VFD Filament Driver Using 555

I recently salvaged a vacuum fluorescent display (VFD) from a piece of old test gear. The VFD is a 13 digit 7-segment multiplexed display and I thought it would look great in a custom digital clock or something similar. While it has the model number FUTABA 13-MT-54NA, I could not find any information on the internet specifically for this model.
Of course, before I could put this vacuum fluorescent display to use in my final project, I needed to first build a driver circuit to drive this display. Unlike driving a multi-digit 7-segment LED display, driving a VFD is a little bit more complex due to the multiple voltages involved and the relative potential requirement for the filament. Of course there are many specialized chips we could use to build a VFD driver with, but like with many of my previous projects I love to build things from scratch so I thought I would build the filament driver and the VFD driver myself.
VFD Filament Driver Using 555One of the key design considerations for the filament driver is the need to “float” the filament potential above the ground. This offset voltage is necessary to prevent ghosting (see this app note for more detailed explanation). Also in order for the illumination to be uniform, an AC drive current waveform from a center-tapped transformer is desired. A typical filament driving waveform is illustrated below:
Fortunately, we can generate this waveform by biasing the center tap of a pulse transformer above the ground and feeding a square wave through it. The following circuit design shows such a filament driver circuit using a 555 timer and a center-tapped transformer. The 555 timer is configured as an as table oscillator. Given the RC values chosen (1k, 4.7n), the operating frequency is right around 100 kHz. The diode across R1 ensures that the output waveform from pin 3 has a 50% duty cycle.

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