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The circuit board containing the motors (DC fans) is subject to extreme humidity, possibly condensation. There is a concern of corrosion disabling the fans without actually causing an open circuit (short circuits are inhibited by the power supply). The heater in the system relies on the fans to prevent overheating of the heat sinks, and thus must be protected in the event of increased impedance (not necessarily an open) disabling the fans.
This circuit converts the current through the fan to a voltage at the drain of the PFET. Using a calibration resistor (50 ohm used here) and a 12V DC input, the circuit can be calibrated by sweeping the potentiometer until it turns off with the calibration resistor selected. To calibrate the upper comparator, one motor should be set to 100%, correlating to a stalled motor in reality.
The parallel RC after the NFET at the bottom op amp output allow maintaining a DC (with significant ripple) level sufficient to enable the heater if PWM or DC are present.
There is one other concern: If a fan stalls for some reason (e.g. worn bearings), it will draw *increased* current. Thus we need a window for comparison with an upper bound near the normal operating current of both fans.
Finally, it is important to tune the current monitor's output resistor so that it does not rail (0V or 12V) under normal operation
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