A circuit that automatically makes your cabinet fans running at the lowest speed, while keeping the temperature down:

1) analog circuit, for use with small blowers
I have used this for blowers up to 450mA ( x 12V = 5.4W)
For higher power blowers, the control circuit would generate to much heat itself, so then a digital version should be used.

      12V supply        NTC sensor         blower connection
42 42
I used a Darlington transistor from my scrapbox, and an aluminium pipe from an ancient charger circuit.
The transistor is mounted with a thermal pad to isolate the collector from the heatsink.
Note that the spring will be compressed in the pipe and conducts the heat from the transistor.

The schematics looks like this.

The picture to the right shows the sensor mounted in the airflow of a cabinet blower, where the blower is situated in the top of the cabinet,
so the temperature will be measured correctly when the blower just runs very slowly when the cabinet air is at room temperature.

schematic stigs nas

2) digital circuit, for use with highpower blowers

coming soon