Re: Power adapter for Hitachi X Mobile 250G USB hard disk

Man-wai Chang to The Door (33600bps) wrote:
This beast, with 2 USB ports (A-B) in its cable, draws a lot of power. I found that most older PCs just can't power it. So I plan to buy a DC power adaptor for it, as this beast has a DC in!

What approximate voltage & current should I look for?


You'd start by looking at the raw drive mechanism power specs.

Unfortunately, some of the drive manufacturers are dishonest, and
won't tell you the spinup current for their 2.5" drives. The current
can be in the 1 ampere range. Normal operating currents (once
the drive is spinning), will be less than that (0.5A or less).
So when a 2.5" drive refuses to work, it could be that it is
having trouble "spinning up".

I would buy a 5V @ 2A adapter, to have plenty of power. If one
was not available, I would buy a 5V @ 1A adapter. Anything with
less current rating than that, probably isn't worth having.

There are some sample specifications here. Only the "start-up" is
a problem. Any parameter below 2.5W is OK on a USB port hosted
right on the PC itself. This one is actually over the 1 amp mark
by a bit.

http://www.hgst.com/internal-drives/mobile/travelstar/travelstar-7k320

Start-up (W, peak, max) 5.5W which is the same as 5V @ 1.1 amps
Read/Write (W, avg) 1.8W
Low power idle (W, avg) 0.8W
Standby (W, avg) 0.2W

Having two power sources, with slightly different 5V values,
isn't really a good idea. I hope the barrel power jack
inside the disk drive enclosure, disconnects the USB bus power path,
when the AC adapter is in place. Otherwise, current could flow
from one 5V supply to the other.

Any adapter rated to provide more current than the "start-up" value,
should be sufficient. Even a 5V @ 1.5A adapter would do.

Paul
.

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