Circuit diagram:
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.
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.
