Automatic Temperature Controlled Fan Circuit using Thermistor

This is a simple automatic temperature controlled fan circuit using NTC thermistor. The main components of this temp control fan circuit is a negative temperature coefficient thermistor (NTC) which detects the temperature, a comparator which is used to compare the preset temperature value and a relay circuit to drive the fan. Power supply for this circuit is given by a step down transformer and a bridge rectifier. A filtering capacitor is used to get perfect DC. We will be posting a circuit on how to control fan speed as speed control of fan is not discussed in this thermistor applications circuit.

Circuit diagram of Automatic Temperature control Fan

                                               Click on the circuit for enlarged view

Components required

  1. IC LM324
  2. Transistor BC 548
  3. Diode 1N4007 (4 Nos)
  4. Resistors (1K 2Nos,4.7K)
  5. Variable resistor(10K 2Nos)
  6. Thermistor NTC
  7. Capacitor (1000uF/16V)
  8. Relay 12V
  9. Transformer 230V/0-12,500mA
  10. FAN 230V

Working of Automatic Temperature Controlled Fan

  • Step down transformer reduces 230V AC into 12V AC. It is then converted into DC by a bridge rectifier circuit. A capacitor (1000uF) is used to avoid the unwanted AC ripples.
  • IC LM324 has four comparators each having an inverting and non inverting terminal. Here we use the first comparator, inverting terminal of which is connected to a reference voltage made by two voltage divider resistors R1 ,R2 and a variable resistor RV2.
  • Non-inverting terminal is connected to thermistor and resistor RV1. The temperature at which fan becomes ON is set by the variable resistor RV1.
  • When there is a temperature increase, thermistor resistance will decrease as it is a negative temperature coefficient NTC thermistor. Then the voltage drop across RV1 is increased and if it becomes greater than the voltage on the inverting terminal of the comparator, then it output will go high(+Vsat)
  • The high comparator output will turn ON the transistor and relay will switch ON the fan as phase of AC supply is connected to the NO (Normally Open) connection of the relay.
  • When transistor is ON, the resistors R1 and RV2 become parallel decreasing the voltage drop(reference voltage). Hence fan will not be OFF though fan ON time temperature is reached. This difference of temperature can be varied by using the variable resistor RV2.

Components pin out

33 thoughts on “Automatic Temperature Controlled Fan Circuit using Thermistor

  1. Its a nice circuit but i can’t understand that “when thermistr resistens decares who voltage drops accros RV1 is incrased???”and i can’t understand RV2’s duty.

    • Hi Ali Sher
      1. When Thermistor resistance decease then current through thermistor and RV1 will this make more voltage drop across RV1.
      2. Once fan is ON then it will not OFF when the temperature decrease from threshold(it may make oscillation of relay), it OFF when temperature reach a lower level, this difference can be achieved by making RV2 parallel with RV1.

    • Hi punith,
      Hope I explained working of this circuit in detail. Due to the large volume of emails and comments I didn’t get time to support projects. As a professional I hope you may understand my situation. Please make a Google search for the datasheet of components. You will get more information form the datasheets of components.

  2. Hi sir, will the fan’s speed increase when the temperature is high. Then it will decrease as well when the temperature is low right? Will these be a good basic example of a closed loop control system?

    • And can u please elaborate more about the functions of P1 and P2..

      If i purchase a thermistor ntc.. doesn’t it have any value?

      thanks! regards

    • Hi Tenzin
      The function of this circuit is limited to switching action of the fan only, speed cannot be controlled by this circuit.

  3. sir, i need report related to the thermistor base temperature control project.
    kindly provide me report if you can, i ll be thankful to you..

  4. please post the speed controlling circuit for this fan sir. i gave the connections.Is there any other way to control the fan’s speed.we tried a lot like heating the thermistor etc, but none of them worked.

  5. Hello sir,
    V made all d connections..but d fan strts rotating as soon as d current is passed through d circuit..d thermistors role is invisible… It’s not detecting d temp, v Evn tried to cool d thermistor by giving it cold waves n den pass current but Evn den d fan was rotatinG. In either way d thermistor is not sensing n making d fan rotate. Plz do help wid dis…

  6. Hello sir, is there anyimage regardibg thermistor NTC as i try to buy the components, only thermistor NTC they dob’t have any clue how does it look like. Thanks 73.

  7. Thanks alot.
    Nice work.
    Pls can u tell us what we can do to control the speed of the fan with increasing temperature and decrease as temperature drops?

  8. hello, may i know what to do if i am applying 12V dc fan instead of 230V ac , what would be the change in the circuit for that ?

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