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
If you intend to supply the 12v from a 45v supply there will surely be a lot of dissipation if you had to use the first circuit with 2 transistor. As you can clearly see the original supply voltage is 15v on the schematic.
On the other hand the supply voltage of the second circuit is also 12v but I guess the NE555 is controlling the fan speed by pulse width modulation. This type will handle dissipation in the switching transistor more safely although I can see the voltage feeding it is still quite high.
Using a dropper resistor still has to be quite large in wattage as it will get very hot due to the voltage difference is large.
These are your options
1) You either use a small iron core 12v transformer to supply the fan/s
2) You can tread a couple of turns around the main transformer center core of your smps (if you have space) to provide the 12v-15v needed. I do not know the turns ratio of your transformer but they can be easily calculated according to the primary turns. The wire can be ECW or better still CAT5 solid core insulated wire. This you wind on top of everything and there will not be the need to tape it up, just twist the ends together with enough length to arrive to the place needed to feed the fan control circuit.
Regarding the schematics I think they are copied from circuits of amplifiers and they are surely tested and work fine. I am adding a schematic which is a part of my 2KW smps. This was supplied from an auxiliary winding from the main trafo. It was tested with 3 X 80mm fan motors and can take them without any trouble. The darlington transistor was an old stock of mine but you can substitute it with TIP122 or similar.
Regards Silvio