Temperature-controlled 12V dc Fan

Posted in Temperature sensor, on 2015-08-14

Circuit diagram:

Temperature-controlled 12V dc Fan


  • R1______________15K @ 20°C n.t.c. Thermistor (See Notes)
  • R2_______________1K5 1/4W Resistor (See Notes)
  • R3_______________1K 1/4W Resistor
  • R4_____________270R 1/4W Resistor
  • R5______________22K 1/2W Trimmer Cermet or Carbon
  • R6_____________680R 1/4W Resistor (Optional, see Notes)
  • R7_____________470R 1/2W Trimmer Cermet or Carbon (Optional, see Notes)
  • C1_____________100µF 25V Electrolytic Capacitor
  • D1______________LED (Optional, any shape and color, see Notes)
  • Q1____________BC547 45V 100mA NPN Transistor
  • Q2____________BD140 80V 1.5A PNP Transistor
  • M1____________Fan Motor 12V 700mA max.


Requested by some correspondents, this simple design allows an accurate speed control of 12V dc fan motors, proportional to temperature. A n.t.c. Thermistor (R1) is used as temperature sensor, driving two directly coupled complementary transistors wired in a dc feedback circuit. An optional circuitry was added to remotely monitor fan operation and to allow some sort of rough speed indication by means of the increasing brightness of a LED.


  • R5 must be set to allow motor just starting at the desired temperature.
  • Any n.t.c. Thermistor in the 6K8 - 22K range value might work, provided R2 value is one/tenth of Thermistor's value.
  • R6, R7 and D1 are optional: R7 must be adjusted until the LED glows faintly when the motor is just running.

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