2.6.2 Power Constraints of USB Port

The required power level of USB devices is defined in 100mA increments from 100mA to 500mA. When a USB device is first connected to a computer, it informs the USB Host of its required power requirements. In theory, the actual power used by a USB device should not exceed its self-defined limits.

The USB ports on the Raspberry Pi are designed to withstand loads of at least 100mA each, which is sufficient to operate low-power devices such as mouse and keyboards. However, devices such as Wi-Fi adapter, USB hard drive, and USB pen drive may consume much more power depending on the USB device. Thus, if Raspberry Pi’s own power is insufficient, it is good to use an external USB hub with separate power supply. Of course you can connect a USB device that uses 500mA power directly to the USB port of Raspberry Pi, and operate the USB device by supplying enough power to the Raspberry Pi, but if the power is insufficient, it is difficult to guarantee reliable operation.