USB current limits

[konradIC13]

Hello,

From what i know the USB 2.0 has current limit of 500mA and USB 3.0 has limit at 900mA

I want to power from USB a device that requires 1200mA of current. I want to use a USB cable that can be attached to 2x USB to get more current 2x 900mA > 1200mA but i have question.

What will happen when i plug my device that requires 1200mA into 2 USB ports (so the current provided by USBs will be enough) and then like by mistake remove one of the USB cables so ill end up with one USB that can provide 900mA and a device that requires 1200mA? Can it damage my USB port/laptop motherboard or it will just make my device not work since it requires 1200mA?

 

[konradIC13]

Also i heard that if you short together DATA+ and DATA- of USB you can get 2x its current (so 1000mA for USB 2.0 and 1800mA for 3.0) but i dont know if its true. And also if it will work for USB 3.0 if i would use USB 2.0 cable.

 

[KeepItSimpleStupid]

Usually the A side will shut down the port. I have a USB serial server which has a limited amount of power that can be shared between two ports. The manufacturer said one port would shut down. e.g I can draw 1 A from 1 port, or 0.5 A each from both ports.

 

[MrAl]

Hi,

On XP Pro i've gotten messages that say "The USB Port Has Drawn Too Much Current and Will Not Operate Again Until Reset" or something very similar to that. So i suspect that the port gets shut down, but it could vary with the type of motherboard too. MSI for example has detection circuits and i guess others do to, but you might have to find out from the maker of your motherboard.

Another real question is if you try to draw twice the current by using two ports what happens if there is an imbalance and one port tries to deliver more than 900ma. It might shut down. You'd have to be able to ensure that the circuit draws equal power from each port or close to it. That's usually not as simple as connecting them in parallel.