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Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
What is the max current for a USB port? Is that the max current for all of the ports or for each USB port? What would happen in the event of a short circuit or overload?
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
The answer: It depends, it depends, and it depends.
The maximum current from a single USB 'host' on a mobo is typically 500mA combined across all ports connected to the specific host. A motherboard can have anywhere from 1 to 4 hosts onboard, more if you're adding PCI/PCMCIA/Express card expansion cards.
Many hosts, particularly on the new netbook laptops where power conservation is vital, can accept external power from a 5v adapter. These ports can often source nearly 2A. Some external hubs and expansion cards also support external power supplies.
Particularly in the netbooks (one of the two ports on the HP 2133 for instance) the ports that support external power supplies also do not support the full 500mA output. On my 2133, it cannot drive a device that claims to require 300mA unless you provide the external power supply, while the other non-supplemented port can drive it just fine.
As for failure, the hosts are typically carefully protected and regulated. If you attempt to draw more than the 500mA from the motherboard, the device could see marked voltage drop and either doesn't work at all, or is very unstable. I'm not sure about devices utilizing supplemental power, but I'd imagine they are similarly regulated.
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
I've never seen a 3.5" hard drive powered by the port itself. I've only ever seen 2.5" caddies powered straight from the port.
The rule of thumb when designing a tower is to allow 15w per 3.5" hard drive. 10w is more reasonable, but you're only getting 2.5w out of your USB port. You will definitely require an externally powered USB hub/port, and if you're going to have the external power adapter anyway, why not just hook it straight to the drive so it can go anywhere?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
It is all quite complicated, this is what you are allowed to use within the specifications.
2.5 mA if the bus is suspended. 100 mA if the device is not configured. 500 mA if the device is configured. 1500 mA if both the device and the hub are compliant with the battery charging specifications.
These numbers are conditional so different conditions may render the numbers invalid. As Dave mentioned the hub is free to refuse to deliver more than 100 mA.
You can always use several ports to get the power you need but it hardly seems worth the effort.
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USB 2.0
