Written by: Preeti Das
With the help of this circuit the speed of a fan motor elf adjusts depending on the temperature of a heatsink that is intended to be controlled. Here a standard thermistor device is used as the temperature sensor specified with a resistance value of 10 K at 25 degrees ambient temperature.
The motor to be controlled is powered by the PWM pulses from the IC 555 whose pulse rate cycle goes down from around 34% at room temperature (minimum speed) to 100% (maximum speed) when the temperature has reached a high. These pulses are generated by 555 which is rigged to work as an integrated voltage controlled oscillator circuit. On the control voltage pin 5 a varying voltage is applied determined by the resistance of the thermistor which in turn depends on the temperature generated over the heat sink.
In order to ensure an immediate transfer of temperature, the thermistor must be attached or glued to the heatsink appropriately. The shown 100uF capacitor connected in parallel with the thermistor shorts the supply with pin5 of the IC simulating a high temperature state for a few seconds during power switch ON so that the motor gets an initialization torque and is prevented from getting stalled.
The voltage to the IC 555 is regulated by the zener diode of 9,1V so that it allows the IC to work regardless of the input supply fluctuations.
To adjust the temperature triggering threshold at which the motor may be expected to speed up, you can change the value 2.7K resistor connected to pin 5 of 555 or even use a potentiometer for setting up the same.