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Subject: RE: V1R running on USB power possible.
From: Stuart Tedford (stuart.tedford_at_piresearch.co.uk)
Date: 2004-09-02


> You have to make sure that you draw a maximum of 500mA on the USB 5V,
> if you draw more you could damage the USB port on your PC, or the USB
> host PC (or hub) might decide to shut this port down to
> prevent damage.
>
> So, it's not just a simple diode.
>
Yeah, in theory. But in practice, most USB ports ignore the standard and
provide 1A or more.

I have an external USB enclosure for a 2.5 inch HDD and out of maybe 10 PC's
I've connected it to, only 2 refused to provide the 1A or so it needs to
spin up. Both of those PCs didn't even provide the spec'd 500mA!!!

The enclosure has a USB cable with a second dummy plug on it on the PC side
so you can connect it to 2 USB ports simultaneously (only one of them
provides the data link, but both provide power) That way it is within the
USB 500mA spec. As a side note, those same PCs refused to power the disk as
well, even with both ports connected! As a second side note, the 2 PC's
that refused to work are both toshiba laptops.

I also have a couple of Compaq laptops, and both of those can supply over
1.3A on a single USB port, I measured it when connecting a CD-ROM drive over
USB.

So I'm not really concerned about the current draw on the USB port, and it
will only draw current when the batteries go below 4.7ish volts, and the
disk will already be spinning which only consumes about 500mA anyway - (it's
only the disk spinup that draws 1A).

Stu.

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