I've to thank some instructables members (dog digger and Skater_j10) for the help their ibles gave me to understand the connections.
Please if you're not familiar with electronics, inform about danger of electricity on some trustworthy sources (a simple but comprehensive one could be www.dummies.com)
Step 1: The original brick
My brick is the third generation model, because is less powerful than the first two, but it's even so very powerful. Indeed you can read from the back side data that it generates 12V DC at 150W, It also gives a secondary 5V 1A line, which is a very pleasant peculiarity.
The 5V line is always on, since it's the standby power source of the console, but the main line is turned on with a small current flowing between two contacts of the plug.
Pay attention that the psu retains high current in his big capacitors for about 10 seconds after disconnection, and probably some current remains also for more time, so please be careful to not touch circuits with fingers!