power over USB

Dist · 27 Aug 2008 at 17:51

According to wikipedia, USB can provide 5v, and up to 500mA, but how do i work out what this is in Watts?

I ask because for part of my coursework i have to determine what a set of HDDs could be used for, and i need to work out if one of the drives is suitable enough to be powered over USB, or if it would need an external power source.

Ben M · 27 Aug 2008 at 18:07

current x voltage
(500x10^-3) x 5 = 2.5W

HDD drives use around 8-12W, so no is your answer I think.

Dist · 27 Aug 2008 at 18:21

Ben M said:
current x voltage
(500x10^-3) x 5 = 2.5W

HDD drives use around 8-12W, so no is your answer I think.

Thanks for that, it appears the out of the 4 models of HDD in my question, 3 of them are under 2.5w (2 5400rpm drives operate at 2.2w and 2.1w when seeking, which is the max power requirements for them, and a 3600 rpm drive which max power usage is 0.75w when in the process of read/write)

Ben M · 27 Aug 2008 at 18:26

Be aware that it says "up to 500mA" which may not mean that it can provide that at 5V

tigerstyle · 27 Aug 2008 at 18:26

you proably already know this but notebook 2.5" drives can be USB powered

Justintime · 27 Aug 2008 at 18:32

Ben M said:
current x voltage
(500x10^-3) x 5 = 2.5W

HDD drives use around 8-12W, so no is your answer I think.

Or simply 5Vx.5A = 2.5W

BTW i've got a usb hub externally powered by a 11A 5V source, got 8 Laptop hdds via USB enclosures, works a charm