TEMPERATURE CONTROLLED DC FAN (Thermister Characteristics)

Circuit diagram of temperature control DC fan

Heat Sensor:

  • Temperature controlled DC fan is a temperature based fan.
  • It can cool the 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.

Components  Required:

—Resistors  (4.7kΩ)

—Op Amp (LM 358)

—Transistor (BC 548)

—5V DC Motor

—9V DC Battery

—NTC  Thermistor (4.7k Ω)

—Potentiometer (4.7k Ω)



—LM 358 Op amp is used as a Voltage Comparator,  it compares two input voltages at pins 2 & 3.

—The output of LM 358 collects at pin1.

—BC 548 Transistor is used as a Switch to control Fan.

—BC 548 is a NPN transistor

—5V DC motor is used to run the Fan.


Thermistor Behaviour:

—Thermistor is a Resistor whose  resistance varies with respect  to Temperature.

—Negative Temperature  Coefficient(NTC) Thermistor,  resistance Decreases as  temperature rises.


Thermistor Characteristics

Thermistors can be classified into two types depending  on the sign of k.

If k is positive, the resistance increases with increasing  temperature, and the device is called a positive temperature  coefficient (PTC) thermistor, Posistor.

If k is negative, the resistance decreases with increasing  temperature, and the device is called a negative  temperature coefficient (NTC) thermistor.

Circuit Diagram:

Circuit diagram of temperature control DC fan
Circuit diagram of temperature control DC fan


  • The basic working principle of temperature

controlled dc fan is based on the thermistor.

  • LM 358 compares the voltage across the NTC thermistor and voltage across the 4.7k Ω resistor.

—At room temperature the voltage across the NTC  will be greater than or equals to the voltage  across the 4.7k Ω resistor. So the Op Amp output  will be either negative or zero voltage which can  not turn ON the Transistor.

—If the temperature rises above the preset value, it  leads to decrement of Thermistor resistance and  voltage across it.

—Then if this voltage lesser than the voltage across  4.7k Ω resistor,  the output of  Op Amp will be  positive voltage which can turn ON the Transistor.

—When transistor turns ON, the motor(FAN) will be  connected across the DC supply and starts to  rotate.

—The motor will be turns OFF when the  temperature returns back to room temperature.


—It is very economical and easy to handle.

—Motor fan runs automatically, so that it controls  the temperature without using it manually.

—It is very easy to install at heat dissipating devices  to cool down them.

—Saves the energy by turns OFF the motor fan at

normal or low temperature automatically.


—Temperature Controlled Fan can be used to  control the temperature of devices, rooms,  electronic components etc. by monitoring the  temperature.

—The circuit can be used for Car Engine to reduce  the heat.

—Cooling Pads in personal computers.


  • The electric fan operates automatically according to temperature rises in order to compensate the rise in the temperature and  turns off when the temperature returns back  to the norml temperature.

Leave a Reply

Your email address will not be published. Required fields are marked *