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10937p-50 chip?
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hydrolisk1792:
Okay. I have some common cathode displays, and I usually go by the 2.1v 10ma thing when dealing with unknown leds.

Quick question, when using these chips with the actual vfd glass, what is a good filiment voltage to use? AC or DC? Working on another one because I found that one of the glass are still good that I have.
Ian.M:
LED Vf is highly dependent on colour  Your 2.1V 'guesstimate' will only be typical of GaAS (Gallium Arsenide)  amber, yellow and green LEDs and *SOME* GaAS red LEDs. 

However if your LEDs are over 2V Vf, due to the voltage drops in the 10937p-50 segment drivers and the ULN2003/2803 inverting cathode drivers it looks like there wont be enough headroom to run the LEDs between the +5V rail and ground and still have enough drop across the segment resistors to accurately set the current , so you'll have to return the  cathode driver to your -10V rail.  If you wanted to run them between 5V and Gnd, you'd need to use MOSFETs (with gate pulldown resistors) for the cathode drivers to avoid the high Vce_sat drop of the Darlingtons in the ULN2003/2803.

Whether you need to run a VFD filament on AC or DC depends on the filament length, and how much brightness variation you can tolerate across the display due to the voltage drop across a DC filament resuting in a different anode-cathode voltage for each digit.   Get it going on DC, and if you don't like the results, build an AC filament driver.

To find the filament voltage for an unknown VFD with no data,  if possible, measure the RMS voltage across the filament (using a scope or true RMS meter that can handle the filament drive frequency) in the equipment it came out of to give you a starting point.  Otherwise, in a dimly lit room, connect just the filament to a PSU that goes right down to 0V. Observe the VFD,  shielding it from as much external light as possible, and *SLOWLY* starting from 0V turn up the voltage till the filament has a barely perceptible dark red glow.  If you can observe the glow under normal domestic room lighting, you are almost certainly over-driving the filament.     
hydrolisk1792:
So for the vfd question, I went ahead and measured one of the original displays in working condition.  It measured spot on 5vac with my fluke 87 true rms DMM.
Ian.M:
So, next check that as described above with a bench supply, wind the voltage up to 5V DC (while keeping an eye on the filament to make sure it doesn't get hotter than a very dim glow) and measure the filament current, which you'll need to design a supply for it.

Also, while you can, measure the supply rails to the VFD driver,  the DC bias on the filament (check both ends), and the negative voltage feeding the grid and segment pulldown resistors,  to get an idea of the anode-cathode voltage it needs, and the extra grid bias for cutoff.   See Fig.4 Partial System Schematic in the  10937p-50 datasheet to get an idea of what voltages you are looking for.

That gives you enough data to put together something that should work first time - either a 6.3V filament transformer + a dropper resistor, or drive a MOSFET H-bridge powered from your 5V rail, with a 50% duty cycle squarewave, then capacitively couple its outputs to the ends of the filament so you can bias the filament to a suitable   negative voltage.  If you use a BJT H-bridge, you'll need a somewhat higher supply voltage to compensate for its Vce_sat voltage drops.

Either way you'll need a resistive divider across the filament to fake a center-tapped 5V transformer.   I'd start by using resistors of about 5x the filament hot resistance.  Again see fig.4 - your drive circuit replaces the CT transformer and AC source shown directly below the VFD.  The center tap of the two resistors goes to the cathode of the bias Zener Ek.

The pro-grade alternative of a Royer or similar power oscillator driving a miniature transformer to provide both a CT 5V RMS secondary and a suitable output to rectify to get the -Vdisp supply is just too much design + experimental work for a one-off to make sure the filament voltage doesn't change excessively as the load on -Vdisp varies, unless you can simply reverse engineer the oscillator + transformer circuit from the donor equipment and reuse the transformer frim it.
hydrolisk1792:
Actually the last part of your post is exactly what I'm doing for the one off vfd I'm doing. I was given another one off a jukebox that had a busted glass and I'm going to use that transformer. My mate also gave me a schematic for that display, so I don't even need to reverse engineer the board.

As for the LED verson in the OP, I'm waiting on more displays to come in the post.

Now I'm assuming on the LED verso I don't need the negative bias for the pull down resistors, I can just put them at ground, right? Or should I keep that true to the datasheet?
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