at a time, looking for drama. I was staying out of the thread so you can work with pqass, haven't followed all the tests.

--- End quote ---

Eventually, bostonman will need to test all GRIDn and SEG-m pins directly attaching to the +27VDC source since that's what the driver does; no resistor in series.  Measuring the current first through a 10K, then 100R was just being overly cautious.  We talked about shorts as what to look for but really any excess current >25mA spells doom as that will destroy the driver chip. The algorithm involves connecting only one grid and one segment at a time, recording the current through each.  But, he covers all combinations so if there was some leakage current to an adjacent grid or segment then it will be reflected in the recorded current measurement.  Okay, it may still be <25mA with leakage currents, so as bostonman takes the measurements be on the lookout for multiple lit segments.

Agreed, once a round of measurements are done with NO resistor in series (with one grid and one segment on), he can then do a final multiple segment on test by shorting all segment pins together (tying them to +27VDC) and measuring the current through each GRID-n (to +27VDC); total of 12 measurements.

bostonman:

--- Quote ---Given how low the current readings are in your tables, it'll probably be fine to skip from 10K to 100R.  I was just being overly cautious by trying with 1K too.
--- End quote ---

I'm sure you also don't want to give advice that may blow up the property of others, so it's good to take caution. My next test will be 100ohms on both SEG and G lines. This time around maybe I'll just skim along without noting the currents and make a mental note of the highest one.

A few reasons why I took extra time to note the current values was to provide a clear chart on my measurements to help dissect any possible VFD issues, and for anyone else who comes across this thread who needs help. As most may know, it's extremely time consuming to stop and write each current value and then enter them into Excel.

Also, my initial current measurements included looking at the display each time to check the associated segment was lit, and I didn't see any others that were lit simultaneously.

floobydust:

--- Quote from: pqass on May 28, 2024, 05:29:38 am ---
--- Quote from: floobydust on May 28, 2024, 04:22:27 am ---1kΩ is the lowest I would go to work with the tube. Operated as a triode, heater to screen-grid current limit is that I find.

I don't see you checking for shorts between "G-lines" (as you call the screen grid)? You've checked between segments I think.
Another check for shorted elements is to energize one segment and see if any others will light. Same for the grids, I tie a bunch of segments together high and energize each grid one at a time, looking for drama. I was staying out of the thread so you can work with pqass, haven't followed all the tests.

--- End quote ---

Eventually, bostonman will need to test all GRIDn and SEG-m pins directly attaching to the +27VDC source since that's what the driver does; no resistor in series.  Measuring the current first through a 10K, then 100R was just being overly cautious.  We talked about shorts as what to look for but really any excess current >25mA spells doom as that will destroy the driver chip. The algorithm involves connecting only one grid and one segment at a time, recording the current through each.  But, he covers all combinations so if there was some leakage current to an adjacent grid or segment then it will be reflected in the recorded current measurement.  Okay, it may still be <25mA with leakage currents, so as bostonman takes the measurements be on the lookout for multiple lit segments.

Agreed, once a round of measurements are done with NO resistor in series (with one grid and one segment on), he can then do a final multiple segment on test by shorting all segment pins together (tying them to +27VDC) and measuring the current through each GRID-n (to +27VDC); total of 12 measurements.

--- End quote ---

Last tests I did on a HP VFD, connecting a screen-grid to +HV through a 1k yielded unexpected results.
The filament stopped glowing to the right of that grid  :o  The display works fine. I'll see if I took pictures of that, had me stumped.
I figure cathode current was being pulled. The tube can be run as a diode- so screen current must be limited and 1kΩ was low enough for me.

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