A number of USB devices require more power than is permitted by the specifications for a single port. This is a common requirement of external hard and optical disc drives and other devices with motors or lamps. Such devices can be used with an external power supply of adequate rating, which is allowed by the standard, or by means of a dual-input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device. Some external hubs may, in practice, supply more power to USB devices than required by the specification but a standard-compliant device must not depend on this.
Some non-standard USB devices use the 5 V power supply without participating in a proper USB network. These are usually referred to as
USB decorations. The typical example is a USB-powered reading light; fans, mug heaters (though some may include USB hubs
[24]), battery chargers (particularly for mobile telephones), miniature
vacuum cleaners, a miniature Lava Lamp, and even toy missile launchers are available. In most cases, these items contain no digitally based circuitry, and thus are not proper USB devices at all. This can theoretically cause problems with some computers—the USB specification requires that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host's permission, into high-power mode.