Building a VFD filament driver: Part 1

For my next project I would like to do a big VFD display using Russian IV-25 tubes. This has lead me to read a lot about these tubes and their notorious filaments. Long story short: they are very fiddly and require a low AC voltage which is quite hard to obtain without specific transformers.

Another way to power it is through straight DC; but the problem is that one side of the display ends up being more luminous than the other side.

The third way is to feed it a high frequency square wave floating slightly above the ground level. Ideally it should be ~25Khz or above to avoid the human hearing range. This is the way I have been exploring and I believe I have managed to do quite well! Here’s the method:

  • Generate a 1V DC signal.
  • Generate a 5V square wave (using whatever method you prefer. A 555 timer if you’re old school. I use a Attiny13 because they’re dirt cheap and require no external components).
  • Pass these 2 signals in a non-inverting summing amplifier.
  • Buffer the resulting signal using a BJT power transistor.

Very simple isn’t it? Here’s the schematics:

VFD filament driver schematics
VFD filament driver schematics


I built a quick prototype of this using perf board, SOIC to DIP adapters for both the ATTINY13 and the op-amp in quad package form:

VFD filament driver prototype
VFD filament driver prototype

Note: I use a simple N23904 BJT NPN transistor here instead of a power transistor. A puny BJT can source enough current (less than 100mA) for a single VFD tube so it is good enough for a prototype.

Originally, I used a LM2902DR2G quad Op-Amp from On Semiconductor, a proven design of the LM290 family of operational amplifiers. Here’s the resulting signal:

Note that the signal has been shifted down a bit to better see it. Ground level is where the “1” on the left is. Signal is floating 1V above ground.

At barely 5Khz, the square wave already doesn’t look very square… Clearly the LM290 won’t cut it for this application. It’s good enough to light up the tube though –with a very annoying buzzing electricity sound:

This power supply is looking good! In the next post I’ll discuss op-amp selection!



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