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5.1-5.2V power supply?

Posted: Sun Sep 16, 2012 3:05 am
by jeff
It seems that the Raspberry Pi requires at least 5.1 volts of power to allow ethernet, keyboard and mice, USB, HDMI, and the SD card to even function correctly, without crashing everything due to low voltage. Is it safe? I need an expert to answer this, preferably Gert van Loo. Thanks! :D

Re: 5.1-5.2V power supply?

Posted: Sun Sep 16, 2012 7:36 am
by obcd
I am not the expert, but I couldn't resist.
The Pi voltage is usualy referenced to as the TP1 - TP2 voltage.
When that voltage lies between 4.85 and 5.25V, your Pi should work normally.
There is no solid evidence that increasing the supply voltage above 4.85V improves stability or usb functionality.
The voltage that comes out of your supply adapter can get lower by 2 main factors:
1. The cable with the micro usb connector. Some have really thin wires with a 2 high resistance.
2. The Pi main polyfuse. Some of them also have a voltage drop between 0.2 - 0.3V
The current needed for connected usb devices also passes this fuse.
So connecting a 500mA usb device can overload that fuse.
To my understanding, they never really 100% recover.
Some 5V power adapters with micro usb connector are only intended to charge a cellphone and deliver a very unstable voltage. Some also can't deliver the current indicated on their label.
That's were most of the Pi supply confusion comes from.
For completeness, you also need to be aware of the fact that hotplugging an usb device can cause a small short voltage dip on the 5V supply rail. This dip can also interfer with the normal operation of the Pi (it can reset due to that.) Some supplies handle the situation of current change better than others. There is no way to know for sure if you will face those issues with your supply and usb devices.

Re: 5.1-5.2V power supply?

Posted: Sun Sep 16, 2012 2:19 pm
by johnbeetem
RasPi is supposed to work with any standard USB power supply, which deliver 5.0V +/- 5%, which is 4.75V to 5.25V. obcd gives a good summary of voltage drops from the Micro USB cable and polyfuse F3. A USB power supply of 5.1V is good so that it drops down to 4.9V or so inside RasPi.

What you actually need depends on your peripherals. Many peripherals use 3.3V parts internally, so RasPi's 5V0 just needs to be high enough to keep their 3.3V regulators happy. My own RasPi works fine down to 4.65V -- below that I start getting HDMI anomalies. However, YMMV as some peripherals may need more.

As long as your peripherals work, having a lower 5V0 is actually an advantage since the linear regulators will run cooler, especially 3.3V regulator RG2.

For more details on RasPi power, I recommend the RasPi Hardware wiki:

Re: 5.1-5.2V power supply?

Posted: Sun Sep 16, 2012 2:29 pm
by mahjongg
There is some truth in the OP's post.
In some cases (using the older rev1 board) with USB polyfuses (F1 & F2) that had previously been tripped, and so had even higher than normal resistance, the USB peripherals stood a better chance of getting a sufficiently high working voltage if the voltage on the PI (between TP1 and TP2) was a bit higher than normal.

But that is about the limit of it.

The PI itself does NOT need anything above 4.8 Volt, (and possible even a lot less) and is still happy with levels up to 5.25V. Power problems almost always existed (pre rev2 boards) because the USB peripherals didn't get enough Volts, not because the PI itself didn't get enough.

Re: 5.1-5.2V power supply?

Posted: Mon Sep 17, 2012 12:56 am
by jeff
So would a 5.25 volt power supply suit peripherals better or would a standard 5 volt supply work better? :?:

Re: 5.1-5.2V power supply?

Posted: Mon Sep 17, 2012 2:26 pm
by pinoccio
As stated in the posts earlier, every usb compliant device should work without any problems at up to 5.25V. If your peripherals draw more current than the PSU can provide, or the board (due to polyfuses etc.) or cable causes a voltage drop - you either have to supply more power, or fix/bypass the cause/s (replace polyfuses, use better cable, stronger PSU, powered hub...).
Check voltage on TP1&TP2. Anything below 4.75V will most probably cause problems, because it will be even less for the usb devices (if you have the board with polyfuses).
So you cannot go wrong here... Once your board has trouble caused by low voltage (like mine), this is the easiest and safe way to go. Just try another -reliable PSU, or make your own ;) And get a solid usb cable.
Here is my temporary/testing construction: I'm running it at 5.25V, but that's what the supply provides, TP1&TP2 say 4.82V. My plan for later is a (homemade) switched PSU with fine-tuned voltage and protection circuitry...