I am also curious: with the multiplex scan being relatively slow and close to mains frequency, do you observe any instability of brightness, like a beat frequency?
czwartek, 2 kwietnia 2026 o 18:58:27 UTC+2 Tomasz Kowalczyk napisał(a): > " The A/C filament drive is connected at the left hand end of filament, > the other end to GND." > I need to make sure: how exactly was this applied? The result is in line > with what I would expect if you hooked up a transformer tap with one end to > the left side of the display, other end to the right side, and grounded the > right side. Which, for half of the cycle, would mean the cathode is > negatively biased compared to the grid and anodes, letting some current > through. > > As Gregebert said, if your filament transformer has a center tap, use it. > I think you should hook it up to a positive voltage of at least half the > filament peak-to-peak voltage to prevent any part of the filament ever > going below ground, so if your transformer produces 4,5 V, the center tap > must be at least 2,25 V. Slightly more might be needed to get rid of off > digit glow completely, it's display dependent. > These displays are basically triodes, after all, and to shut off a triode > completely you need to pull the grid below the cathode voltage. > > For chopped 5 V, I think dedicated filament drivers are not strictly > necessary, any H bridge capable of operating at the filament current should > be OK. Check what frequency the dedicated ICs produce as a reference. The > filament is a thin, long wire which probably can resonate mechanically at > some frequency, and I have absolutely zero idea what the range might be. > > > czwartek, 2 kwietnia 2026 o 07:35:52 UTC+2 Richard Scales napisał(a): > >> I acquired a bunch of Itron FG2013A1 VFD displays. These are 16 segments >> (plus DP/comma) 20 character displays. >> >> I wanted to use them as a reason to try and grasp the nettle of >> multiplexing (which has eluded me for some time). I hooked the filament up >> to 4.5V and applied 12V to a grid and 12V to some segments - all good so >> far. >> I had a bunch of HV5812 to hand and tried driving the display with these >> to turn on grids and segments - still good. >> >> I made up a board with 2 x HV5812 drivers cascaded to give me a total of >> 18 segment drives and 20 grid drives. All I had to do then was to crack the >> multiplexing thing. >> >> After some time and with assistance I got an interrupt routine firing >> every 1mS which would set the segments and grid I wanted to turn on and an >> SPI.transfer later - the display showed what I wanted. 1mS x 20 characters >> gives a refresh of 50Hz (if my math is right) which looks just fine. >> >> .. however, when I want to show all 20 characters (in my test case 0-9 >> twice) I see some 'interesting' results. >> >> The A/C filament drive is connected at the left hand end of filament, the >> other end to GND. >> >> I am using 12V for VPP to the HV5812 drivers so I expect this to be what >> is going to each grid and segment that I want to activate. >> >> The ISR just sets the segments for the current grid and turns them on in >> one hit (the SPI.Transfer is sending 40 bits - 20 for the grids, 18 for the >> segments and two are wasted) >> >> Everything checks out using a logic analyser and there is plenty of time >> left to do other stuff - I am using an ESP-32 from AZ_Delivery for the >> testing and am using a CD40109 to convert the 3V3 logic to 5V. >> >> I can see that it really is showing the right segments on each digit - >> it's just that there is a whole bunch more illumination of other unwanted >> segments which is far worse at the A/C supply end of the filament and >> completely gone ant the GND end. >> >> I call upon those more experienced in these things to see if anyone can >> point me in the right direction? >> >> I plan to try a DC filament drive next just to rule that out. >> >> - Richard >> >> -- 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 view this discussion, visit https://groups.google.com/d/msgid/neonixie-l/06eb7bc4-c36e-4818-be43-8542373eb409n%40googlegroups.com.
