This circuit controls a load (in this case a dc brushless fan) based on a temperature compared with a setpoint. The transduced is a diode in the forward polarization regime. In fact when forward biased, the forward voltage drop across a diode has a temperature dependence, in particular has a negative linear slope. This because of the Boltzmann distribution, causing electrons to pass to the conduction band thermally, lowering the voltage drop across the diode.
Anyway this circuit compares 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 setpoint, 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 setpoint is adjusted with the potentiometer, and you can use a LM3914 led driver to make a temperature setpoint 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.
Important Points
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 continuosly).
You can control temperatures up to 140°C (150 max diode temperature), but linearity is not ensured.
Possible uses? Heatsink cooling, computer emergency cooling (but a linear device would be better than a on-off), metal cooling when drilling etc.
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