That would be what I would infer too (that the anode voltage would have to be suitably higher to maintain the same brightness).
This reference design shows an interesting way to produce an oscillating filament voltage - basically using an additional secondary winding of the transformer used in the boost converter for the anode voltage: https://pdfserv.maximintegrated.com/en/an/AN4363.pdf. The transformer doesn’t seem to be made any more, which seems to be an issue with this approach as VFDs become increasingly obsolete. > On Sep 15, 2019, at 6:03 AM, Alex <[email protected]> wrote: > > I am trying to get my head around that, but I see the diagram you mention: > > "If the filament potential is lower than the anode and grid cut off voltage, > thermionic electrons can reach the anode and cause illumination of the > phosphor. The filament > bias voltage should be increased to prevent this problem. Generally, a zener > diode is used to obtain the bias voltage." > > I guess it could be summarised as advantagous to put the center tap of the > filament supply a diode drop or two above ground, such that when the grids or > anodes are pulled to ground (to turn them off) they then appear slightly > negative to the filament and thus are fully disuaded from illuminating. Mind > you, I would think that any raising of the filament center point would also > require a slightly higher anode potential to give the same brightness? > > Another interesting point on that page is that VFD displays designed for DC > applications (e.g. pocket calculators (!)) can have sloped filaments to > mitigate the brightness gradient... > > >> On Saturday, 14 September 2019 13:22:36 UTC+1, Paul Andrews wrote: >> Papers I have read also emphasize the need to bias the filament above the >> cutoff voltage of the grid or anode, so I assume if the center tap is >> attached to ground, that the cutoff voltage of the anode or grid is >> significantly below that? In fact it would have to be below the negative >> swing of the AC. It might be easier to connect the center tap to a positive >> voltage? See the diagram in the "Cutoff bias voltage" section here: >> https://www.futaba.co.jp/en/display/vfdinfo/vinfo_kudo_1.html > > -- > You received this message because you are subscribed to a topic in the Google > Groups "neonixie-l" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/neonixie-l/R89iFm3Ndhg/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > [email protected]. > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/794103de-d808-477d-9a26-161d15d9f3cc%40googlegroups.com. -- 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 on the web, visit https://groups.google.com/d/msgid/neonixie-l/F5A5197E-20BA-42D9-A8B6-7FE57DC3EA99%40gmail.com.
