[TriLUG] Raspberry Pi Power Supply

[Pete Soper]

On 03/13/2013 03:04 PM, Grawburg wrote:
> Anyone see a problem with using a 5V x 2A power supply (from my Lenovo pad) for the R.P?
>
>
> Brian Grawburg


This area is murky, to say the least, but mostly 'cause there is so much 
lattitude between minimum and maximum requirements and two potential 
power supply sources, plus the USB specs concerning current limits and 
the hardware design changes that Raspberry Pi has undergone. Here's a 
first pass at clarifying the situation.

I'm going to ignore the lower cost, model A board, 'cause I've never 
seen one and believe us fat rich Americans who want to use that board 
will know what we're doing and this blurb will be irrelevant by the time 
model A finds its niche.

The RPI spec calls for five volts at 7/10 amperes via the microUSB for 
normal operation. If you aren't actually using HDMI and/or wired 
ethernet presumably you can get by with less, but most supplies come in 
increments of 1/2 amp, so pick the one amp one.

Mea Culpa, I misinterpreted the September, 2012 "removing the USB fuse" 
temporary ECO as if it implied one could just push bags of current into 
the post-ECO RPIs made from 9/2012 onward, but that is *mistaken*.

What they did was replace the fuse with one having a larger value. So 
current RPIs have a self-resetting (PTC) fuse that limits current flow 
from the microUSB connector to less than one ampere, and opens at 1.1 
ampere. This means feeding more than a one ampere supply into the 
microUSB connector is a waste of money and effort. That is, if a passive 
USB hub with a bag of peripherals connected to it suck so much current 
that the microUSB-supplied supply is asked to provide a bit more than an 
ampere, the "lights will go out", causing momentary shutdown until the 
load is reduced, the fuse cools off and closes again.

Although the RPI designers suggest that an RPI can be run off a powered 
USB hub this flat out does not work with a relatively new rev 2 RPI I 
have. To repeat, I cannot make an RPI work with just a powered hub: a 
power supply connected via the microUSB is required where I live. Maybe 
there are special powered hubs that back-feed greater than the 1/2 amp 
per socket that is typical for the USB spec?

Anyway, as I've preached for many moons, a powered USB hub is your 
friend. If you're running a USB wireless adapter, webcam, USB hard 
drive, yada yada with Raspberry Pi, USE A POWERED HUB.

OK, as I said this is a first cut based on what I sussed out at 
elinux.org and raspberrypi.org.

Sorry for the cross-post, but RPI discussion seems to splob all over the 
area lists these days.

-Pete