usb: can you get 5v 2a out of a laptop usb port?

[Gregory Heller]

anyone know? i am wondering if i could power a little fon router off
my macbook usb port.

--
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Gregory Heller
AIM/SKYPE: GregoryHeller
http://www.CivicActions.com

Changing the world one node at a time!

[John Aegard]

According to http://en.wikipedia.org/wiki/USB#Power:

"The USB specification provides a 5 V (volts) supply on a single wire
from which connected USB devices may draw power. The specification
provides for no more than 5.25 V and no less than 4.75 V (5V +/-5%)
between the positive and negative bus power lines.[11] Initially, a
device is only allowed to draw 100 mA. It may request more current from
the upstream device in units of 2 mA up to a maximum of 500 mA."

So it doesn't sound like you can get a full 2A out of a USB line.

I am not a hardware engineer tho.

zo.

[Rogan Creswick]

On Fri, Apr 25, 2008 at 5:04 PM, John Aegard <joh...@sff.net> wrote:
>
> According to http://en.wikipedia.org/wiki/USB#Power:
>
> "The USB specification provides a 5 V (volts) supply on a single wire
> from which connected USB devices may draw power. The specification
> provides for no more than 5.25 V and no less than 4.75 V (5V +/-5%)
> between the positive and negative bus power lines.[11] Initially, a
> device is only allowed to draw 100 mA. It may request more current from
> the upstream device in units of 2 mA up to a maximum of 500 mA."
>
> So it doesn't sound like you can get a full 2A out of a USB line.
>
> I am not a hardware engineer tho.

It scares me a bit how close we came to sending byte-by-byte identical
replies at almost exactly the same time.

I do wonder if you could do it with 2 ports though. (It sounds like 4
ports would do it, but can it be done with fewer if there's a voltage
converter?)

anyway, I'm not a hardware engineer either.

--Rogan

[Jon Dugan]

John Aegard wrote:
> On Fri Apr 25 2008, Gregory Heller wrote:
>> anyone know? i am wondering if i could power a little fon router off
>> my macbook usb port.

As John pointed out the limit is 500 mA. However, are you getting the
2A requirement from the wall wart that comes with the router? My guess
is that it draws less than 2A, but it may or may not be as little as 500
mA.

Do you have a multimeter? It would easy to measure the actual power
consumption. If you'd like I could bring one with me to SH tomorrow and
we can see what it is consuming.

[Dan Vogel]

Voltage converter? 5V 0.5A + 5V 0.5A == 5V 1.0A (max). Ohm's Law
(V=IR) keeps you limited.

The more interesting question is whether you really need 5V 2A and how
compliant your macbook is.

Off the top of my head, I imagine the worst that would happen by
trying it would be that it wouldn't work.
Trouble typically happens when you apply too much current/voltage to a
device, not too little. You've
got the right voltage available, just maybe not enough juice for peak.
Most stuff should just operate at
lower power (ie, a blackberry will charge off USB about 5x slower
without a proper USB driver installed)
or fail to operate (I've had external laptop hard drives that would
only spin up without external power source
on /some/ computers).

But then, I'm not a hardware engineer. I used to build robots, but
I've seen too many puffs of magic
blue smoke from components to be trusted even that far.

-Dan.

[Tanjent Tanjent]

The USB controller is smart enough to shut down a port automatically if you try and pull too much juice from it (and you'll even get a helpful little warning bubble if you're running Windows) - go ahead and try it, you won't break nuthin'

-tanjent