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How USB Charging Works, or How to Avoid Blowing Up Your Smartphone

Edit: Shenzhen Keyuantai Industry Co.,Ltd      Date:Aug 18, 2017

How USB Charging Works, or How to Avoid Blowing Up Your Smartphone

The tech world has finally coalesced around a charging standard, after years of proprietary adapters and ugly wall-wart power supplies. Well, sort of: We’re already seeing some fragmentation in terms of the new USB-C connector, which could eventually replace USB, as well as what is thankfully turning out to be a short-lived obsession Samsung had with larger USB micro-B connectors for its Galaxy line. But aside from that, and with the obvious exception of Apple’s Lightning connector, micro USB has destroyed the industry’s penchant for custom ports.

Ten to 15 years ago, you always had to make sure you had the correct power supply for each of your gadgets. Usually, that power supply wasn’t even labeled. Today, you can charge your phone at your friend’s house, plug your tablet into any computer, and display photos from a digital camera directly on your TV, all thanks to a standardized connector. In its place, though, there’s a new problem: USB power. Not all USB chargers, connectors, and cables are born equal. You’ve probably 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. It turns out there’s a method to all this madness — but first we have to explain how USB power actually works.

New specifications

There are now four USB specifications — USB 1.0, 2.0, 3.0, and 3.1 — in addition to the new USB-C connector. We’ll point out where they significantly differ, but for the most part, we’ll focus on USB 3.0, as it’s the most common. In a USB network, there is one host and one device. In almost every case, your PC is the host, and your smartphone, tablet, or camera is the device. Power always flows from the host to the device, although data can flow in both directions, such as when you copy files back and forth between your computer and your phone.

Okay, now the numbers. A regular USB 1.0 or 2.0 socket has four pins, 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. USB 3.0 ports add an additional row of five pins, so USB 3.0-compatible cables have nine wires. 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); with USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1,500mA (1.5A). USB 3.1 bumps throughput to 10Gbps in what’s called SuperSpeed+ mode, bringing it roughly equivalent with first-generation Thunderbolt. It also supports power draw of 1.5A and 3A over the 5V bus.


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