View Full Version : USB 1.1 - max current output?

Robin S_
08-03-2012, 11:30 PM
What is the maximum current output for a USB 1.1 port? (eg it is 500mA for USB 2.0).


Speedy Gonzales
09-03-2012, 08:27 AM
Look under USB root hubs in device manager (under USB controllers). Click on the power tab

09-03-2012, 09:00 AM
Just FYI
Most of the longish USB cables & extention cables are hopeless for transmitting(wrong word) USB power.
Often you'll get too much of a voltage drop in the longer cable & the USB device wont power up, especially with USB powered external HD's

09-03-2012, 09:18 AM
USB 1.1 and USB2.0 specifications both define a maximum draw of 5 unit loads, with a unit load being 100mA (therefore maximum draw 500mA). USB3.0 defines a unit load as 150mA, and a maximum draw is 6 units loads (900mA).

09-03-2012, 05:03 PM
I had cause to plug in a USB powerd dialup modem today, which was being kept on hand for emergencies.

It almost caused an emergency itself when it went up in smoke.

On the first computer (USB 1) it produced "Power Surge" warnings from the OS when plugged in.

It was a computer known to have flaky responses to USB 2 gear, so I plugged it into a USB 2 computer, and it very quickly roasted itself. My skin still stinks from handling it, despite washing repeatedly.

Pulled it apart, and it was all micro components like a cellphone has. Too tiny and toasted to determine which part failed.

Robin S_
10-03-2012, 12:51 AM
Thanks for the answers and suggestions. Paul.Cov's experience suggests that the modem tried to draw too much current for USB 1.1 and triggered an overload response, whereas USB 2.0 was able to provide enough current to fry it. On the other hand Inphinity's info indicates that both provide up to 500mA. This is interesting as I had assumed that some (? most) laptops were fitted with USB 1.1 ports well after USB 2.0 became standard in desktops because 1.1 presumably used less current and therefore gave better battery life.

10-03-2012, 09:09 AM
Motherboard manufacturers don't seem to limit themselves to the specification, it's often possible to draw much more than 500mA and some even mention this as a feature (my gigabyte board has some kind of blurb about fast charge for usb devices).
I've seen it described on one tech website as motherboards using current limiting circuitry across several usb ports so that 4 ports for example can supply 2A total with individual ports being able to exceed 500mA depending what else is in use across the ports.
It makes sense as individual current limiting circuits would cost more.

I think manufacturers are relying on the devices being plugged in having their own limiting circuitry if required, or simply not drawing too much current to begin with.