Powering from USB

[Pete Hobson]

Hi all,

Ive been powering my Pi for several days now under fairly heavy use, directly from the USB connector on my NAS drive.

I know your not supposed to do this, but everything seems tickety-boo so far.  Can anyone explain what the danger is of this, is it to the Pi or the device its suckling from, or both?

NB im only using the Pi as a server, it has no display, or USB devices attached, which means power requirements are as minimal as i can make them (I believe).  Network is via ethernet.

[Paul Hayes]

From what I understand, USB devices are meant to draw no more than
500mA, but the PI can end up drawing 700mA without anything other than a
keyboard and moused plugged into it. Obvious this means the usb port you
are drawing the power from is working over capacity, and many usb hosts
refuse to work when this happens. Potentially you can damage the usb
host chip drawing that much power, but I think that's a rarity nowadays.

I've been meaning to ask. Can anyone lend me a raspberry pi from now
until December 22nd?

yours
Paul

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[Josh Emerson]

I can if you pop into where I work.

Studio 2, 28 Kensington Street

Brighton

BN1 4AJ

United Kingdom

Josh Emerson

[Pete Hobson]

Ok.

Im also cross referencing this:

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=2&t=6050

my reading of the above is that the Pi was drawing about 200mA (0.2 amps) in their tests.  Am i reading this  wrong (my knowledge of amps and watts is basic)

I dont have a keyboard or mouse plugged.  And im assuming this combined with the results above places the pi well under the 500mA limit...?

[Paul Hayes]

My reading of that is that it uses about 400mA when actually processing, without keyboard or mouse. Of course the network port will take up current too, so your right on the line.

[Paul Clarke]

I guess power consumption must also depend on clock speed

 

Are you using one of the “allowed” overclocking rates Pete or using factory settings?

 

 

Regards,

Paul.

 

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[Pete Hobson]

deliberately un-overclocked (?) so running at the default speed.  CPU speed in this case is way less important than low power draw.

(FYI - ive noticed that USB3 is rated at 900mA, i dont have those available, but assume they would be perfect)

[OricTosh]

The USB 1.0 HOST (PC, Device, Pi) port will supply 100mA to an attached device by default .
To get 500mA you need to request it from the HOST having a pull up resister in the attached device will make it give 500mA and have a USB 2.0 port.

Pete Would I be thinking that this is how it looks

Mains power > NAS > USB > Pi >

Without knowing the spec/model of the NAS, I'm sure it's a USB 2.0 port so is able to supply the current needed by Pi.

With a Model B rev1 board there was power problems as the USB ports attached to Pi had 100mA ployfuses, which if you tried to draw more or needed more current those fuses would break the circuit and your attached device wouldn't get noticed by the Pi during boot. Most important part as the Pi checks devices attached, then goes I got my keys, cash, head attached, OK i'm ready to run the OS.
So as you can see if the power draw is too much it won't find the keys and have a kernel painic (linux way of saying I just crashed)
On my Pi I can show a prime example of this as my keyboard draws 250mA so if I plug it directly in the Pi it won't boot because of the power draw.

Chris

[robin48gx]

Some advertisers offer power supplies for the pi where they give 1A.
The USB spec is 0.5A from a USB port.
I have seen this problem with the pi, and it is really a problem
that the USB spec did not anticipate people needing more than
2.5W.

[OricTosh]

>> (FYI - ive noticed that USB3 is rated at 900mA, i dont have those available, but assume they would be perfect)

Yes the only thing I seen USB 3.0 is fairly high end Mobo, might be wrong as not bought any sort of computer hardware for a while.
Most devices have USB 2.0 ports which gives you 500mA and I'm even sure that some cheaper devices only have USB 1.0 ports because they are cheap. It pays to read the specification sheet to find out which port you have.
Just to add to the previous answer I see no problems running a Pi of a NAS USB port and can't understand why they would say there is one. Maybe it was just confusion on which host is supplying power.
When USB was first launched Microsoft placed 137 devices in series on a USB Host port with no problems as USB Stranded was designed for.

Chris  

[OricTosh]

>>I have seen this problem with the pi, and it is really a problem
>>that the USB spec did not anticipate people needing more than
>> 2.5W

Don't you mean amps (current) flowing though the device?
Watts is a measure of power such as for example my machine produces 2.5 watts of power in an hour

Also cheap USB devices didn't/don't follow the USB standard current draw of 100mA or 500mA.
This was also the reason why the 100mA ploy-fuses have been removed in a Rev2 Pi (has mounting holes)

Chris

[Pete Hobson]

well powering from a USB2 certainly seems doable, if as i am, your extremely frugal in whats connected.  Reading a bit deeper the 500mA although part of the spec will be implemented in different ways depending on the device, so some motherboards can apparently supply 1000mA without blowing a fuse...  And others may bottle out at 450mA

I can understand the foundations recommendation not to power via USB now, but in my case the risk to reward ratio is good enough for me :)

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