[Microsoft Surface] SixSixSevenSeven:
The surface uses a 7.4V battery pack. Attempting to connect a 5v supply to charge without a stepup converter will not charge the surface at all and instead lead to damaging your 5v supply as the surface attempts to supply 7.4v to it, which may also damage the surface too.

In electronics nothing is more important than the correct voltage. Take a raspberry pi, many people (including myself) use them for electronics projects. Both me and a friend added ultrasonic rangefinders to our pi's except I observed the fact that the module in question uses 5v signalling and my friend did not. We both connected the +5v and ground lines to the corresponding points on our pi's. I connected 1 GPIO pin of the pi to the trigger pin of the HC-SR04 (module in question) as did friend. The pi uses 3.3V on its GPIO pins, the HC-SR04 is safe with a 2.7v-5v trigger voltage, thats fine. The echo pin however is +5v. I ran my echo pin through a voltage divider to give myself a 3.0v output instead, this is safe for the pi (it is fine with 2.7-3.3). My friend did not, he connected the echo pin direct to the pi, tried to use the sensor for the first time ever and found he didnt get any results, do a few test lighting an LED and he found he could no longer use that pin, he damaged it.

I would be more concerned about the surface damaging the supply than the other way around in this case.

However you are correct in believing the device charges at 12V (2A or 4A depending on whether its an RT or pro charger). The surface would have the correct voltage regulation to charge the battery and supply 5v to USB internally. There are 4 wires within the charger. Red: +12V, Black: Ground, Blue: charger detection, Yellow: signal for the charger LED. I don't think its known *exactly* how the yellow and blue wires function, the surface does charge without them. Blue is rumored to be something to do with the RT charger identifying itself as a 2A device and telling the pro to not draw more than 2A (it draws 4) hence damaging the charger but exactly how it does that is unknown. If the 12V supply can supply more than 4A your golden. A supply capable of higher currents than the device needs

is

actually perfectly safe, current is drawn by the device from the supply, the supply does not force the current at the device. If a device requires 2A and the supply is capable of 10, the supply will still only give out 2 as that is all the device is drawing. A device drawing more than the supply can cope with on the other hand IS DANGEROUS. Lets say you were to use a 5v>12v converter from a computer USB port to charge a surface, a computers USB only supplies half an amp under normal conditions (and by shorting D+ and D- it can do 1.2), the pro trying to draw 4 combined with the inefficiency of 5v-12v stepup converters will cause a current draw so large on the USB port that I would be surprised if damaging the USB port is the only thing to happen, you could fry the entire USB controller easily, damaged USB controllers can then cause their own issues, primarily with this amount of current the possibility of fire.

If you want to charge your surface. Find a voltage source *above* 12V (but not too high above) and use a voltage regulator to make a clean 12V. Then make sure both the regulator and battery can cope with the current. The regulator will determine how far above 12V you are safe to go, some might safely regulate 100V to 12 (most likely a switch mode regulator which although highly efficient won't output a clean 12V, it will have alot of voltage drops and spikes) whereas some might only do 20 to 12 (linear, much less efficient but a very clean 12v, stick a heatsink on it as they get warm). A car battery btw is not 12V, its 12.6 nominal and when fresh off the charger as much as 14.