[TriLUG] Soon-to-be new RasPi owner (DC power requirements)

[Pete Soper]

On 07/08/2013 12:10 AM, Brian wrote:
> Hi Gang,
>
> Just polling the list for personal Raspberry Pi anecdotes; I've got a 
> Model B on order (plus a 10" capacitive touchscreen kit from 
> chalk-elec.com) that I'm planning to use for a home automation 
> project, and wanted to hear about peoples' experiences/troubles/cool 
> projects.
Let's talk about troubles for a minute. I think one hazard that has 
tricked and bothered a number of Pi users has to do with DC power 
requirements.

My experience, and that of everybody I know, is with the model B, which 
for a long time has come with 1/2gb of RAM and so uses the most DC power 
of the three basic flavors (current B, original B, model A, in my 
estimated order of power consumption from highest to lowest). All that 
follows is about the current model B.

An easy trap is to think a small power supply like a cell phone charger 
will run both the RPI and USB peripherals. If it's a 1/2 amp charger, it 
may not even reliably run the board itself. At one amp it will power the 
board and may run most flavors of USB keyboard and mouse. Adding an 
unpowered hub and a 3rd item is likely to overtax the power supply. When 
it's oversubscribed the power supply can do a few different things, but 
the overall effect is failure to boot at all, failure to complete 
booting,  or random misbehavior later that may cause a person to become 
superstitious ("Don't buy WIFI Dongle X", or "If you run program Q while 
doing this and standing on your left foot it causes the operating system 
to crash" or "This [existing/new] USB thing I added is defective and I'm 
taking it back to the store".)

Some peripherals really suck current. WIFI dongles and external USB hard 
drivers of the "portable backup" variety can use a lot of current. I 
found that a two ampere power supply was not sufficient to run a 
keyboard, mouse, Seagate 2 1/2" backup drive, and WIFI dongle reliably. 
This of course became evident in the middle of a demo while teaching a 
class at TechShop (Murphy: we can always count on you!)  These days I 
use a used three ampere powered USB hub I got very cheap from Intrex 
along with a one amp supply to power the RPI. However most of you would 
find a two amp hub and one amp board power supply sufficient. I'm just 
finishing wiring up a three amp supply to try out with an unpowered hub.

-Pete

>
> There've been some occasional disturbances on the list regarding 
> what's on- and off- topic lately; if anyone wishes to contribute but 
> feels it's not good material for the group at large, by all means feel 
> free to reply off-list.
>
> Cheers,
> ~Brian