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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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    80 Responses to "Temperature Controlled DC Fan"

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

    5. urgent please

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

    7. 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..

    9. Pleas tell me about working of this project

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

    11. This works GUYS!

    12. need a working model of this project???

    13. I want full data of this project.

    14. please send me the data of this project

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

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

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

    18. 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. can i get the part number for the thermistor?

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

      • 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. is this circuit working properly?

    22. 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 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. I want the data in more easy manner.

    24. hello,1

      someone please send me the detailed explanation about this project. pl its urgent !!!!!

      mail me ASAP

    25. Hi All,
      Interesting discussion, but can anyone tell me what I have to change for proper function with 24V= fan?
      Please also a changed scheme if the original drawing is not correct.

      Thank you all!

      • Kuberkoos says: on October 6, 2014 at 7:41 pm

        Replace the fan with a 12V relay, and use the relay contacts to switch your 24V fan circuit

      • Hi Kuberkoos,Thanks for your reaction.
        Using a relay is not the problem for the fan. But I have 24V= only and not 12V=, so the circuit should be changed or I need for instance an extra resistor to downgrade the 24V to 12V for the electronic circuit.
        Which resistor can I use for downgrading the voltage?

        Thank you, Telebob

      • Kuberkoos says: on October 7, 2014 at 4:40 am

        You need to use a 12v voltage-regulator to obtain 12v from your 24v supply. If the 24v supply is not stable, Google ’12v regulators’ for the necessary application and size of capacitors on the voltage-regulator circuit.
        Hope this helps,

    26. No no no no… You’re all doing it wrong! Seriously guys. They best way to do this is go down to your local DIY store and buy whatever it is your trying to build. Job done.

    27. Please Email me the data information and circuit diagram of a 24VDC automatic fan controlled temprature regulator

    28. Can anyone explain about the working of this new circuit or why the diode 1n4007 is connected in reverse ?
      I have tried this circuit but it is not working in my case..:(

      • Kuberkoos says: on December 5, 2014 at 8:31 pm

        Google “Free Wheeling Diode” for the explanation of the diode connected “in reverse”

      • Thankyou and can you tell me why the circuit is not working..i’m using
        500K pot
        4.7K,47 ohm resistances
        4.7k NTC thermistor(i also have 5k ntc)
        12 V(0.25 A)DC fan
        12 V battery
        I connected the circuit according to schematic given above,heated the thermistor(body temp.),adjusted the POT but the output is still not working.
        Voltage across thermistor is 6.05,by heating thermistor i adjusted it to 5.03 as in the video …
        Sir please tell me where im doing it wrong.

      • Kuberkoos says: on December 6, 2014 at 8:09 pm

        I would measure voltage at terminal 6 of the 741 while adjusting the 500k pot; if it does change from 0v to almost 12v, you have either the transistor or the diode connected wrong.
        If not, the problem lies in the first part of the circuit. Re check all your connections – especially those on the 741.

    29. Thank you for the guidance Sir,i changed the connections with transistor and adjusted the pot,it’s working now.

    30. Hello, I would like to use 2 fans with a 5v power supply and to control the fan speed via the thermistor therefore changing speed by temperature. How would I go about doing this?

    31. During execution of the circuit, overload is shown even when input is 2V. What is the remedy?

      • Kuberkoos says: on January 13, 2015 at 9:05 pm

        You must have something connected incorrectly. Recheck the whole circuit – especially your transistor and diode connections. Also ensure that your fan is not faulty.
        I would disconnect the transistor and switch on; if fine, your problem is in the transistor/fan/diode circuit. If still in over-current, the problem is in the first part of the circuit.

    32. please can you send me the program for the above.

    33. How can i use the 12vdc automatic controller to control a 24vdc brushless fan.

    34. Sir,
      What was the total cost of this project?

    35. Sir,
      Can you send the Data of this project to my email, its urgent

      • Kuberkoos says: on January 27, 2015 at 6:34 pm

        Hi Ravisankar, you have the schematic, as well as a good description of how it works. It is not clear to me what else you need.

    36. Sir,
      . I want deacription about each component that how it works because i am going to project expo on this

      • Kuberkoos says: on January 27, 2015 at 8:43 pm

        There are numerous basic electronic courses freely available on the internet that explain the working of all the components.

    37. can we change the thermistor with ptc 5k ohms??

    38. good day sir, i just want to ask what are the alternative replacement of the 4.7kohms ntc thermistor? because we cant find that specific thermistor that is required in the project… thank you very much!

    39. Good day Sir, what was the connection of the thermistor? Is it okay to use 5k instead of 4.7k NTC thermistor?

    40. Can somebody email me the whole project literature.

    41. hello sir..i did same connection as above,,,but i have used 470k pot instead of 500k,,,,do v have to use 500k pot only??

    42. well i used 10k thermistor instead of 4.7k and isnt working . please help me.

    43. Usually we give a -VCC supply for the terminal 4 of an opamp. But here it is grounded. Can you please explain ?
      Is there any equation for the output of this circuit?
      How do we get a high voltage at terminal 3 when the resistance of NTC increases?

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

    45. 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 ???

    46. 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?????

    47. 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

    48. 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?

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

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

    51. 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 says: on November 6, 2015 at 1:58 pm

        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.

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

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

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