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While the quick start guide says not to power the device from a USB hub or from a computer, the only requirements for power listed are:

Micro USB power supply – make sure you use a good quality one, capable of providing at least 700mA at 5V

When trying to power the device from a non-dedicated source, how can I make sure it provides a correct amount of power to the device?

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I'm trying to figure out what the voltage tolerance is, but I'm not having any luck. It seems that it doesn't regulate the power supply, which makes it that much more important to have a range of acceptable voltages. –  AlanSE Jun 12 '12 at 20:32
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4 Answers

It's not clear what you mean by 'non-dedicated'.

Most reasonable power supplies will indicate how much current they can supply on their label - 700mA is the same as 0.7A. More is fine. No label? I'd have concerns about the unit.

The USB spec gives some room for small variations in the voltage which is nominally 5V.

The common pitfall for using a power supply and a separate cable is that some cheap cables use very thin internal wires that are unable to deliver the current required.

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Non-dedicated means the purpose of the power supply isn't exclusively to provide power to the device (like a 5V 700mA AC adapter). –  jandjorgensen Jun 12 '12 at 22:49
    
So, a clarification: the USB port may not have a label since it isn't an adapter. –  jandjorgensen Jun 13 '12 at 1:26
    
In that respect, a USB port is spec'd to provide a minimum of 100mA but can do more on request of the device plugged in, if it is designed to be able to deliver more, you'd have to look at the spec of the USB port/device you are using which in the majority of cases I doubt you'll be able to find easily. –  Nick McCloud Jun 22 '12 at 15:38
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If the power source (dedicated or not) provides 700mA @ 5V, then you're in the clear and should be fine. That's the crucial piece of information.

FWIW, I successfully powered my Pi by connecting it straight to a USB port on a PC (which technically is only rated for 500mA) and had no noticeable issues, but I wouldn't really recommend that (also worth noting I only had a keyboard and mouse attached, no other peripherals.)

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I think as with most consumer electronics, there is some tolerance for voltage error. I think any USB adapter that is providing a properly regulated output will work fine, provided it can supply the necessary current.

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A simple "Power Budget"

Raspberry Pi 1 Amp (at full steam ahead) USB Mouse 100mA USB keyboard 250mA USB Wireless Bluetooth 150mA USB Wireless LAN 300mA optional GPIO additions 300mA (say) ------- total required 2100mA -------

Clearly you can't draw all this through the micro USB power connector that's only rated at 1 Amp and has a 1.1 Amp fuse. The additional power must come from a powered Hub. The above values are only a guide. Work out your own budget based on your devices. Then get a powered hub with a supply big enough to do the job, and some. For those who used to be on BT internet, dig out your old router, I'm fairly sure that had a 5Volt supply rated at over 2 amps.

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While your conclusion is right, your numbers seems to be much to big. For example RaspberryPi by itself (or connected to wired mouse and keyboard) should never take more than 0.7A since it's 1.1A fuse is not really designed to handle more current for long times. –  Krzysztof Adamski Nov 28 '12 at 7:00
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