This is a circuit for controls a load (in this case a dc brushless fan) based on a temperature compared with a set point. The transducer is a diode in the forward polarization regime. In fact when forward biased, the forward voltage drop across a diode has a temperature depends, in particular has a negative linear slope. This is the figure of the circuit;
Anyway this circuit comparator a precise voltage reference (zener) with the forward voltage drop of the diode forward biased with 11mA of current. The comparator is simply a LM158/258/358 working in open-loop mode, the inverting input is connected to the diode sensor, and the non-inverting to the reference voltage. Se when the temperature rises above the set point, the forward voltage drops under the voltage reference and the comparator output is vcc turning on the transistor and so the fan.
Higher power transistor can be substituted for bigger fans, or you can substitute a relay, IGBT, mosfet etc to control higher loads (and higher voltages). The set point is adjusted with the potentiometer, and you can use a LM3914 led driver to make a temperature set point indicator (needs careful calibrations and the use of excel to calculate slope and intercept). Many modifications can be done, but the circuit works very well in its basic form. The comparator can distinguish 10uV differences so approx 0.01°C differences (carefully adjusting the potentiometer can allow to feel body heat from 1/2 cm from the sensor, or feel ambient heat, making to turn the fan on and off continue). You can control temperatures up to 140°C (150 max diode temperature), but linearity is not ensured.
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