I've used a ptc thermistor - the sort that changes from low to high
resistance abruptly at a set temperature. Run some current through it so
it self heats, the airflow cools it so it stays below the trip point.
The thermistor is about the size of a match head, so the mechanics are easy.
Many years ago a friend tried a variation on this - putting the sensor
in the output air so it tripped if the air got too hot, whether that was
from lack of flow or excess dissipation.
Steve
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