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Fan Speed Controller Circuit for Heatsink

Fan Speed Controller Circuit for Heatsink

In this post we study about a automatic fan speed regulator circuit for controlling the temperature of a heatsink and from preventing th etemperature to erach dangerous levels. This approach is to ensure safeguardign of the connected devices wit the heatsink.

Written by: Preeti Das

With the help of this circuit the speed of a fan motor self adjusts depending on the temperature of a heatsink that is intended to be controlled.

How it Works

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.

Circuit Diagram

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About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!



21 thoughts on “Fan Speed Controller Circuit for Heatsink”


  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hello sir, i try to gather all the components for this circuit, but was unable to get thermistor 10k, i only find one written 47D, Can it serve same function?
    2. Second question, can i use 5k or 10k preset in place of 2.2k?

    • sorry I am not sure about the value of the 47D you may have ask your nearby component dealer for the details…

      yes 5K perset will do instead of a 2.2K preset.

  3. Meaning that i have to reverse the pin connection right? According to my understanding the motor’s negative will connec to collector and to ground while the emitter to the positive of power supply? Pls correct me if misunderstand u sir.
    Also, should i maintain the base resistor as the 10k or i will change (increase or reduce) value. Pls help. Thanks.

  4. hi bro, i,ve made a circuit for pwm using ic 555 for controlling a dc motor( 4.5v). am able to vary the speed but problem is motor is not starting by itself. Initially motor is making a sound & when i rotate it by hand it starts. am using 1k resr & 10n cap for RC and 100k variable pot. ive even added two diodes in antiparallel. am using tip 31 to drive the motor.
    so can u suggest any ideas for making it self starting?
    and also can i replace transistor with a mosfet(irf 540) for driving.

    • Hi bro, it could be because the lowest PWM is too narrow to provide the required amount of current to the motor, you can try using a IRf540 mosfet or a TIP122 Darlington BJT and see if that improves situation. If not then you may have to add an extra resistor having 1/10th value of the pot value and add it in series with the pot terminal at the zero PWM side. This will restrict the PWM from becoming too much narrow and keep it at a level which may be just sufficient to initiate the motor movement at the lowest PWM

  5. hi swagatam, can i swap the thermistor with an lm35 chip as that is what is easily obtainiable in my country

    • Ho Oladipo, yes you can try it by removing the thermistor and connecting the LM35 +/- pins with the supply and its out with pin#5, and see how it responds

    • you can probably do it by making the 2.7k resistor variable, and adjust it appropriately for achieving the results

  6. Hello. Please, tell us which zener is the correct, the 5.1V from the diagramm or the 9.1V from the text. Thank you, George.

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