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.

Futaba 13-MT-54NA

Futaba 13-MT-54NA

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:

Filament Voltage Waveform

Filament Voltage Waveform

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.

Filament Driver

Filament Driver

The pulse transformer was hand-wound using a torroid core. Because the operating frequency is relatively high, only a few turns are needed for each winding as a result. In my case, I used 40 turns for the primary winding and two 15~20 turns for the secondary (depending on the voltage required for the filament). Using the values given, the measured RMS voltage across the filament is at around 2.2V.

VFD Filament Driver

VFD Filament Driver

Because the center tap is tied to +5V, the filament potential will stay above ground level at all times and thus preventing electrons reaching the anode when the grid voltage is 0. This in turn eliminates the ghosting effect.

Figuring out the actual pinout of the module is relatively easy. Since I couldn’t find it anywhere, I thought I would share the pinout information here.

13-MT-54NA Pinout

13-MT-54NA Pinout

segments

segments

Also see A DIY Vacuum Fluorescent Display Driver.

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7 Comments

  1. [...] DIY Vacuum Fluorescent Display Driver uses a 555 Timer to drive it. Most of us have used a 7 segment display before and they are very [...]

  2. azog says:

    This is a great post. I have a module that is very similar, if not the exact same part number. It’s been sitting around for a couple years, never knew what to do with it, cause I didn’t know how to drive it. Can you give a bit more info on the coil? I’ve never wound one like this before; would I be able to use an off-the-self component? Do you know the voltage to drive the grids and the segments, are they just +5vdc? Sorry so many questions. I was just excited when I saw this.

    • kwong says:

      I will post the remainder of the driving circuit in a couple of days, so stay tuned :-)

      Regarding the coil, yes, in my case it’s roughly 4:3 (40 turn on primary and 2×15 on secondary). If you are not sure what the driving voltage is, you can play safe by trying 2:1 first, the 2:1 configuration will provide a filament voltage of roughly 1.5V RMS (assuming 50% duty cycle) which is safe for most of the VFDs.

      Anyway, I will post the entire working circuit soon.

  3. Alex Nelson says:

    I’ve had success doing this with a 555 and a capacitor in series with the output. The effective resistance of the capacitor changes with the frequency. And I count 12 digits.

  4. Scott O'Connell says:

    Nice post Kerry. I lucked out and found a supply of LM9022 ICs that I’ve been playing with for various VFD glass modules. I didn’t have the knowledge to calculate the transformer turns. You’ve re-motivated me to learn more about home brew transformers. Thanks!

  5. [...] This DIY Vacuum Fluorescent Display Driver uses a 555 Timer to drive it. Most of us have used a 7 segment display before and they are very simply to power since they are just LEDs and it is very simply to power LEDs. Kerry Wong had a VFD but no driver, he built one using a 555 timer, a hand wound transformer and a hand full of other components. [...]

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