Always wanted a temperature controlled fan for you power amplifier etc. Nice and quite while idling but brute force when things get hot under the collar.

Well here’s one done on a budget. Up to 3A output for less than $10 and you’ll have some spare NTCs and zeners to pass around your mates.

Based on a LM2596 DC-DC Switching Regulator (LM2596-DC-DC-Switching-Adjustable-Step-Down-Voltage-Regulator-Converter-3V-40V available on eBay), modified to be controlled by a 10K negative temperature coefficient resistor, 10K-Ohm-Temperature-Sensor-Thermistor from eBay. The thermistor, you’ll need,  is 10K at 25 deg C with a beta of 3435 approx.

Remove the pot and the adjacent 330R SMD resistor and install the circuit in the photo.

The voltage to the fan is about 6V at 25 deg C and provides 12+ V at 50 deg C.

The zener prevents the output voltage going higher than 13.5V when regulator is feed from source greater than 14V. Zener should be low leakage 400/500mW type. If supplying the regulator from 12-13.8 V the zener is not needed.

For 24V Fan from 26V supply, the zener is not required, the resistor is then 68K. I used a 33K across the thermistor to fine tune to 12V at 25 deg C and 24V at 50 deg C.

The NTC should be inserted in the PA heatsink 2.5 mm hole with thermal grease. Sleeve the legs of the NTC with 1mm tubing. NTC can be remote connected using shielded cable. It is better to mount regulator on heatsink and keep leads short.