Any interest in VFD tube power supply boards?

[theworldbuilder]

**Not Variable frequency Drives

Hey all,

I'm currently doing some work with Vacuum Fluorescent Display Tubes and after getting sick of repeatedly bread boarding the power supply for them  I decided to layout and order some power supply pcbs.

The board takes ~12V supply and has a 1.25 V - 3 V output for driving the tube filaments and 15 V - 40 V output for driving the anodes and grids.

These boards should be able to power a number of VFD tubes but I have yet to receive and test them

Since I will have a few spare I was wondering if anyone had any interest in buying some populated boards?

If there's any interest in these boards for any use I will post again in the future after I have got a chance to do some testing.

[Rolo]

I like VFD display's, also working on a VFD psu board. Could you give more details, like is the filament DC or AC. Has it a (virtual) center tap for even brightness on longer display's. Show us more!

[theworldbuilder]

To keep things simple the board is based around two MC34063s. So DC only  
One in a buck configuration one in a boost configuration.

Why I chose MC34063s is because they're extremely simple to work with and quiet tolerant to hobbyists (like myself).

It also means that the output range can easily be changed by simply changing one resistor in the feedback voltage divider; so for tubes with 4-5 V filaments I can just change a resistor and increase the voltage range to be able to supply 1.25-5 V.

As I'm sure you can guess this has no virtual center tap although if you can tell me more on how virtual center taps work I would love to learn about it!

I'm currently working with individual IV-11 tubes which would explain why this board is so simple; it's only 66 mm by 30 mm.

It's intended as a more generic kind of board that I can throw into any project where I might use these IV-11s; the next board I will be laying out will be a driver board for these tubes.

[Ian.M]

Unfortunately AC filament drive is a requirement for longer filament multi-digit VFDs.   

Once you have more than a few digits, the voltage drop across a DC filament becomes significant compared to Vg-k so affects the  brightness.  With AC filament drive, either synchronous to the multiplexing frequency, or at a much higher  frequency,  the variations in Vg-k average out over the digit's on time so all digits are of similar brightness.

However the AC drive means you still need to provide a DC return path for the cathode current.  Traditionally that is done with a center-tapped secondary on the filament drive transformer, so that if you hold that center tap at a DC level, that's the DC potential on the center of the filament.   If the filament drive is capacitively coupled, for a virtual center tap, you can put a pair of resistors, typically of about five times the filament hot resistance, in series across the filament and apply the required DC potential to the junction of the two resistors.  However that's equivalent to adding a series cathode resistor of half of one of the CT resistor's value, and if the voltage drop across the cathode resistor is significant compared to the static Vg-k, you are back to uneven brightness, this time varying with the number of segments illuminated in the digit.

Thoughts on how to provide transformerless AC drive, preferably from a supply as low as 5V would be appreciated.   I know I can 'brute force' it using a low pin count MCU, with a PWM to alternating pins driving a small MOSFET H-bridge, capacitively coupled to the filament.   By varying the ON time to dead time ratio, I can even control the effective RMS filament voltage.   However it would be nice to have a single chip solution that doesn't require firmware, so I'm wondering if any of the commonly available push-pull SMPS controllers can be coerced into it, *IF* one can be found with a low enough minimum operating voltage and enough drive current capability.

[Rolo]

I'm experimenting with 8 and 13 digit dotmatrix VFD's at the moment. My PSU "design" (mostly from datasheets) has a  transformerless AC drive, virtual center tap, HV supply. Both HV and Filament can be enabled/disabled with logic control lines. The HV output and Filament power can be adjusted using other resistors on the board. So it adaptable for a different display's. The PSU works on 5V DC.
The board is at the PCB manufacturer at this moment. If there is interest I can post a new topic about this project. First I want to build one, just to see if it works like I expect it to work. Won't hijack this topic further, OP thanks for the details. Keep the VFD's glowing!



 

[Ian.M]

@Rolo: please update this topic with a link when you get your board built and tested.

[Rolo]

Boards arrived today, made one : https://www.eevblog.com/forum/projects/showing-my-vfd-psu/