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Would something like this work to drive a VFD display ?

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MathWizard:
I salvaged a VFD display from a Toshiba DVD player, I think a SD-3950. I have the VFD PCB, with it's driver IC/MCU. Also the PWR/IR remote PCB, and the main PCB with the main MCU and main video IC and eeproms for copying I2C to the VFD IC.

I don't have the SMPS PSU, but I did manage to find a PCB layout diagram, but without any values, and maybe zener's are just labelled as diodes for all I know.

The only rails the VFD PCB uses are +5V, -20V, and some cathode/filament rail tied above the -20V rail.

But I made a PSU layout model in LTSpice, and I can see where and what most of the PSU connections to my PCB's are for. And now , without much calculation, I made a "working?" model of the negative rails part in LTSpice, with just a JFET to sort of act like the VFD, maybe with some reasonable currents. But IDK that much about designing SMPS or VFD's.


For now I was just going to use my bench PSU for +5/-20V rails. And I came across a little 555 driver circuit for making some AC waveform for the filament.
http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/

I really don't know what the original SMPS waveform would be, but does this 555 circuit look reasonable ?? I just guessed at some inductor values, I'd have to try to calculate all that later. IDK yet about torriod and/or non-air-core inductors really anyways.


I think I found the VFD IC datasheet too, but trying to learn that will be another big challenge.

pcprogrammer:
For as far as I know, the filament is just for heating the display and can be driven with just an AC or DC voltage of around 6V or so. Basically much the same as old vacuum tubes that use a separate 6.3V winding on a transformer.

I have done it like this with a VFD a long time ago. Just made an adjustable DC voltage to a max of 6.5V and connected it to the filament. Also have another VFD I tested recently again, but that one has all the voltages made on the board itself. Will look at it to see what it does there.

HalFoster:
A filament is best run with AC - DC will cause a brightness slope across the display.  Google for information on driving them as well as the theory of their operation - I know Futaba and Noritake have good white papers available.

pcprogrammer:
Did some measurements on the VFD module I have, and it shows that there is indeed a form of AC voltage across the filament, but it uses a DC offset and counter phase signals on both pins.

I measured with the scope connected with two probes connected to ground and a tip on each end of the filament.


--- Quote from: HalFoster on March 25, 2024, 09:18:20 am ---A filament is best run with AC - DC will cause a brightness slope across the display.  Google for information on driving them as well as the theory of their operation - I know Futaba and Noritake have good white papers available.

--- End quote ---

It has been to long ago that I did the DC voltage on the filament, so no recollection if it indeed showed difference in brightness. Have to do the setup again to see if this is indeed the case with that display.

Edit: I hooked up the second module I have, and unfortunately it has a defect. Some driver ic has lost a bunch of pins, so not all of the pixels light up.  |O Can't recall what happened to it or where I bought it. Talking about > 30 years ago.  :o

A picture of the working module can be found here.

Attached are screen captures of the measurements. One with AC coupling on the two filament signals and one with DC coupling. The DC offset is around minus 40V.

MathWizard:
Yeah I've read a few articles on VFD, and most all of them barely mention real details like current and voltages. I know it depends on the size and number of elements, etc. And they can be driven in different ways. Whatever I try, I'll limit the filament current to 10's of mA. If the VFD wasn't attached to the PCB and IC, I'd would have tried it already.

Later today I should hook up the +5 and -20V, and see if the gird is pulsing between -20V ,and +5V or GND, or if maybe they pulse the anodes. I'm forgetting that a bit of DC won't kill it. The cold filament resistance is 7Ohms, that will change too.


I was thinking of just making an op-amp oscillator, and driving some BJT(s) between 2 low voltages near the -20V. But yeah I'd really have to watch the DC current whatever I try.


But yeah some VFD's allow for a bigger DC drop across the filament, by changing the geometry of the device. But on mine, the filament looks pretty level.




But that VFD IC, that thing is huge, it really is a huge MCU/CPU, not just some simple voltage/current driver.

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