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jamesjung21 3rd June 2006 08:13 AM

Max current output of USB port...?
Hi :)
Any idea about how much current I can draw up to with USB2...?

I currently have an external 3.5" HDD with external mains PSU, but I want to get rid of that PSU if possible....

Do you think I could achieve this by having two USB connetors, one with Data+power connection and the other with power only..?


sangram 3rd June 2006 09:26 AM

500 mA is the quoted USB spec AFAIK, per port.

The bigger problem is that 3.5" hard disks require a 5V and a 12V connection, at 1 amp each.

2.5" disks are a little more benign in their power requirements, and external cases using those disks don't need an external supply.

mirlo 5th June 2006 04:16 AM

USB devices may draw up to 100 mA before "negotiation" with the host, after which they may draw up to what the host gives them permission to draw, which is limited to 500 mA due to physical interface requirements (USB cables, connectors, etc are designed for no more than 500 mA)

The negotiation requires exchange of information between the devices over the bus. I don't know what the standards for that exchange are.

-- mirlo

Spasticteapot 5th June 2006 06:33 AM

IIRC, many 3.5" drives require 12 volts in addition to the five put out by the USB bus power. I would strongly avoid not using bus power for a 3.5" drive; you're likely to damage something.

Your best bet is to get an extension cable from one of your PC's internal Molex connectors, which are what are used for hard drives internally. Then, get it out to the hard drive, so that it can use your PC's internal PSU.

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