The standard USB port, be it USB 1.1 or USB 2.0, delivers 5 volts of electricity and allows your connected device to draw 500 milliampere. As this is standardized among USB devices, any device that can charged via USB is safe to plug in to your computer. However, you need to be more careful when using a USB adapter for a wall outlet.
Certain tablets and smartphones actually require more power than is delivered by the average USB port. You're most likely to encounter this when using an older computer to charge modern devices. However, some computers are capable of delivering more power to devices when necessary. Other high-power devices will charge when connected to a 5-V USB source, but will charge slower than they would via outlet adapter.
While USB ports deliver 5 V and 500 mA by default, newer computers have USB ports that can detect a device that needs more power. According to Dan Frakes of Macworld, most post-2007 Intel Macs can detect when a connected device requires more power and can deliver 12 V and 1200 mA for as long as a high-powered device is connected to the computer. Once disconnected, the port reverts to 5 V.
USB Outlet Adapter
A popular alternative to charging your devices via computer is getting an outlet adapter with a USB port for charging your devices. These are convenient because they work with any USB cable, but they often have different voltages than a USB port; for example, USB outlet chargers from Apple have 12 V instead of the standard 5 V. Check the adapter you're using before using it with your device.
Universal Phone USB Chargers
Phone manufacturers and retailers committed to creating and implementing a universal micro-USB charger for mobile phones in 2012. These chargers will have the same voltage and compatible with multiple devices, regardless of manufacturer. Micro-USB is already used in a number of devices, including Sony's line of handheld PSP devices and the PS3 controllers.
How USB Chargers Work
USB plugs have four pins: two that communicate with your computer and two that supply power to the device. When connected to the computer, your device gets both power and data. Usually this means that you won't be able to use the device unless it's ejected from the computer. When charging, only the power pins are engaged, which allows you to continue using the device while it charges.
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