Fan controlled by temperature circuit

Temperature Controlled DC Fan

This circuit can cool your heat generating electronic devices by operating a DC fan when the temperature in its vicinity increases above the preset level. Its operation is fully automatic and turns off when the temperature returns normal. It uses a small 12V DC brush less fan used in computers.

Schematic of DC Fan Controlled by Temperature Circuit

Fan controlled by temperature circuit

Note by P Marian: this is an updated version of the old circuit designed by D Mohankumar that didn’t function at all. I had to remove the leds and use different values for R1 and R2 and now it works as you can see in the video below.

Video presentation of the tested circuit

The circuit exploits the property of Thermistor to operate the DC Fan. Thermistor is a kind of temperature dependent resistor and its resistance varies depending on the temperature in its vicinity. There are two types of Thermistors- NTC and PTC.

Negative temperature coefficient (NTC) Thermistor decreases its resistance when the temperature increases while Positive temperature coefficient (PTC) increases its resistance when the temperature increases. Thermistors are bead like resistors available from 100 ohms to 10K or more values. Here a 4.7K NTC Thermistor is used.IC uA 741 is used as a voltage comparator to switch on the DC fan. Its INV input (pin2) gets an adjustable voltage through VR while its Non-INV (pin3) input gets voltage through a potential divider comprising R1 and the Thermistor. Thus the voltage at pin3 depends on the conductivity of the Thermister.

When the temperature is normal (as set by VR), pin3 gets higher voltage than pin2 and makes the output of IC high as indicated by Red LED. This high output keeps T1 off since its base is positive. DC fan remains off in this condition. When the temperature increases above the value set by VR, resistance of Thermister decreases and the voltage at pin3 decreases. As a result, output of IC becomes low to switch on T1.

A small brush less DC fan (one used in computers) turns on to increase the air circulation. When the temperature returns normal, Fan automatically turns off. Diode 1N4007 is necessary to remove back EMF when T1 turns off.


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  • hessah

    Hello sir,
    I need your help please!
    How can I find transfer function for this circuit?
    Please I need help ASAP!!

  • scrern

    is it okay if i will use 2N2907 instead of BD140?

  • Deku10

    I tried measuring the resistance of a DC fan and I got 45kOhms and a little over 1MOhm depending on the way I connected the multi-meter to the fan’s two inputs.

    • Jim Keith

      Brushless DC fans contain active circuitry — a DVM or multimeter cannot measure the proper resistance of such due to the low voltage applied. However, the resistance can be calculated from the load current measurement at rated voltage.

  • dasdsd

    Is it possible to alter the circuit so that we can use a 10k (or some other value) ohm thermistor in the circuit?

    • dasdsd

      Thanks (voltage divider right?), I was also wondering if we need a 500k potentiometer or could we do with 10k or 100k?

    • Jim Keith

      @dasdsd, Yes, simply make R1 equal to the base thermistor resistance.

  • sridhar

    why a 47ohm resistor is connected to base of pnp transistor?

  • College

    Need help at the the project Sir. It seems that 10k NTC Thermistor won’t work for the circuit, because after turning the voltage supply on the fan worked. Or is my circuit connection wrong?

  • tuhin das

    Sir i have a 12v dc fan . when i connect the 12v power supply the switch of the fan gets heated. I have to change the switch for twice .
    But still there is a heating problem.
    Plz upload a MOSFET switch circuit .
    Thank You

  • Kuberkoos

    I need to switch a pump to my solar pool heater on or off when the temp. difference between my pool and the heater outlet is more or less than ± 5ºC.
    My idea is to replace the 500k pot connected to pin 2 with a circuit similar to the one connected to pin 3 with the 4.7k resistor replaced by a trimpot. This should give me the option of setting the difference between the two NTC’s to say 5ºC. Right?????

  • Hamza

    NTC depends upon temeperature but above info says that change the value of VR to operate it………..How NTC depends upon VR rather than temeperature ???

  • Johnny

    Does anyone have a picture of the breadboard? I am having trouble finding where mine is not connecting.
    Thanks in advance!

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