Summary of VFD Filament Driver Using 555
Summary: The author salvaged a FUTABA 13-MT-54NA 13-digit multiplexed VFD and built a custom driver. Key challenges include multiple voltage rails and floating the filament above ground to prevent ghosting. They designed a filament driver using a 555 astable oscillator, diode-forced 50% duty, and a center-tapped pulse transformer to produce the AC waveform with the filament biased above ground.
Parts used in the VFD Filament Driver project:
- FUTABA 13-MT-54NA vacuum fluorescent display (13-digit 7-segment)
- 555 timer IC
- Resistor R1 (1 kΩ)
- Capacitor (4.7 nF)
- Diode across R1
- Center-tapped pulse transformer
- Drive transistors or buffering stage for transformer (implied)
- Power supply capable of providing filament bias voltage
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
- What VFD model was salvaged for the project?
The VFD is FUTABA 13-MT-54NA, a 13 digit 7-segment multiplexed display. - Why must the filament be floated above ground?
The filament is floated above ground to prevent ghosting on the VFD. - What waveform is desired for uniform filament illumination?
An AC drive current waveform from a center-tapped transformer is desired for uniform illumination. - How did the author generate the required filament waveform?
By biasing the center tap of a pulse transformer above ground and feeding a square wave through the transformer. - Which IC is used as the oscillator in the filament driver?
The 555 timer IC is used as an astable oscillator. - What frequency does the 555 oscillator run at with the chosen RC values?
With R1 = 1 kΩ and C = 4.7 nF the operating frequency is around 100 kHz. - How is a 50% duty cycle achieved on the 555 output?
A diode across R1 ensures the output waveform from pin 3 has a 50% duty cycle. - Is a specialized VFD driver IC required for this design?
The author notes specialized chips exist but chose to build the filament and VFD driver circuits from scratch instead.
