I think you want to drive the filament from a center-tapped transformer, and ground the center-tap. This minimizes the voltage gradient across the display, basically half of what it would be if single-ended.
With your current setup (single-ended), one side is grounded, and the other going positive/negative via AC, you are still setting up a voltage gradient across the display. The other thing to consider is applying either a negative bias onto the grids to turn them off better, OR...if you follow the above suggestion with the center-tap transformer, apply a small positive DC bias to the center-tap. This will effectively make your grids negative relative to the filament (cathode) and improve the cutoff. You will also want to increase the segment voltage roughly by the same amount of the DC bias at the filament transformer center-tap, otherwise the brightness will be reduced. Be careful about driving the grid positive relative to the filament; it will definitely increase the brightness as well as causing current thru the grid. Normally, grid-current is close to zero, because it's intent is to cause electrons to be repelled (blocked) from reaching the anode (segments). The datasheet should have details about this. NIMO tubes behave similarly to VFDs, though the anode voltage is quite a bit higher. BTW, I dont see any ghosting so that's a good sign. On Wednesday, April 1, 2026 at 10:41:09 PM UTC-7 Richard Scales wrote: > [image: vfd.jpeg] > > This is supposed to show 0-9 twice - it is there but mostly masked by this > 'effect' on the left. > - Richard > > On Thursday, 2 April 2026 at 06:35:52 UTC+1 Richard Scales wrote: > >> 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/87609a6e-90ac-4a53-8c18-47c8b1d572b4n%40googlegroups.com.
