Temperature-controlled 12V dc Fan

Parts:

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.

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Comments:

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.

Notes:

• 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.