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7/27/2019 Stdnt Projct_TempControlFan
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Temperature controlled DC fanDescription.
Here is a simple circuit based on two transistors that can be used to control the speed of a 12
V DC fan depending on the temperature.A thermistor (R1) is used to sense the temperature.
When the temperature increases the base current of Q1 (BC 547) increases which in turn
decreases the collector voltage of the same transistor. Since the collector of Q1 is coupled to
the base of Q2 (BD 140), the decrease in collector voltage of Q1 forward biases the Q2 more
and so do the speed of the motor. Also, the brightness of the LED will be proportional to the
speed of the motor.
Circuit diagram with Parts list.
CKT-1
Notes.
The R1 can be a 15K @ 20C ,N.T.C thermistor. The M1 can be a 12V,700mA fan motor. The capacitor C1 must be rated 25V. The circuit can be powered from a 12V PP3 battery or 12V DC power supply. Assemble the circuit on a good quality PCB or common board.
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CKT02
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 usedin 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.
DC Fan Controlled by Temperature Circuit Diagram
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