I'm trying to build myself a network attached storage system using the rasberry pi. I'm planning to use an external drive for storage and the pi as the server. But I want to power everything from one DC jack that I'm going to get from RadioShack. I'm going to use a DC to DC converter to step down the 12v for powering the pi. Now the problem I have is how I should wire the micro USB cable. I looked at the Micro UsSB schematics and it shows only a +5v pin. There's no -5v how does that complete a circuit if I only run a wire into the pi and not have one coming back out?
USB has VCC (+5V) and GND (0V) pins; they are usually red and black respectively but cheap cables can ignore the standards so I'd recommend double checking the connector with a continuity tester before soldering anything. (Note that if it was +5V and -5V the device would be receiving 10V, not 5V - voltage is otherwise known as "potential difference" as it's a measurement of the difference in electrical potential between two points).
The Raspberry Pi is very sensitive to insufficient power and consumes a fair amount of current so make sure it's getting enough power from the power supply by measuring the voltage across TP1 and TP2 - it should be above 4.8V. You may need to add a powered USB hub to your arrangement to reduce the load on the Raspberry Pi.
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