I am using a Microchip TC648VPA chip which does all the work for us.
You can find the datasheet of this chip here: http://ww1.microchip.com/downloads/en/devicedoc/21448c.pdf
The matérial needed for this setup is as follow:
1x tc648vpa chip
1x 7805 voltage regulator
1x n-channel mosfet
1x 100uF capacitor (polarised)
1x 10uF capacitor (polarised)
1x 1uF capacitor (ceramic)
1x 10nF capacitor (ceramic)
1x 100Kohm NTC resistor
1x 37Kohm resistor OR 1x 50Kohm trim pot
1x fan (12 to 24Vdc)
1x 100nF capacitor (noise absorption on the fan) might vary depending on your fan!
This chip has a fault pin which can be used if you want to, it will go LOW when the temperature mesured is higher than the maximum preset.
If you want to use this feature you will need as well:
1x LED (I use red)
1x 330ohm resistor
1x 1Kohm resistor
1x 2n2222 NPN transistor
These will give you a temperature setting from 25 to 55degrees where the fan will vary from 0 to 100% duty.
Above 100% duty and the fault output will be pulled LOW, turning on our LED in this setup but it could as well be used on a microcontroller input pin etc etc...
In place of the 37Kohm resistor you can use a 50Kohm trim pot, which will allow you to modify the temperature zone at which the fan will work. I added this possibility in my setup but was not using it on this particular one where the pictures were taken.
I will not go too much into the calculations details for the temperature settings as they are very well explained and documented on the datasheet of the chip, but in a few words here is how it works.
The pin 1 of the chip, called Vin, is where your voltage will be inputed to regulate the fan. It has a minimum value of 1.26V and a max value of 2.65V which will be your 100% fan duty.
It is then with your NTC specs and resistors (or pot) that you will set the fan speed wanted accodring to a set temperature. Using the OHM LAW it is fairly straight forward to calculate.
I have made up some little boards, they mesure 38mm by 33mm, you can find the layout used on them in using the schematic which is here as well. It works great and I am even using them in the professionnal world out there at some customers of mine.
You can drive pretty big sized fan, all you have to do is have a MOSFET which can put up with the current and an appropriate capacitor used as a filter to smooth down the perturbations created by the fan.
Feel free to contact me if you would like me to add some infos or answer any questions. I have as well possibility to ship a few boards, complete setup or board alone for a small fee.