Coloured block in top-right corner

I’ve just acquired another Raspberry Pi and have just set it up. The resolution I am using is 1280×1024 on a 17" monitor. Strangely, I am getting an occasional display of a tiny coloured block in the top-right corner about 8mm square. This block stays for about 3 seconds and then fades to nothing after changing all its colours slowly. What is the meaning of this please.

after changing all its colours slowly

Raspberry 3? Then it could be the low power alert…

I get the coloured block on my Pi 3, even when using the official power supply. No issues on my Pi 1.

Thanks for that information, the board is an A+ and is wearing a prototype HAT which is drawing 5V from the Pi. The coloured box is in the top right corner though, not the top left as you state. I will look afresh into the power requirements of its HAT. With thanks.

Yes. That’s the under voltage warning. Although I regularly go to GitHub and grab the new firmware, I did rpi-update in Raspbian this morning and there seems to be another addition to the firmware. Instead of the rainbow it’s a yellow lightning bolt with a black outline. Much more self explanatory!

It’s either your power supply, something drawing too much current, or the MicroUSB cable that’s being used to power the Pi.

My Pi 3 has been a headache for undervolt issues since I got it. I kept upping the ante on the power supply until I gave it a 5 amp power supply. It still did it today when I booted Raspbian! It seemed to be momentary spikes in CPU usage and who knows what else. In spite of trying multiple USB cabes with it and currently using a 1’ long cable to keep length short, I still think it may be the cable. To stop it grumbling for now this morning I just connected a leftover 470uF cap to a +5v and gnd pin on the GPIO to take care of the momentary current spikes, or rather fill in for the voltage drops. Seems to be working fine for now.

I really do need to find an actal good Micro USB cable though. It’s my biggest gripe with the Pi. I would have preferred a barrel connector. I could fit one via the GPIO but I’d rather have the protection of the polyfuse.

I get the coloured block on my Pi 3, even when using the official power supply.

I reported this issue to the Pi Foundation several years ago. I guess they’re still shipping cheap PSU’s that don’t give out the specified mA. Voltage droop can also be issue on the Pi, if you suddenly ramp up the horsepower required.

The polyfuse may be the problem. Most people don’t realise that a fuse has to drop a significant voltage in order to work – be it a polyfuse or a hot wire, there must be a voltage drop in order to cause the self-heating that trips or blows it.

The polyfuse may be the problem. Most people don’t realise that a fuse has to drop a significant voltage in order to work – be it a polyfuse or a hot wire, there must be a voltage drop in order to cause the self-heating that trips or blows it.

Yes and no. The higher the current, the lower the voltage because of that effect. It’s not a linear relationship. There is some breathing space even with the Pi 3 unless it has a lot of current being drawn by addons, whether they are USB or GPIO.
I’m not saying there isn’t voltage drop through the polyfuse. It’s just there’s other causes too which can be way more pronounced.

Jon, I have never used an official supply. How much do they actually handle before running into issues? I don’t think I’ve seen a USB power supply that comes close to it’s rated current, unless you count it as outputting some voltage at the rated current for a little while before it goes pop.

In my case I’m using a 5 amp, 5v switching supply intended for use with LED strip lighting. I have it wired to a USB hub which I have modified to stop backfeed (although nothing is plugged into the USB input). I also went through and added caps to each USB port where they had been removed during a previous design revision, and did a proper job soldering everything instead of the usual minimum pass ussed by manufacturers. The Pi Zero runs off it fine with it’s small demand, Other things can hang off the hub not a problem too. The under volt issue that my Pi 3 experiences has nothing to with other devices on the USB hub. The weak link is between my hub and the Pi. I’ve watched the 5v rail with a multimeter via the GPIO connector. Heavy load definitely causes a momentary voltage drop, and less so as heavy load continues.
To me that points to either the USB cable I’m using for power or something internal to the Pi. I see differences in the condition when I swap power USB cables around, but can never alleviate it because I simply can’t find a good Micro USB cable. They are rubbish! A consumable. I periodically have to buy batches of them for the household as they all fail. I try a different type each time. I’ve got a heap of USB Mini cables still going strong. All I’m saying here is they all seem to be horribly underbuilt with too-thin wires which cause a nasty voltage drop, especially during current spikes. My money would be on a conductor in the cable blowing before the polyfuse.
Yes there are bad power supplies, but the USB cables are so borderline in their ability to supply a Pi 3 that it can be a common issue. It’s actually documented elsewhere.