USB power capacity

GreatWaves

GreatWaves

GreatWaves

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Joined
9 Dec 2008
Posts
387
Hi,
Ok I know that any one USB port is supposed to have a maximum power supply capacity of 500 mA. And if I understand correctly many or all motherboards have two physically separate USB hubs on the mobo. So first of all, is that right?

The reason I ask is that I need to connect a 2.5" HD via USB to a lappy. The power requirement of the 2.5" HD is exactly 500 mA. The IDE->USB adapter has a USB Y-cable in cases where the power requirement may exceed 500 mA. The Y-cable allows the HD to be connected to two USB ports for 500+500 mA power.

SO, for getting capacity in excess of 500 mA, do I need to make sure to connect the USB Y-cable to the USB ports from two different hubs? Would connecting the cable to two ports on the same hub only give 500 mA instead of 500+500 mA? (again by "hubs" I'm referring to the hubs within the mobo like stated above)

OR, do I totally NOT need to use the Y-cable if the power requirement for the 2.5" HD is specified as 500 mA (the max output of a USB port)?

I need to recover data from the HD in question... which might not end up being possible, because the drive might be corrupted... but need to check.

Thanks
 
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Hmm, good point.
I would expect it to chuck out 500mA, but as you say, I am not sure whether that is per port or per bus. I think it is per port, but I have no way of confirming that.
 
IIRC it's per port.

However a lot of external USB hubs don't have power supplies able to supply it for each port, but built in Hubs on motherboards should be fine :)
 
I have had a few issues with laptop drive caddies and using them directly off of a pc's usb port. They kept dropping out etc, no idea if it was due to the lack of current from the usb ports on my pc but in the end i spent £10 on a powered usb hub and the problem went away. So i assume it was power related or lack there of, this was on several pc's plugged into front and rear ports.

My advice invest a few ££ in a powered usb hub to be on the safe side.
 
Thanks all for input. So I just put the Y-plug ends into two ports next to each other since I didn't have an extension (so probably on same hub). Since no one here said that mobo hubs definately do not ensure 500 mA per port, I figured that 500 mA must be ensured for each individual port on mobo's at least... would make sense.

The 2.5" HD by the way is completely nackered. I knew that it was not able to boot up on the lappy that it used to be in, but thought that maybe the data might be accessable. The thing just excelled at being really scratchy and noisy continuously non-stop... like it's stuck in some sort of infinite loop in it's start up procedure or something.
 
One general point about higher power USB devices - I bought some 'Y' splitters and found they would not work with a drive that worked fine with its own 'Y' cable.

I checked the specs and found the original lead had thicker conductors for the power connections and after calculating the voltage drop for the new cables I realised they would lose more volts than the drive needed to start. :(
 
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