Thermal Fan Controller

The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. This circuit designed to be use for 12V computer fans, as these are now very easy to get cheaply. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch. I used a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.

As can be seen, the circuit needs only a 12V single supply. This can be obtained from a small transformer, which need be rated at no more than 5VA or so. The supply does require regulation for the sensor if you want it to be accurate, but a simple zener regulator is sufficient for normal operation.

All diodes are 1N4004 or similar, and Q1 must be on a small heatsink - or may be mounted to the amplifier chassis. Make sure it is properly insulated, and use thermal grease. Maximum dissipation will be about 2 W, but it will overheat very quickly if there is no heatsink.

R7 has been added to ensure that Q1 turns off when the opamp's output is high. Most opamps can't reach the supply rail, and the voltage is usually about 1V less than the +ve supply. Some opamps may have a lower maximum voltage, and R7 will ensure that Q1 can be turned off completely. The same change has been made to the alternative version shown below.

The temperature is set with VR1. Operate the amp until the normal temperature is reached, then adjust VR1 until the fan starts. Then back off very slowly until the fan stops again. Any increase over the normal temperature will start the fan, and promptly bring the temperature back down again.

You can test the circuit without the amp, using a diode (or diodes) out in the air. Adjust as above, then hold the diode between your fingers - the fan should start up almost immediately, and stop again when you release the diode. Just the heat from your fingers is enough to operate the circuit. I tested the circuit with 3 standard 1N4004 diodes in parallel, and even without device selection I could hold any one of them and make the fan start.

Resistor Values For R5 and R8 
You will need to check the current that your fan draws to calculate the resistance. Connect it to a 12V supply, and measure the current. Your supply voltage must be less than 30V (otherwise see below). Calculate the resistance of R8 with ...

R8 = (+ve -12) / I     Where +ve is your supply voltage, and I is the measured fan current
Select the closest resistor value larger than calculated. You will also need to work out the power:
PR8 = (+ve - 12)² / R8
Now you can work out the value for R5 (1/2W will be OK here) - here, use the next smaller value if an odd resistor value is calculated. The zener current is nominally 10mA, so ...
R5 = (+ve - 10) / 0.01   Where +ve is your supply voltage.

For example, if you have a 25V supply available, and your fan draws 200mA at 12V ...
R8 = (25 - 12) / 0.2     = 13 / 0.2   = 65 Ohms (use 68 Ohms)
PR8 = (25 - 12)² / R8   = 13² / 56   = 169 / 68  = 2.5 Watts (use 5W)
R5 = (25 - 10) / 0.01   = 15 / 0.01   = 1500 Ohms (1k5)
PR5 = (25 - 10)² / 1500   = 225 / 1500   = 0.15W
Having worked these out, you can adapt the circuit to any voltage, as long as it is less than 30V and 12V or more. For higher voltages, see the alternative version in Figure 2 (below). Note that with the 10 Ohm resistor for R8, the actual fan voltage will be about 10V.