It’s true that most modern wall wart power adaptors are switch mode supplies, but that doesn’t mean they have perfect regulation. Far from it in fact. To get an idea just how bad some are and how much they vary from brand to brand, have a look at the following article:
The problem is twofold -
Most USB chargers are not designed with good voltage regulation, because it’s not generally required for charging - your Samsung phone doesn’t care whether the 5 volt input has actually dropped to 4.3 volts under full load because it has its own internal regulator and will be charging a 3.7 volt battery anyway. If they can save a few cents by deleting a few components that would have been needed to get better regulation, you can bet they will!
More expensive chargers are not immune to this either - my 12w Apple iPad charger drops to an unusable 4.3 volts trying to run an overclocked Pi model B and won’t even boot my Pi 2.
Even adaptors that are designed as power adaptors rather than “chargers” (different use case) tend to try to cut corners where they can. A power supply with a clean well regulated output takes good design and good quality parts - cutting corners by using cheaper parts, inferior design, or simply leaving out parts that aren’t 100% necessary results in a definitely sub-optimal performance for noise and regulation. Most USB chargers/adaptors are “made to a price”.
Couple that with the fact that the Pi doesn’t have an on-board voltage regulator for it’s 5 volt rail (instead powering USB and network chips directly from the input power) and it’s not surprising that the Pi is very fussy about its power.
In my experience the Pi 2 is the most critical, then the original Pi 1 B and the most forgiving is the Pi 1 B+.
Well summarized, I didn’t want to get into all that technical detail
This is the key, I guess. Not everything with a USB port can be used for charging or as a regulated power supply. That’s what I meant by “decent” - it must be a power supply designed for the purpose of providing a well controlled set of parameters for the output current. An iPad charger will not do, nor will a USB port at the back of a TV. The official power supply is guaranteed to work correctly, I’m also happy with the MMP model. Just don’t forget about that cable
I think part of the issue with the MMP charger is the fact it has a USB socket on the power adaptor and then a cable with a USB plug. That’s handy for trying different cables but I find that on two of mine the the socket is quite loose now, (on any USB plug) and that wriggling it around can cause the low voltage warning to appear.
So a poor connection there may be part of the problem. That’s another reason I like the official Pi adaptor - there is no unnecessary USB socket in the body of the adaptor to wear out and get loose.
Old thread I know, but helpful nonetheless. I was getting the rainbow box momentarily on videos, and as I moved up and down the GUI, on my Upstairs PI - which is a PI2 Model B, 1GB RAM connected to the recommended “ModMyPI” adapter.
Downstairs I have a PI Model B+, 512, which is running on an RS recommended PI adapter.
As a double-blind test I simply swopped the PSUs over to see what would happen. Now neither of them shows the “under-voltage” rainbox box. Go figure eh?
Oh and none of them have anything plugged into the USB.
EDIT: I notice that Dbmandrake had this issue with the ModMyPi charger on the PI2 alone, the same as me.
Voltage drop under load for a charger is somewhat desirable as the internal phone/device regulator will generate less heat dropping the voltage to the 3.* volt level required for most batteries. The Iphone charger is an excellent example, I’m sure the volt drop under load is designed into it so as to generate less heat internally in the idevice
I have rPi2 and rainbow square started appearing few months ago. Nothing changed, same PSU (5V, 2A), same cable.
At first it started appearing if I left it on for longer periods, like overnight. When I turned TV back on it blinked with every action I made. When first it appeared I tried changing few PSU’s, from 1.5A to 3A. Nothing helped. Then I tried few cables, from thin ones to thick ones. Again, nothing changed. In the end I gave up and turned it off. When I turned it on few days later square was gone, again, with same 2A PSU and cable like at the start. First time I left it on overnight square was back and nothing would get rid of it until I left it off for at least overnight. I suspected overheating so I took it out of it’s case (which had open top anyway) and directed fan on it (I have aluminum coolers on CPU and RAM chips). But again, nothing changed.
It intermittently comes and goes, now it doesn’t even have to be left overnight, 20 minutes are enough for it to appear, and when I turn it off, next day it’s away (and comes back after 20 minutes).
I’ve just added “avoid_warnings=1” to config.txt and this removed it. I’m going to leave it at this as at this point I think it’s an defect on rPi2. I’ve just tried USB voltage/amperage checker and at PSU output it shows 5.3V, I’m playing 720p x264 video and amperage is 0.3A. Well below PSU spec. As I mentioned, changing 4-5 cables didn’t help, these were from no name ebay ones to ones bundled with Sony and Samsung mobile phones which I guess are higer quality. Nothing helped.
welshblob thanks for “avoid_warnings=1” suggestion
Be aware that while this hides the square, the turbo mode gets disabled whenever the square would have appeared so may make your Pi seems slower than it should be (running at 600MHz rather than 900MHz). You also may find you get network or usb errors when this is occurring.
You really should use a voltmeter on the Pi itself - it will be dropping below about 4.6V.
You may find you can use the USB voltage checker on a USB port of the Pi (e.g. connect it to a USB stick). That should show the voltage the Pi is seeing after any cable losses.
Good idea, just tried it. At normal usage voltage is 4.93V, when I browse movies and menus it goes down to 4.73. But I’ve ordered this few weeks back so I hope it’ll be powerful enough. Plus it has on/off switch so that’s a plus, no need to mess with the cable.
If you get no adverse effects just disable the warning. What’s going to happen? The warning is about insufficient power not too much. Seriously, I sometimes wonder if people actually want to watch media or are just obsessing about performance for performance’s sake… And the vast majority of people who are obsessing have zero clue anyway. if it works and there are no adverse effects then what are you all whining about? People talking about a multimeter… lol… are you on crack?
A good charger (min 2A) and a good cable should allow the Pi to power a normal 2.5" drive. Your only alternative is a powered usb hub.
And yes, if the drive is not getting enough power OSMC would not recognize it.
You need a voltmeter to be sure, but I’m pretty sure you’ll read less than 4.63V when under load (that is the threshold which triggers the rainbow square).
It could be the PSU or the cable that is the problem. You can confirm with the voltmeter by measuring both at the PSU end and at the Pi end (when under load).
Thank you, @popcornmix for info. I thought it was the current, not voltage.
Voltage was at 4.5, so i’ve adjusted it (PSU has a fine tuner for output voltage) to 5.0V. Rainbow has gone.
