Undervolts on Pi
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As far as I can tell, the "Duet powers Pi via ribbon" is not at all sensitive to current. Adding or subtracting 100 or 200mA makes no difference. (I'm sure there is some upper limit, where adding 5 full amps would cause a problem, or whatever. Pundits, hold your tongues regarding edge cases).
This lack of current sensitivity argues strongly against simple resistance based voltage drop. The Zener sounds much more likely.
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If its not current sensitive then it has to be something else? Its not a constant; and given the sort duration and high frequency i doubt temperature is the issue (unless indirectly caused by load fluctuations).
It could well be its not dependent on the load of the PI though.
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The 5V regulator on the Duet 3 has both internal and external 5V outputs, both through didoes.. On version 0.6 and 1.0 boards, the feedback to the regulator takes an average of those two voltages. So the 5V external output (which is the one fed to the Pi) will see about half the difference in the two diode drops, not all of it. This is likely to mean that at low loads you get 5V and at 2.5A load you get 4.9V. There will be additional voltage drop in the ribbon cable and connectors.
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There's been warnings from Octoprint about running Raspberry Pi's with lower rated supplies than the official supplies. I've got everything ready to go now, and was planning on using an external feed for the Pi and screen as the ratings from the duet seem to cut it very fine. My choice, and I know others get away with it.
I wonder it this has anything todo with the short raspberry pi lifespans one of our fellow users ( @spllg ) is seeing?
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Scrub my last question, that specific user has external power to the PI I think.
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@chas2706 said in Undervolts on Pi:
If the cable is not beefy enough to supply the amps that is being pulled then you would soon know about it because it would start a fire!
Not sure how a .2-.6W will cause a fire, the cable will (and usually does) get slightly worm and that's about it. Again, I'm talking about USB cable, not the flat cable on the duet. Replacing cable with a proper one in all cases solved the problem. Looks like here the 2 diodes dc mentioned are the issue. I doubt there's any issue in the flat cable (have not seen those made out of crap, like USB cables I was talking about) but any drop after 5V reg. like a diode can be enough to trigger the voltage warning on the pi.
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Looking at the schematic and @dc42's comment, I don't think it is the diodes since the regulator feedback is taken after these.
I've just tried starting my system up from cold with nothing attached to the Pi (other than the D3, clearly) and I didn't get power warnings at all (can be observed by either the Pi's red power LED staying lit or looking at the journal logs). As soon as I plugged the LCD in, I begin getting occasional under-voltage warnings that tend to last for ~6 seconds before the voltage shows as normalised again. If I then open a chrome tab on the Pi with DWC then the it's under-voltage warning is on pretty much permanently with only the occasional flick off - not long enough to register as normalised in the journal logs.
Having looked at the schematic, I'm now wondering if the bottleneck is that all the power to the SBC (via J48) seems to go through just one jumper - JP4. While I don't know the exact spec of the part used, the internet seems to suggest that these are typically rated at 2A.
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@T3P3Tony said in Undervolts on Pi:
@Danal said in Undervolts on Pi:
I don't have a D3 schematic
2x 5V pins
5x GND pinsTHANKS!!
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@ChrisP said in Undervolts on Pi:
Looking at the schematic and @dc42's comment, I don't think it is the diodes since the regulator feedback is taken after these.
I've just tried starting my system up from cold with nothing attached to the Pi (other than the D3, clearly) and I didn't get power warnings at all (can be observed by either the Pi's red power LED staying lit or looking at the journal logs).
It may be right on the edge... I do get warnings in the above config. Pi 4B+ 4Gig.
As soon as I plugged the LCD in, I begin getting occasional under-voltage warnings that tend to last for ~6 seconds before the voltage shows as normalised again.
I probably should look at the logs more carefully. Eyeball doesn't see any change when I plug in my 7" LCD... but the Mark 1 eye has its limits. I'll do some math, and see if there is more influence than I think.
If I then open a chrome tab on the Pi with DWC then the it's under-voltage warning is on pretty much permanently with only the occasional flick off - not long enough to register as normalised in the journal logs.
Interesting. I'm letting the Chrome/DWC autostart. So my "nothing attached" baseline has this running. Hmmm....
