Temperature control fan

Temperature control fan

With this simple circuit, you will be able to control the speed of a DC fan based on the temperature measured by a temperature sensor. It is an ideal add-on to your PC cooling fan to eliminate generated noise. At the request of some users, this simple design allows for precise speed control of a 12V DC fan motor, proportional to temperature. A negative temperature coefficient thermistor (R1) is used as the temperature sensor, driving two directly coupled complementary transistors connected in a DC feedback circuit. An optional circuit is added for remote monitoring of fan operation, allowing for some coarse speed indication via increasing LED brightness.

Note:

R5 must be set to allow the motor to start precisely at the desired temperature.

Any negative temperature coefficient thermistor with a value in the range of 6K8 to 22K can work, as long as the value of R2 is one-tenth of the thermistor value.

R6, R7, and D1 are optional: R7 must be adjusted until the LED emits a faint light just as the motor is running.

part:

R1______________15K @ 20°C Negative temperature coefficient thermistor (see note)

R2_______________1K5 1/4W resistor (see note)

R3______________1K 1/4W resistor

R4_____________270R 1/4W resistor

R5______________22K 1/2W Fine-tuning cermet or carbon film

R6_____________680R 1/4W Resistor (Optional, see note)

R7_____________470R 1/2W Fine-tuning cermet or carbon film (optional, see note)

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 maximum