Safe USB power?

Thread Starter


Joined Mar 16, 2006
There seems to be a never ending supply of crazy Japanese USB gadgets that do just about everything using sufficient power right from the port. Is there a safe way to get started with using power from a USB port? The last thing I want to do is fry my PC experimenting.

I've searched on the forums and didn't come up with anything. Google really wasn't my friend either, but I could just be looking in the wrong places.


Joined Oct 14, 2005
The standard says that maximum one unit load (100mA) can be drawn by general devices. Up to five units load (500mA) is available, but it has to be negotiated first. There is also a maximum and minimum capacitance value between the positive and ground on the device side. If I can remember correctly, the maximum is 10uF and the minimum is 1uF. However, if the bus is suspended, the device can only use 500uA maximum.

Any violation on the maximum current value may result in the power being cut-off.