Max current output of USB port...?

jamesjung21 · 3rd June 2006, 07:13 AM

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..?

Cheers,
James

sangram · 3rd June 2006, 08:26 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, 03:16 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, 05:33 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.

3rd June 2006, 07:13 AM	#1
jamesjung21 diyAudio Member Join Date: Dec 2003 Location: New Zealand	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..? Cheers, James

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3rd June 2006, 08:26 AM	#2
sangram diyAudio Member Join Date: Sep 2002 Location: India	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.

5th June 2006, 03:16 AM	#3
mirlo diyAudio Member Join Date: Jul 2002 Location: San Diego	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

5th June 2006, 05:33 AM	#4
Spasticteapot diyAudio Member Join Date: Jan 2006 Location: Wisconsin....what did you expect?	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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