Cheers Exo, thats really helpful.
I've though about this a lot, and decided that the regulator is a better idea, but will not allow me to run it full speed. So I thought that maybe i could use a SPDT switch so I can switch between full speed and controlled speed. A la 'diag1.gif'
A couple of problems you might be able to help me with here - on the 'speed controlled supply for fan' connection, if I was in full speed mode, would current flowing back into the speed controll cause any problems? (bearing in mind that the supply for the speed controller would be cut off)
However, I then thought it would be cool to have an LED to tell me if the fan is running or not. Then I though it would be even cooler if the LED lit up a different colour depending if I was in in full speed or speed controlled mode. Hence, I created 'diag2.gif'
However, as well as with the existing problem detailed above, there is another one. When the fan is in speed controlled mode, current from the speed controller would flow back up the full speed connection for the fan, and travel to the LED. Thus when in speed controlled mode, the LED would light both colours, and is using PWM, would pulse into the other colour (this may look cool, but it would be realy annoying). This could be solved with a diode, but I can't afford the voltage drop.
Any suggestions/ideas will be much appreciated.
Thanks again
Tim