I bought a DC-DC adapter so I can charge my Viliv N5 in my car (while the battery lasts quite long, I may need to use the GPS function when the battery is low). The UMPC needs 5V and 3A, so I just bought an adapter instead of making my own (I am terrible at making cases and a switching power supply would probably cost more in components, while a linear power supply would need a big heatsink). The adapter has selectable voltage and a plug suitable for my camera, so I may be able to also use it with the camera and a SLA battery).
In any case, I decided to take it apart to see how it was made. There is a SO8 chip with "SW" or "MS" written on it, depending on how you rotate it. And I saw one thing that I now have to remember when I'm using this adapter. It connects the positive wire straight to the +12V, while it regulates the voltage on the negative wire to make the difference be equal to the number on the selector. So, this means that if I connected my UMPC to this adapter and my tape deck (which I modded to add a line input in addition to the radio and tape) something would blow up, since "ground" for my Viliv would be 7V above "ground" for my tape deck. I would have to use transformers now.
My other, 5V 2A (USB) adapter does this properly, so I can charge my MD player while listening to the music over the car speakers.