My project in 6th semester……..

“Heaven’s Light is our guide”
Rajshahi University of Engineering & Technology (RUET), Rajshahi

Name: Hemangshu Halder
Dept.:  Electronics  & Telecommunication  Engineering
Roll : 074018
Course No : ETE 300
Course Title : Electronics Project Design   &  Development

Project Name:
Temperature Controlled DC Fan.

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.

DC Fan Controlled by Temperature Circuit Diagram:

Circuit Components:

—    Integrated circuit – uA 741
—    Resistance – 2 pieces (1 k , 470 ohm)
—    Variable resistance – 1 piece (470 k ohm)
—    LED – 1 piece (green)
—    Transistor – 1 piece (BD 140)
—    Thermistor – 1 piece (4.7 k NTC)
—    Diode – 1 piece (IN 4007)
—    12 V DC Fan
—    Power Supply (12 volt)


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 D3 is necessary to remove back e.m.f when T1 turns off. Green LED indicates that the Fan is operating.

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.

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