I am running into the issue of getting the undervoltage detection on my Pi 3 B V1.2 while it is connected to an original psu (which provides 5.1V/2.5A).

The psu definetly works as intended (I tested it with watt meter and other Pi's of the same and different model).

I measured some voltages on the board and

  1. confirmed that 5.15 V are applied on the USB connector pins - measured with PP1 and PP5 (See figure down below) . Same goes for C5.
  2. found out that the voltage between PP7 and PP5 is only 4.5V, hence the undervoltage trigger is justified.

During these measurements the Pi was running an idle Octoprint server and consuming about 1.25 W. So I wouldn't assume that some consumer on the board is e.g. shorted and dropping the supply voltage.

My guess is that something in this black square is broken, but I am not quite sure on how to troubleshoot this. Did someone already encounter such a problem? Any help is gladly appreciated. Thanks everyone. :) reference schematic Pi 3B V1.2

  • Welcome. "My guess is that something in this black square is broken" -> If so, I very much doubt you will be able to fix without some pretty serious equipment and professional skills. If you want someone to analyse a schematic you are probably better off pursing that on our larger sibling site, Electrical Engineering. That said, I'll leave this open for now although it really should be closed as a duplicate of raspberrypi.stackexchange.com/questions/133968/…
    – goldilocks
    Dec 20, 2021 at 18:16
  • You say the 'black box' is the original psu as in an "official" power supply? Is that what you're saying?
    – Seamus
    Dec 20, 2021 at 22:23

2 Answers 2


It's either the cable (as the dupe answer states) or poor contacts in the cable, PSU or the Pi. Try a different cable (as short and thick as possible).

If the contacts are poor, inserting and removing the cable several times in the Pi and the PSU may clean them somewhat.


Unlike most such questions you have performed sensible diagnostic tests which almost certainly excludes most of the normal causes (cables or connectors).

There are 2 components which can cause voltage drop - both indicative of some abuse.

The polyfuse should eventually recover given time. The "black square" is a ideal diode (the MOSFET is a low resistance switch controlled by the balanced transistor pair).

You can distinguish between these by measuring on PP35 (or on the polyfuse pads, which are more accessible).

The good news is neither is essential (especially if you are using an official supply); indeed the Pi4 omits both. You can short the ideal diode (or use a Shottky diode if you want reverse voltage protection) and bypass the polyfuse.

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