External power supply

Hello @nsm,

I integrated the FPAA into a 5U synth panel:

I added two CV converter boards to make use of all inputs (by converting -10V to 10V to 0 to 3V). Now I am about to wire it all up.

Still unsure what is the safest way to power it. I’ve got internal 5V, +12V and -12V rails available. I want to keep the micro USB port free to connect an optional programmer. Can I use the onboard pins for supplying power? But I can see only 5V and -5V. While I could create the -5V by myself, would it be ok if both external power and USB are connected? (with Teensy boards for example, one needs to cut a power line and add diodes to avoid issues between USB and external power)

If not, I would probably have to add a USB power splitter between the USB panel port cable and the ZRNA, so that USB power is supplied by the internal power bus, while data is coming in via the panel port cable.

Thank you

1 Like

You can put in a little linear voltage regulator on the -12 to get it up to -5 since you’ll probably be drawing vanishingly little current from it.

Yes, but it could still be a problem to connect power both via the pins and USB. Also I’d like to know if I can take 3V from somewhere on the board to trim the outputs of cell 3 and 4 (should someone mistake it for an input) to this maximum voltage using diodes.

That, I cannot answer! @nsm will have to get back to you on that!

Just for the record: I’ve now connected the board by inserting Eurorack 5V instead of the power supplied via the external USB port. For that, I cut off the micro USB plug of an old micro USB cable.

On one side, this plug goes into the ZRNA FPAA board. Its wires I soldered to a micro USB-B breakout, but only the GND, D+, D- wires. GND and the VBUS wires are connected to the Eurorack power lines.

This allows me to connect any device to the breakout for uploading configurations, but the power will only be supplied by the rack. That should be the safest option.

1 Like

This sounds reasonable @mcleinn. Unfortunately the current Zrna design is not as flexible in terms of power as the Akso. I think I’ll most likely reuse the Akso power structure in future FPAA designs. Having full +12/-12 rails available and flexible OR’d pin or connector power input would be great for FPAA use.

Is it possible connect a DC input barrel for mount on panel for external power supply. If it is… Which pins should I use?

Thanks in advance

Looks very clean and professional, cudos! May I ask which CV converter boards you used? is that your design?

There is no dedicated Vin pin in the current version of the FPAA board.

You could probably solder something to the board close to the USB plug, but the much cleaner method is to take any unused (micro) USB cable, cut it apart, and then solder your barrel to power and GND lines (red and black, respectively). It is will be a barrel-micro USB power adapter, you can probably even buy those ready-made.

If you want to use the same cable for updating firmware or sending MIDI, you could leave the two data cables (green/white) connected (just make sure to supply your external power, as the computer will no longer power the FPAA board).

A third variant would be to connect both USB and your power barrel, but you need to put in diodes, so that only one of the supplies will be used at a time. You can find information how to do this on Arduino or Teensy forums.

1 Like

Thank you. I just googled for “scale and offset CV” example circuits (there are many) - quickly drew one of them into EasyEDA and ordered 10 boards from JLCPB. You can use my board, but I am just a hobbyist, not a professional, so there are several things which can be improved. It seems to work, but quite likely there are better designs. And one could make it much smaller, obviously, and combine several of the circuits on the same space. https://oshwlab.com/tobias.f.weihmann/cv-level-converter

Thanks for such a deep explanation. It would be great a Vin pin in future versions.