The premise of this MicroUSB Plug to 5.5/2.1mm Jack adapter is simple. There's a MicroUSB plug on one end and a 5.5mm/2.1mm DC barrel jack with center positive polarity on the other. Since this does not have any resistors or other connections inside for the data lines, this won't work for charging an iPhone, iPod or iPad or other device which expects some sort of resistance on the D+ and D- connections. It does, however, work great for Arduinos, BeagleBones, Raspberry Pis, many generic cellphones & Android devices, as well as most other USB-powered gadgets.
There is no regulator or voltage protection in this adapter cable. If you plug in a 9V or 12V power supply in, you could destroy something expecting only ~5V so be very careful!
Contents and Images from Adafruit