How do USB devices draw more than 0.5 Amps without damaging USB ports ?
The USB power specs calls for maximum of 500mA=0.5 Amps, or 2.5 Watts at 5 Volts. This used to work for most low-power devices until the rise of bigger and brighter screens and really power-hungry tablets and spartphones.
Device manufacturers have developed a couple of workarounds to determine whether their device is connected to a charger, computer, or an unknown source. This is necessary to avoid pulling more than 0.5 Amps from a data port on a computer/laptop, which risks damaging the power source. Most devices only draw over 0.5 Amps if they determine that they are connected to a dedicated charger.
This is accomplished using two common methods. The first, and most common method for all non-Apple devices is to simply short the two data pins (pin2 and pin3) together in the charger. Since the data pins wouldn't be shorted in a computer, or any USB data port, the device can assume it is connected to a charger, and it is safe to draw over 0.5A.
The second approach, used by Apple does not fully short the data pins, rather applying different voltages to the Data(-) and Data(+) pins, to indicate to the device exactly how much power it can draw. Below is a table of known D+/D- voltages and the exact power that they can draw:
current: pin2(D-) / pin3(D+)
The standard USB pinout is as follows:
pin1 (red) +5VDC (VCC)
2.75VDC can be achieved by connecting a 10KOhm resistor to +5V (pin1, red)
2.00VDC can be achieved by connecting a 10KOhm resistor to Ground (pin4, black)
USB specs and chargers allow for 0.25V variation, i.e. VCC from 4.75 to 5.25V