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Re: Powerbook 12" USB power

On Mon, 12 Sep 2005 11:04:25 +0200
"Federico 'Pain' Pistono" <fppain@gmail.com> wrote:

> For some reasons my 12" Powerbook does not provide enough energy to
> the USB device.
> I have an external hard-drive, 80 GB USB 2.0, that has two connecting
> cables. It usually works fine with just the connection cable on a
> desktop with a USB 2, but if you have only a USB 1 or you have a
> particular situation you can use the other cable as well, its purpose
> is solely to provide energy when needed.
> When I first used the powerbook on OSX it worked fine with just a
> cable, and the same with Linux. Now it's gradually getting worse, and
> I can't even transfer decently any file. I am sure thr USB 2 works and
> it's configured, since if I plug the energy cable into another
> computer and the connecting cable in my 12" it works fine, meaning
> that I just nedd to provide more energy from my USB ports.

Be carefull with that. USB is specified for 5V/500mA = 2.5W per
Port/Hub. Most hard disks, even the 2.5" Laptop ones, use more power so
you may damage your laptop if you use an external hard drive without
external power supply. Read the datasheet of your hard drive carefully.

Some time ago I replaced my internal hard drive with a bigger 40GB one.
This hard disk has a nominal power consumption of 2.5W (USB limits) but
needs 5W during power up sequence. I put the old one in an external USB
housing. This hard disk has 2.5W nominal power consumtion and also 2.5W
during power up sequence, but nevertheless I don't have the courage to
use it without external power supply at my PowerBook.

I don't know what the manufacturer use to limit the current at USB
ports, but a famous german computer magazin wrote that they mostly
don't use melting fuses. I could imagine that they use polyswitches.
This are reversable fuses. The current through the polymer heats the
material and at a certain temperature it changes its crystal structure
and therefore its resistance rapidly. After some time the polymer
recovers and returns to low resistance state. 

The problem with polyswitches is that they don't recover completely to
their former performance. Furthermore heat and current pulses increase
the resistance and they grew old fast. This would match the behaviour
you observed. The current out of your USB port decreases and would not
be able to power the hard drive any longer.

> I was told that I had to configure this through the open firmware, but
> I have no idea of how to do it.

I don't think this problem could be solved by tweaking open firmware.

 Best Regards

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