USB Power

USB Power

USB devices can be classified as bus-powered or self-powered devices depending on their source of power.

Bus-Powered Devices

USB cables have wires to carry both power and data as shown in this diagram of a USB cable.

Devices that receive power from their USB cable are called bus-powered devices. Bus-powered devices are classified as low-powered or high-powered devices depending on the amount of power they draw from the USB bus.

Device Type	Power Requirement (milliamps)
Low-powered	100 mA or less
High-powered	Between 100 and 500 mA

Self-Powered Devices

Devices that rely on their own power supply (in other words, you plug them into an AC outlet) are called self-powered devices. All devices that draw more than 500 mA of power are required to be self-powered.

You should know the following facts about USB power:

• Like USB devices, USB hubs can be bus-powered or self-powered. You cannot connect high-powered devices to a bus-powered hub (you can only connect low-powered or self-powered devices to a bus-powered hub). Therefore, self-powered hubs that provide 500 mA per port are recommended to ensure an adequate power supply to all bus-powered devices that you may wish to connect to the hub.
• Some USB devices have the option to be bus-powered or self-powered.