| Electronics > Projects, Designs, and Technical Stuff |
| VFD Display 101 |
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| NivagSwerdna:
I have eventually given in and bought an IV-18 VFD display to amuse myself with... I'm planning to use the LM4871 for the filament drive (although MAX628 and uP clock seems feasible) and some kind of boost regulator... There is a nice reference design here.. http://magictale.com/wp-content/uploads/2015/06/VFD_PSU_EXP01_CircuitDiagram_Rev.1.0.pdf and here https://www.eevblog.com/forum/projects/showing-my-vfd-psu/msg1610392/#msg1610392 Anyway... there seem to be two schools of thought... generate +30V or generate -30V. I'm tempted to generate +30 (or maybe +35) but many designs use -30... What is the benefit of using the negative HV? I obviously need to control the grids and anodes... so anticipating using an HV5812... which pushes me more towards +30? Hoping for some Rolo and Ian.M replies :) TIA |
| Ian.M:
Edit: You cant call it "VFD 101" without linking https://www.noritake-elec.com/technology/general-technical-information/vfd-operation which is a must read for any VFD newb. >:D Back in the '80's, running a VFD with its filament cathode at a negative potential had considerable advantages - the HT supply would probably be generated by a blocking oscillator or Royer converter driving a pot core or other small transformer, so an isolated filament supply that could be biassed to approx -25V was no problem, and it avoided the need for level shifting for all the anodes and grids, as a VFD doesn't need a lot of current per electrode apart from the filament they could simply be implemented as open collector lateral PNP transistors, referenced to the 5V rail, without any level shifting in an otherwise 5V ASIC, albeit with a large negative substrate bias. The development of +HV drivers with integrated shift registers makes the traditional negative filament approach much less attractive, and IMHO you'd be crazy to go down that route unless you were using a LSI VFD controller that required it. (e.g. some sort of retro single chip timer/clock build) |
| NivagSwerdna:
It seems LM9022 aka LM4871 is a popular choice... single supply, self-oscillating etc but the output needs to be biased up otherwise on the down side the cathodes will end up more negative than off grids/anodes? One of the above designs uses a pair of 100R to get a virtual middle and then attaches that to a zenner and C... I put that into a simulator and it seemed to seriously squash the output? |
| Ian.M:
Did you try the sim with a pair of current sources feeding a few mA to both ends of the filament to simulate the electron current from cathode to anodes? Without it the Zener wont be properly biassed. |
| Rolo:
I use +HV drivers with integrated shift registers, never worked with negative HV. I use the 20 channel A6812ELW in my designs. They are out of production but can be found on Aliexpress, used several of these and had no issues. I think they are original old stock chips. In case they can't be found anymore they are pin compatible with the Supertex HV5812 and Microchip MAX6921. The A6812 does take 3.3V logic on the inputs, and is cheap. Is used these in static and multiplex operation, works great. For driving the filament I use the LM4871 and for the HV the LM2733-Y. This design has been posted on this forum. Be ware for fake LM4871's, they work but are not up to the original specs. |
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