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Vacuum Fluorescent Display Driver

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132mA filament current is only just a little higher then DC-DC spec of 120mA but it'll be fine for a quick test.

Also good to see the segment currents are <<1mA using a 10K current limiting resistor.   Q1: Are the currents in the table the sum of both Gn and SEG_m currents?  Or just the segment current alone?  See attached; ie. A1+A2 or just A1?

Q2: If it's just A1, are the A2 currents similarly <<1mA using a 10K current limiting resistor? Don't bother with a detailed table of every combination. Just report back the largest draw.

Q3: If the Gn and SEG_m currents are <<1mA, change the grid and segment current limiting resistors from 10K to 1K and confirm that the Gn and SEG_m currents are now <<25mA.  If any Gn or SEG_m current >25mA that's going to burn-out the HV518 driver.

Q4: If the Gn and SEG_m currents are <<20mA (using 1K resistors) then switch to 100R for the grid and segment resistors and report the largest current drawn (from Gn (A2) and SEG_m (A1)).  The object here is to sneak-up on the 25mA limit. If we go over, then the display is bad because a good one will never demand more than that from a driver output pin.  Normally, there is no resistor in series with a driver output.

If the Gn and SEG_m currents are <25mA using the 100R series resistors, then finally remove the grid and segment resistors completely (each connected directly to the +27VDC supply).  Q5: What's the largest current drawn from Gn (A2) and SEG_m (A1)?

I will set aside time sometime this week to perform the measurements you're asking. Unfortunately it may not be until late in the week or maybe the weekend.

The numbers I provided were the SEG current only. Gx was connected directly to 10k resistors (I'd clip lead the 27V to each resistor) without a current meter - I didn't have a spare current meter handy.

Attached is the 38V and 5V (38V was taken from a via next to E4 and 5V from pin 10 on the ribbon cable on the main PCB side).

Also, this is without the VFD installed and the VFD IC removed. Per what I remember seeing when the display was working with the new IC (until I turned off power and turned on power only to find out the IC blew), the +38V discharges slowly.

The scope capture shows after 12s the voltage is still 10V while Vcc has long since turned off.

I pieced together the segment locations (see attached) that I noted while bench testing the VFD.

Maybe this is applicable to other VFDs, but thought to piece this together for anyone interested.


--- Quote ---Q2: If it's just A1, are the A2 currents similarly <<1mA using a 10K current limiting resistor? Don't bother with a detailed table of every combination. Just report back the largest draw.
--- End quote ---

Since I don't quite understand how the VFD works (although I've watched videos), I wanted some clarification: Do I need to measure all the Gn lines for every SEG combination (i.e. SEG-A with G1>G12, SEG-B with G1>G12, etc...), or can I just power a single SEG line, measure all twelve G lines, and those numbers will be the same for all SEG lines?

Not sure what I have for resistors, but maybe I'll use a decade (resistor?) box. If I remember correctly, they are 2W resistors, so that may make things easier.

The intent is to confirm that at NO TIME does any Gn x SEG_m combination, that EITHER current (A1 or A2) draw is >25mA. 
First using a pair of 10K, then a pair of 1K, then a pair of 100R resistors, then no series resistors. 
1/2 watts should be fine given the brief on-time to check currents.

So, I envision two ammeters. One lead from each ammeter starts from the +27V supply.  Then the other lead (with series resistor)... first on G1, second ammeter lead (with series resistor) on SEG_A, then move second to SEG_B, .... SEG_P, SEG_DP1, SEG_DP2, SEG_COM.  Each time lookup up briefly to check if the Gn or the SEG_m currents exceed 25mA.  Don't bother documenting each reading; just note if it comes close to 25mA.
Then switch first ammeter lead to G2, second on SEG_A....SEG_COM.  Then switch first ammeter lead to G3, etc.
It should just be a couple of seconds each lookup at the ammeters to see.
I don't anticipate that the grid current will change much with each different segment lit.  I just want to be sure.
If you find that as you try every combination with 1K then every combo with 100R, and you're seeing >25mA currents then stop there; don't bother to go lower since the display is FUBAR.

If you only have one ammeter, then you'll have to do every combination of Gn x SEG_m twice for every series resistor combo; once with the ammeter monitoring the A1 current then monitoring the A2 current for 10K, then again for 1K, etc.

I don't also want to rush going from 10K series resistance directly to no resistor because it might zap something inside. Although, if any A1 or A2 current >25mA that pretty much guarantees that the display is a driver chip killer so it'll be useless anyway.

Again, we're just ruling out that the display will never draw >25mA from any A1 or A2 current when no resistor is in series.

VFDs are low-voltage vacuum tubes. 
You must keep the filament (heater) constantly lit by supplying the recommended voltage that does not allow it to exceed its max. current. 
Then also supply a positive voltage (with respect to the filament) for the grid AND segment (anode).  Higher grid or segment voltages will cause more electrons from the filament to strike the phosphor on the anode and cause it to get brighter.


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