AC Adapter - Use of USB

Use of USB

The USB connector (and voltage) has emerged as a de facto standard in low-power AC adapters for many portable devices. In addition to serial digital data exchange, the USB standard also provides 5V DC power, up to 500 mA. Numerous accessory gadgets ("USB decorations") were designed to connect to USB only for DC power and not for data interchange. The USB Implementers Forum in March, 2007 released the USB Battery Charging Specification which defines, "...limits as well as detection, control and reporting mechanisms to permit devices to draw current in excess of the USB 2.0 specification for charging ..." Electric fans, lamps, alarms, coffee warmers, battery chargers, and even toys have been designed to tap power from a USB connector. Plug-in adapters equipped with USB receptacles are widely available to convert 120 V AC or 240 V AC power or 12 V DC automotive power to 5 V DC USB power (see photo at right).

Portable USB battery packs deliver USB power from an internal battery, and is also charged by a powered USB port when not in use. Most of these devices are sized to recharge smartphones multiple times on its single charge, and even support higher current demands for faster charging. USB battery packs is a viable alternative to spare batteries for smartphones, which requires a complete shutdown of the device. The trend started with iPhones that made batteries inaccessible to the user without special screw drivers.

There is even a popular DIY Open Hardware USB charger which can be built from a kit.

The trend towards more-compact electronic devices has driven a shift towards the micro-USB connector, which is backward compatible in function to the original USB connector but physically smaller. In 2009, the International Telecommunication Union (ITU) announced support of the Open Mobile Terminal Platform's (OMTP) "Common Charging and Local Data Connectivity" standard which specifies the use of micro-USB receptacles on mobile phones and standard USB receptacles on (common / interchangeable) chargers. The hope is to markedly reduce the profusion of non-interchangeable power adapters needed for each year's new output of mobile phone models.