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How USB charging works, or how to avoid blowing up your smartphone

25 January 2012

Amazing strides have been made in recent years towards standardized chargers and connectors for smartphones, tablets, MP3 players, and many other gadget species. In just five years — with the obvious exception of Apple’s iOS connector — Micro-USB has single-handedly destroyed the industry’s penchant for wildly customized and proprietary connectors. Today, you can charge your phone at your friend’s house, plug your Kindle into any computer, and download photos from a digital camera directly to your TV, all thanks to a standardized connector.

In its place, though, another problem has arisen: USB power. Not all USB chargers, connectors, and cables are born equal. You might’ve noticed that some wall chargers are stronger than others. Sometimes, one USB socket on a laptop is seemingly more powerful than the other. On some desktop PCs, even when they’re turned off, you can charge your smartphone via a USB socket.

Believe it or not, there’s a method in all this madness — but first we have to explain how USB power actually works.

The Specification

There are three USB specifications — USB 1.0, 2.0, and 3.0 — but we’ll be focusing on USB 2.0, as it’s by far the most common variant. We’ll point out where 1.0 and 3.0 are significantly different. The other important fact is that in any USB network, there is one host and one device. In almost every case, your PC is the host, and your smartphone/tablet/camera is the device. Power always flows from the host to the device, but data can flow in both directions.

OK, now the numbers. A USB socket has four pins and and a USB cable has four wires. The inside pins carry data (D+ and D-), and the outside pins provide a 5-volt power supply. In terms of actual current (milliamps or mA), there are three kinds of USB port dictated by the current specs: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two can be found on your computer (and should be labeled as such), and the third kind applies to “dumb” wall chargers. In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); in USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500mA (1.5A).

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