@ Tomasz: With the filament on DC, no visible beat frequency at all. There is a lot about this which is outside the scope of my current knowledge - though I am learning all the time thanks to the good people here.
I would like to crack it as I have a number of these displays that I would like to put to use. I also plan to do the same trick with a large panaplex display that I have (16 digits of 16 segments) - at least that should be easier to drive (famous last words!). - Richard On Friday, 3 April 2026 at 05:31:50 UTC+1 Richard Scales wrote: > Very many thanks for the tips - a short blanking interval made no > difference but the switch to DC showed that the multiplexing was working > well. > > I need to look at the setup of my current A/C filament drive. > > As for the grid/segment voltages, I am currently using 12V - perhaps I > need to change that? > > - Richard > > > On Thursday, 2 April 2026 at 12:47:39 UTC+1 Grahame Marsh wrote: > >> I've always had success using a LM9022 VFD filament driver. It produces a >> square wave AC on two pins at 5V which you connect across filament. So the >> filament gets biased to about +2.5V. Go look for the datasheet and you'll >> see how easy it is to use. You can also feed the outputs into a Dickson >> charge pump to generate 25V for the grids and anodes. >> >> This chip is now unobtanium but the LM4871 (about £1 from Mouser) is >> identical although not listed as a VFD filament supply. >> >> Hope this helps >> >> Graham >> On 02/04/2026 12:03, David Pye wrote: >> >> Does a short blanking interval fix the issue? >> >> On Thu, 2 Apr 2026, 07:20 'Richard Scales' via neonixie-l, < >> [email protected]> wrote: >> >> Many thanks indeed. >>> As part of my debugging/research, I tried using 5V DC for the filament >>> and got this: >>> >>> >>> >>> Here you can see that the brighter digits are at the end nearest GND and >>> the less bright digits are at the end which is at 5V. I can understand that >>> - at the 5V end there is only 12-5=7V between the filament and the >>> grid/segments whilst at the GND end there is a full 12V between them - I am >>> guessing that this is entirely relevant. >>> >>> - Richard >>> >>> >>> On Thursday, 2 April 2026 at 07:06:21 UTC+1 gregebert wrote: >>> >>>> 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: >>>> >>>>> >>>>> >>>>> 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/323902ee-1ed3-4235-8979-b38e7e63cd2bn%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/neonixie-l/323902ee-1ed3-4235-8979-b38e7e63cd2bn%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- >> 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/CAOQ6x0GwffP7q6gUZkY5peRvRW47_Ah4oXoaG7TPT%3DPJr9_NwA%40mail.gmail.com >> >> <https://groups.google.com/d/msgid/neonixie-l/CAOQ6x0GwffP7q6gUZkY5peRvRW47_Ah4oXoaG7TPT%3DPJr9_NwA%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> >> -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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