More VFD characteristics

I've pretty much settled on using the 12-digit vacuum fluorescent display (VFD) that came with the P170-DH calculator. Trying to squeeze a 2-line, 20-character VFD in its place is more work than I want to undertake. Besides, I'm having too much fun figuring out how to drive the original tube.

Late one evening I decided to figure out the lead spacing on this VFD. It didn't fit my 0.1" (2.54mm) breadboards, but it didn't seem to fit the common 2.00mm metric grid spacing either. Then I noticed that the PCB had the end pins labeled "1" and "60" which works out nicely to 1.75mm pin spacing. This matches the holes in the original PCB as closely as I can measure. I've updated the pinout chart in my earlier post to reflect this pin numbering.

[edit: see later entry to see why this pin spacing is wrong]

I've also taken another look at the VFD filament drive. The original board uses a custom transformer to generate +5 VDC for logic and active anodes & grids, −25 VDC for inactive anodes & grids, and 5 VAC for the filament. However, the 5 VAC filament supply isn't really 5 VRMS due to the non-symmetrical shape and the discontinuous operation of the oscillator. Let's take a closer look at the filament drive... or better, two looks.

First let's look at the filament drive over a short time scale. The yellow trace shows the "bottom" end of the transformer primary winding; it is actively pulled low by transistor TR1. When TR1 turns off it rebounds as the transformer field collapses, rising above the supply voltage until it is limited by Zener diode ZD1. The blue and green traces show the two ends of the secondary winding that are connected to the filament, and the pinkish trace is the difference between these two signals and thus voltage across the filament. Note how the absolute value of this voltage is greater when the transformer primary is being driven (about 7 volts) than it is during the flyback period (about 3 volts). Also note that the driven period is shorter (about 3.6 us) than the flyback period (about 5.2 us). If my math is right, this comes out to be just about 5 VRMS.

However, the voltage regulation operates by cutting off the oscillator when the voltage rises and allowing it to run when the voltage falls. Looking at the same waveforms over a longer period shows this clearly. This looks like about a 35% duty cycle, which would cut the power delivered to the filament significantly. One caveat here is that I took these measurements this after I removed the VFD from the board. [Edit: And with too high a V+ supply! The second calculator provides 7.85V which is low enough to keep the regulator loop out of discontinuous mode operation.] To avoid throwing the measurements off too much I replaced it with a 50 ohm resistor I had handy, but the filament seems to be about 30 ohms hot. I would think this would draw more power from the transformer and cause the oscillator duty cycle to rise somewhat, but I definitely saw discontinuous operation before I started disassembling the thing.

I've read that most VFDs operate with a filament voltage of between 3 to 4 volts RMS. As a test I drove the filament with 5 VDC with the lights turned off and the filament was glowing faintly, which shouldn't happen in a properly driven VFD.

Finally, I wanted to quantify the grid and anode currents. With the filament connected to a DC power supply, I drove one anode segment and the grid of a digit nearest the negative end of the filament with +25 VDC. As I raised the filament voltage above about 3 VDC I saw the appropriate segment of the appropriate digit light up. As I continued raising the filament voltage slowly I could see the combined grid and anode current rise from about 5 mA to about 6.5 mA then level off; I expect that's the maximum brightness filament voltage. I'll probably operate somewhat below that.

However, my goal was to put a number to the grid and anode currents. I'd read that, unlike most tubes, a VFD operates with significant grid current and not much anode current. This is indeed what I observed. Each additional anode connected increased the combined current by about 0.5 mA, which puts the grid at about 6 mA. This lines up with application notes I've seen that suggest the combined grid and anode current will be no more than 10 mA.