MB6HC + Pi 3B+ low voltage since moving from DuetPi Lite to GUI?

fractalengineer

Hi,

Wanting to install an HDMI screen I've just flashed a new card with DuetPi

I've faced some issues with powering the screen that I've fixed on the bench PSU; mostly the 7" screen pulling a little over the 1.2A limit of the Pi 3B+ USB ports

For that, I disabled the 5V pin on the screen's USB cable with a strip of klapton tape, making the touchscreen work without sending power,

And running a 5V line directly from the pi's 5V pin header to the screen.

This setup worked as expected on the bench, pulling 1.6A total with no voltage alert on the screen.

I was confident that it would be no big deal for the 3A rated 5V rail of my MB6HC,

Unfortunately moving on to it, I'm definitely getting low voltage alerts.

And not only with the screen on, but even with no accessory attached, accessing the pi through VNC

Could the GUI DuetPi extra current draw overwhelm the Duet regulator?

Or perhaps is it not even a new issue, and my previous Lite install has been running voltage throttled the whole time?

Why can't the MB6HC supply the pi alone, pulling only 0.6A, 5x less than the rated capacity?

Any tips on diagnosing this much appreciated!

jay_s_uk

@fractalengineer the pi likes 5.2v and that's where it struggles I think. Ultimately you're best of powering the pi separately

dc42

@fractalengineer the 3.3V regulator on the RPi isn't a low dropout one, which is why PSUs sold as suitable for RPi are designed to output 5.2V, not 5V. The ribbon cable and connectors between the Duet and the RPi are not designed for carrying high currents, so they have significant voltage drop. All this means that if you add significant load to the RPi, you need to use a separate 5V PSU.

Since the RPi 4 came out, we no longer recommend powering the Pi from the Duet.

fractalengineer

@dc42 Interesting.

But sad, would you call 1.2A a significant load for a 3A rated rail?

What would be the least janky way to power up this pup then?

I can't see myself putting a wall socket in the cabinet just to plug in an official pi wall plug then wiring the pi by usb...

Likewise, the GPIO is fully taken by the plug; not practical using a 24 to 5.2V adapter either; unless soldering power directly to the B side of the Pi?

Lastly, would it be possible to drop-in replace the 5V pi buck converter IC by a suitable, pin compatible unit set to 5.2V? I can do board level work alright

dc42

@fractalengineer I think you will find that the Pi plus screen draw rather more than 1.2A. The Duet 5V rail may well be at 5V but the voltage at the Pi may be somewhat less because of the voltage drop in the ribbon cable.

You could try running 5V and ground wires from one of the IO port connectors on the Duet to the Pi USB power input.