OK I'm building a fan control which switches between 12 and 7V see picture for rough diagram (sorry about my paint job ) Here is what is very strange. I don't have a spare PSU at the moment so I tap into my server's PSU, when testing. If the computer is running it works great I can switch between speeds (voltages) no problem. If I take out the ATX connector and "start" the PSU by connecting pin 14 & 16 (I'm not sure those are the correct pins the computer is running right now so I can't check) I can run the fans at 12V but if I switch to 7V the PSU dies. After unpluging the PSU and the circut and putting it back together again it runs just fine at 12V but dies every time I try to switch to 7V. Anyone know why this is??