I answer myself because it might help somebody else , I finally found exactly what i needed, the Hama 46685 which can deliver till 2,5 A on 5V an 1,8A on 7,8V and it have the 5,5/2,1 mm plug with the plus inside (and no polarity change so no mistake possible ) and the plug is long enough and curved which is perfect for the Risc modul .
Regards