hi silivo ,thanks for the reply . which is better between the 2 circuits .
i wanted to use ntc 10k but the 12volts i wanted to use +/-45vlts from the amps power supply .
do you have or please draw a simplified schematic using 10k ntc and +/-45vlts for supplying the 12v fans power
Well as you can see from all the schematics that the pass transistor controlling the fan speed need a lot of gain in the base/gate. I guess you can use a fet instead but I am not sure how it would behave. You see a fet needs more voltage in the gate to start saturation. You can use a small transistor like 2N2222 or any small NPN transistor and hook up the TIP41 in the darlington fashion. This will boost the gain at the base of the power transistor. You can build the circuit on a breadboard and try it out. It could be that you may need to change the value of some of the resistors to get the circuit working to your need. Be careful to monitor the temperature range and at what temperature the fan gets the maximum voltage (12v). I guess this should be around 45-50 degrees.