"JG2U" wrote in message
...
On Wed, 13 Feb 2008 08:35:31 -0500, "Eisboch" wrote:


wrote in message
. ..
On Tue, 12 Feb 2008 21:38:22 -0500, "Eisboch" wrote:

I saw a platform with two fans built into it the other day at Best Buy
or
some store like that. You're supposed to place your laptop on it and
the
fans in the platform are supposed to help cool the computer. But, it's
USB
powered, so whatever energy is used by the two fans is provided by the
computer. Doesn't make sense.

USB is pulling power from the 5 volt rail and that is the biggest
supply in the system. Case fans are usually 12v but they don't really
pull that much power.
Back in the olden days the 5" disk drives were the power hog. Now it
is the CPU, memory and processor on the video card.

True enough, but watts are watts, regardless of which output of the power
supply they come from.

Don't forget that P=ExI, where
P=power
E=voltage
I=current

Since bus powered USB devices are typically limited to about 400mA
(0.4A), that's only 2 watts. Insignificant compared to the typical
capabilities of a PC power supply.

A USB printer uses no power from the USB port. A USB card reader is
typically bus powered.

Well! Just goes to show how little I know about computers. I'll stick to
audio amps.

Eisboch