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    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.

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    29 Responses to "Temperature Controlled DC Fan"

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    1. Pentaconto says: on September 21, 2010 at 2:11 pm

      This circuit can not work properly because the LED D1. Is not compatible in the circuit diagram the operation of D1 LED with the operation of T1. D1 can be connected to ground (0V) with a serial resistor for the correct operation of D1 LED (when output of IC1 goes high), and the correct base polarization of T1 when output of IC1 goes low.

      Please correct the diagram circuit, and… please, for any circuit, verify the circuit diagram with the explanation of how work the circuit before publish this, for avoid errors in the circuit an/or explanations.

    2. please send the data of this project

    3. Yes agree with Pentaconto, the LED should parallel with 10 ohm/5 watt resistor, or depend on the fan current.

      • Connect Collector of T1 directly to common (-). Insert 1K ohm resistor in series with D2 and connect this combination across the fan motor with D2 cathode toward T1 emitter.

    4. adamaya says: on July 14, 2012 at 8:30 am

      how to set the temperature by voltage regulator?
      then, can u give all detail or data about the project running??

    5. adamaya says: on July 14, 2012 at 8:31 am

      urgent please

    6. Any suggestions on how to wire this with an 12vdc 80w load (i.e. a radiator fan)?

    7. Jim Keith Jim Keith says: on July 23, 2012 at 4:00 pm

      I have not built this one, but this is what I would do to give it more guts for your high current load:
      1. Eliminate LEDs as shown–replace with jumper
      2. Replace T1 with high current PNP Darlington such as 2N6667 (10A) or On Semiconductor BDW47 (15A).
      3. Replace uA741 Op Amp with LM358 (neglect additional, unused section). The LM358 saturates much better to the negative rail.
      4. Add a little positive feedback (approx 1M or so resistor between op amp output and non-inverting input) to make switching more secure and reduce output noise.
      5. Reduce R2 to 47Ω
      6. Add “ON” LED and 1K series resistor across load

    8. i want full detail about this project?
      can someone give me..
      urgent..

    9. Pleas tell me about working of this project

    10. Kuberkoos Kuberkoos says: on January 23, 2014 at 5:31 pm

      What would be the typical hysteresis of this circuit as it stands now?

    11. Muhammad Asim says: on January 31, 2014 at 2:44 pm

      This works GUYS!

    12. gopi sainadh says: on February 8, 2014 at 3:08 pm

      need a working model of this project???

    13. srinivasa says: on February 17, 2014 at 5:36 pm

      I want full data of this project.

    14. ganesh kumar says: on March 8, 2014 at 6:57 am

      please send me the data of this project

    15. lokesh naik says: on March 11, 2014 at 6:46 pm

      this ckt is working perfectly only remove the in4007 in my case its working

    16. lakshmidhar says: on March 13, 2014 at 4:06 pm

      if i were to use 2 fans how should i connect them (series or parallel) to maximise the speed of both fans?

      • Kuberkoos Kuberkoos says: on March 17, 2014 at 6:54 pm

        If the fans are 12v they should be in parallel, but make sure the current they draw is less than what BD140 can handle.

    17. lakshmidhar says: on March 17, 2014 at 4:12 pm

      the input voltage is 12v. what is the input current for the circuit?

    18. Kuberkoos Kuberkoos says: on March 17, 2014 at 6:56 pm

      Current will vary; between “on” and “off” you will have to measure it, because the “on” current is dependent on the wattage of the fan you use

    19. lakshmidhar says: on March 17, 2014 at 8:35 pm

      can i get the part number for the thermistor?

    20. lakshmidhar says: on March 18, 2014 at 8:17 pm

      i’m using two fans each of 12v and 0.25 amps. what should be my input current?

      • Kuberkoos Kuberkoos says: on March 18, 2014 at 9:55 pm

        Your input current will be 2 x 0.25A plus the current drawn by the rest of the circuit. To get an exact number you will have to measure it with the circuit in operation.

    21. lakshmidhar says: on March 21, 2014 at 7:03 pm

      is this circuit working properly?

    22. ephraim says: on March 27, 2014 at 8:25 am

      can i make this with a bigger fan.?
      do i need to change the value of the components if i change the value of the fan? bcause i want to use a bigger fan.

      • Kuberkoos Kuberkoos says: on March 27, 2014 at 8:22 pm

        You could replace the fan with a relay whose coil draws less current than the BD140 can handle, and use this relay’s contacts to drive a much larger fan

    23. IRFAN AKHTAR says: on March 28, 2014 at 4:13 pm

      I want the data in more easy manner.

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