thanks for your answer.
But I think to took the 12V PSU also because its easier to connect the fans directly to the PSU isn't it?
You also write itÄs better to need a fuse. Is it really necessary?
But to modifie the firmware is not necessary? because I will use my own prusa build with completly different firmware.
Do you think I will be able to rebuild it with your 10-EUR-instruction? I am good in soldering :D