How do I wire the extruder fan on melzi ? the wiki says it should be on all the time but I cant possible connect it directly to my 12V 17A supply right ?
No, devices only draw the current they need. Your nightlight is connected to a 15 amp supply, but it doesn't shine like a floodlight, does it?
200w super conducting cooling fan.... WoW Sorry just kidding. All components have internal resistance that limits their current. In the case of your fan, it may indicate the current that it will draw from your supply. If you are playing with electrics and electronics it would be worth at least watching some online videos about basic electronics AND learning about insulation and circuit protection (fuses) you don't need a fuse for your fan, but it gives you a better understanding of why and how things should be done. Be careful even low voltages can cause fires.