Pull 500mA from USB port

Thread Starter


Joined Jul 24, 2007
Hi, I'm working on a project where I need to get a 5V 500mA power source from the computer. Usually no problem, just connect it to the hard-drive power cables. The problem is I wan't to use my laptop, which only has a couple of USB ports.
Does anyone have any experience in doing this? I know 100mA is easy to get, but I'm probably gonna need all 500mA.


Joined Oct 14, 2005
You need to request the 500mA during USB enumeration. To do so requires a USB device that can be enumerated.

Another possibilities is to share the current across five ports, 100mA each. Just be careful not to exceed this limit, the controller is liable to shut the port down. Especially in a laptop.


Joined Aug 24, 2007
I've read somewhere that the absolute max current you could draw is 500mA.
I agree that using multiple ports is somewhat safe.
If you want to use the 5v for mechanical purposes (fan...) do not do it from the usb port of any laptop, hence seek an alternative power solution.


Joined Aug 27, 2007
Buy a good quality self-powered USB 2.0 HUB. I purchased one for $35 that has 6 ports and a 3.5 A @ 5V switching power supply. 500 ma is no problem.

The Hub will also provide your laptop with an additional layer of protection from any nasty events that may happen in an experimental circuit. Better a smoking Hub than a smoking laptop :)


Joined Jan 17, 2007
The duty cycle is another question. Do you need 500mA all the time? Once upon a time I’ve built a USB-only-powered device that was consuming 1000mA for about 5% of the time, and for the other 95% of the time it was consuming 30mA and charging a NiMH battery pack with 70mA.