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Bus powered USB Hard Drive power consumption

G

Goi

Diamond Member
Hi,
AFAIK a USB port supplies up to 100mA to a device or 500mA to a hub. However, 2.5" HDDs already consume up to 1A or more. So, I'm wondering how bus powered USB drive enclosures manage power? Most of the ones I've seen have a USB connector that splits into 2 plugs, which I assume draws twice the current from 2 ports. Does this equate to 200mA or 1A? Most also have an optional power plug for self-powered mode. Is plugging in both USB plugs into 2 separate ports required or is it merely a "nice to have"? If only 1 plug is needed, again, how do such enclosures manage to supply itself and the HDD with enough power with just a 100/500mA current draw from a USB port?
 
My 4-port HP USB laptop Hub provides 5 VDC at 3 amps. My external 2.5" Firewire drive is happy with 5 VDC and 2.6 amps. It won't run off of the USB hub unless I connect two cables.
 
A USB host will supply up to 500 mA per port to any client device, regardless of whether it is a hub or not. Some hosts will let the client draw more, up to a point, even though it's out of spec.

Similarly, the host is supposed to set a lower limit (it may be 100 mA, I'd have to look it up) and then increase it if the client requests a higher limit, but many hosts do not enforce this.
 
Originally posted by: Goi
Well, so I guess most bus powered 2.5" USB enclosures are running out of spec?
Click to expand...

That is why I prefer Firewire. It also has a faster sustained transfer rate.

 
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