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Subject: Re: power from a USB source. schematics from another website & question...
From: Andrew Jamieson (ajamiesn_at_optusnet.com.au)
Date: 2002-12-03


USB ports can supply up to 500mA, according to spec, but it's not quite as
simple as that (is it ever?). Over 100mA is _supposed_ to require special
enumeration by the host, but the power switches most commonly used are
indiscriminate when it comes to current levels below 500mA, so this is
really moot. Also, the more dodgy USB hosts/hubs use simple PTC fuses to
limit the current, which potentially allow more than 500mA to be passed.

So. 500mA from a USB port is easily obtainable, even if it is not in
compliance with the standard. To power the Archos from this, you would need
to step the 5V available from the USB port to ~9V, which is best done with a
switch-mode power supply. Practically, you could expect to get ~250mA from
a 9V switcher powered from a USB port, which is marginally not enough
according to my measurements of my player whilst charging. You could use 8V
to get a slower charge and have it work I would guess.

BUT. You'd need to buy/build/steal yourself a switchmode power-supply,
which is not quantum physics type hard, but can be troublesome. I would
recommend one of the many National Semiconductor parts if you want to persue
this, they have good application information, and you can get free sample
off the web :)

A

----- Original Message -----
From: "George Michaelson" <ggm_at_apnic.net>
To: <rockbox_at_cool.haxx.se>
Sent: Tuesday, December 03, 2002 5:55 PM
Subject: Re: power from a USB source. schematics from another website &
question...

>
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/960
>
> is a schematic to get a 5v and a 3.3v from a powered USB socket without
> (apparently) ruining the port for datacomms.
>
> Is it plausible that this could be tweaked to give the required volts to
> re-charge a rockbox via its power-in (not by USB, I realize that isn't
really
> on the table right now) because I have empower for my laptop, so can drag
in
> volts from a plane, and if I could feed them back out I'd get the 14+hours
> needed for a trans-pacific flight...
>
> cheers
>
> -George



Page was last modified "Jan 10 2012" The Rockbox Crew
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