Hello,
I am designing a two digit wristwatch similar to those cool nixie watches you can find on
cathodecorner.com. Since I am quite an electronics newbie (I am more a programmer dude) I wanted to try something different and maybe easier. My idea was to use two LD8025e VFD display instead of the two nixies.
These tubes are very different, they don't need 180V to work, but only 12V. Id like to design a power supply for my watch, and since It's the first time I do something this complex, I would like to share with you my findings and maybe you can guide me a bit through the path.
My objective is to learn as much as possible.
Talking about power absorbed current, the LD8035e have the following specifications:
- Filament: 26mA @ 0.8V
- Segments: 0.85mA @ 12V
- Grid: 70uA @ 12V
Furthermore, I need 3V for the microcontroller.
Now, In this project I have a big restriction, which is the available space. I'd like to use a single battery, and since the microprocessor will be always on and the VFDs are on only on demand, I was thinking that maybe the better idea was to use a 3V battery with a DC/DC step-up controller.
Being the first time I have to design such a "complicate" (for me) power supply, I was thinking to follow this flow:
- Estimate power requirements
- Decide which "topology" of power supply to use
- Search the market for integrated circuits
Is this a good approach? I am now at the step one, and this is my thinking:
Filaments use 26mA@0.8V. I have two filaments, if I put them in series I can say filaments use 26mA@1.6V, which is 0,042W.
Segments use 0.85mA@12V. I have max 7 segments which can be on at the same time (I will use multiplexing). This makes additional 0.00085*12*7 = 0.0714W
Grids use 0.00070A@12V. I have only one grid on at a time. 0,0084 additional watts
I will use a TD62783 8Ch Source driver to drive the segments. The datasheet specifies 0.52W max power consumption in recommended operation conditions.
So for the two VFDs and their driver, I have a total required power of circa 0,642W.
I still have to consider the power needed for:
- Power supply conversion
- microcontroller
- +20% safety factor
Am I thinking more or less right?
Thank you for your support
Enrico