Summary of VFD Filament Driver Using 555
The article describes designing a driver circuit for a FUTABA 13-MT-54NA vacuum fluorescent display (VFD), a 13-digit 7-segment multiplexed display. Unlike LED displays, VFDs require complex driving circuits involving multiple voltages and a floating filament potential to prevent ghosting and ensure uniform illumination. The author built a custom filament driver using a 555 timer configured as an astable oscillator at around 100 kHz, and a center-tapped transformer to generate an AC drive current waveform with a filament bias offset above ground. This approach allows control of the filament illumination needed for proper VFD operation.
Parts used in the VFD Filament Driver Project:
- FUTABA 13-MT-54NA vacuum fluorescent display (VFD)
- 555 timer IC
- Center-tapped pulse transformer
- Resistor 1 kΩ
- Capacitor 4.7 nF
- Diode (across resistor R1)
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.
One 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 astable 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.
For more detail: VFD Filament Driver Using 555
