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When running X and when it's not content that it can draw enough power from the adapter, my RPi displays an RGB colour square on the screen. (At least, I THINK that's what the RGB colour square indicates.)

How does it sense that there's not enough juice available?

It doesn't die or reboot: it seems to be able to sense when there's not enough current available. Is it sensing a voltage drop in the supply, for example?

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    If you search you will find a more detailed answer. If the voltage drops below 4.65V the warning is shown. – joan Apr 26 '16 at 21:14
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    Check out this question. It's not shown in the circuit diagram in my answer to that question, but I think the output of the APX803 comparator IC is connected to a GPIO input on the CPU. It's also hard-wired to the red LED which makes it blink when the voltage drops (and the voltage drops as a result of high current draw). – jDo Apr 27 '16 at 14:55
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The Pi (at least all since B+) has a power monitor - the APX803 chip triggers if voltage falls below 4.63±0.07V. This drives the power LED and is connected to a pin on the chip.

In fact the Pi will run at lower voltages as it requires 3.3V and has an on-board regulator. The lower voltage may impact peripherals.

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  • Many thanks. That's bothersome. I was hoping to tap into it to try to calculate the power being drawn for the max available, but it rather looks like it's a fixed hardware trigger. – KDM Apr 27 '16 at 21:20

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