Temperature Triggered DC Fan Speed Controller Circuit

This fan speed controller works by sensing the temperature of the engine and is accordingly used for the triggering. As the temperature increases so does the speed of the fan motor and vice versa.

Operation of the proposed temperature controlled fan may be understood as follows:

The speed of the DC motor alters as the temperature increases which is converted into a proportionately rising voltage and applied between its terminals.

To measure temperature thermistor (R1) to be placed as close as possible to where you want him to sense temperature is used.

In the diagram, one can see that the thermistor (R1) and the resistor (R2) are employed to form a voltage divider network. It is recommended that the value of R2 is around approximately one tenth of the value of R1.

As the temperature thermistor value decreases it causes the transistor Q1 to saturate harder proportionately.

Since the collector of Q1 is connected to the base of Q2, the voltage at the base of Q2 also responds to the above and decreases

The voltage decreases at the base of Q2 which becomes saturated harder, causing the collector-emitter voltage (VCE) to lower thus intensifying the voltage on the collector terminal of the motor.

The maximum speed of the motor will be slightly less than its rated specification.

To add to this, it may not be crucially necessary for precise circuit operation, to know the temperature in order to control engine speed an LED may be used as given in the diagram. This LED will proportionately get brighter as the engine speed increases.

 

Parts List

R1: 15K thermistor
R2: 1.5K
R3: 1K
R4: 47
R5: 680
VR1: preset 22K
C1: 100uF/25V
Q1: 2N2712 (NPN) or equivalent
Q2: BD140 (PNP) or equivalent
D1 LED
M: Motor DC brushed or brushless

Note: The DC motor can be different from a computer motor. Make sure that the current rating of the motor does not exceed the rating of the transistor Q2. (maximum current 1.5 amps). It is recommended not to exceed 1 amp and use sink.

5 thoughts on “Temperature Triggered DC Fan Speed Controller Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. As the temperature thermistor value decreases it causes the transistor Q1 to saturate harder proportionately.

    Since the collector of Q1 is connected to the base of Q2, the voltage at the base of Q2 also responds to the above and decreases

    dear sir , i cannot understand the following line please explain simple words(The voltage decreases at the base of Q2 which becomes saturated harder, causing the collector-emitter voltage (VCE) to lower thus intensifying the voltage on the collector terminal of the motor.)

    thank you sir

    bala

    • Dear bala,

      as R1 decreases T1 conducts more due to more positive voltage through R1, which in turn provides more negative voltage to T2 base and enables it also to conduct more until both the transistor are fully ON

  3. Dear Swagatam,

    In the parts list, VR1 is shown as preset to 22K. Why is this a variable resistor? Also, in order to be able to adjust the initial speed of the fan, would that be done by replacing R4 with a variable resistor and if so, would an appropriate value be 100K with the initial setting to the mid point?

    I want to thank you for providing this wonderful resource to the world. I have seen comments from many countries and I want you to know that your knowledge and willingness to share it is greatly appreciated.

    Pat
    Tigard, Oregon, USA



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