Simple Temperature Controlled DC Fan Circuit

A Temperature Controlled DC Fan Circuit is like a smart fan controller.

It uses a special sensor to feel the temperature around it.

If things get too hot, the circuit kicks the DC fan into high gear, just like turning up the AC.

But when things cool down, the fan slows down or even turns off to save energy.

This circuit uses parts like a thermistor a special temperature sensitive resistor and a control circuit to make these decisions automatically.

So, your electronics can stay chill and keep working their best.

Circuit Working:

Parts List:

This circuit is designed to cool down electronic devices that generate heat.

It operates a DC fan when the temperature in the vicinity exceeds a preset level and it automatically turns off when the temperature returns to normal.

The fan used is a small 12V DC brushless fan commonly found in computers.

The circuit utilizes a thermistor, which is a type of resistor whose resistance changes with temperature.

There are two main types of thermistors: NTC Negative Temperature Coefficient and PTC Positive Temperature Coefficient.

An NTC Thermistor decreases its resistance as the temperature rises, while a PTC thermistor increases its resistance with temperature.

Thermistors are available in various values, typically ranging from 100 ohms to 10k or more.

In this circuit, a 4.7k ohm NTC thermistor is used.

The IC 741 functions as a voltage comparator to control the DC fan.

Its inverting input pin 2 receives an adjustable voltage from VR, while its non inverting input pin 3 receives voltage from a potential divider formed by R1 and the thermistor.

Thus, the voltage at pin 3 depends on the resistance of the thermistor.

When the temperature is at the normal level set by VR, the voltage at pin 3 is higher than that at pin 2 causing the output of the IC to go high.

This high output keeps transistor T1 off since its base is positive.

In this state, the DC fan remains off.

When the temperature exceeds the value set by VR, the resistance of the thermistor decreases lowering the voltage at pin 3.

Consequently, the output of the IC goes low, turning on transistor T1.

The brushless DC fan then turns on to increase air circulation.

When the temperature returns to normal the fan automatically turns off.

The diode 1N4007 is essential for removing back EMF when transistor T1 turns off.

Formulas:

Guidelines and formula for creating a temperature controlled DC fan circuit with an IC 741 comparator.

Voltage Divider Output (Voltage at Inverting Input of IC 741):

V_in−​ = V_CC​ * R_bias​​ / R_NTC​ + R_bias​

where,

• V_CC​ is the supply voltage of 12V.
• R_NTC is the resistance of the NTC thermistor.
• R_bias​ is the resistor used for biasing the thermistor.

Output Voltage of IC 741 (Open Collector Output):

When V_in−>V_ref V_out​ is low(near 0V)

When V_in−​<V_ref​ V_out​ is high(close to V_CC)

Base Resistor for Transistor:

R_base​ = V_CC​−V_out​​ / I_base​

where,

• I_base: The base current that the transistor needs, which is based on the transistor gain and fan current.

Collector Current (Fan Current):

I_collector​ =V_CC​−V_CE(sat)​​ / R_fan​

where,

• V_CE(sat)​ is the collector emitter saturation voltage of the transistor.
• R_fan​ is the resistance of the fan.

Note:

Using an IC 741 comparator, transistor switching and NTC thermistor for temperature sensing, this framework offers an organized method for creating and comprehending temperature controlled DC fan circuits.

Modify component values according to the particular needs and features of your application and fan.

How to Build:

To build a Simple Temperature Controlled DC Fan Circuit below are the steps:

• Connect pin 3 of the IC 741 to a potential divider consisting of the NTC thermistor and R1.
• The other end of the thermistor should connect to ground, and the other end of R1 should connect to the positive supply.
• Connect pin 2 of the IC 741 to the wiper of the Variable Resistor.
• Connect one end of the VR to the positive supply and the other end to ground.
• Connect the output pin 6 of the IC 741 to the base of the PNP transistor BD140 through a resistor R2.
• Connect the collector of the transistor to the ground and the emitter to the negative terminal of the DC fan.
• Connect the positive terminal of the DC fan to the positive supply ,connect the diode 1N4007 parallel to the fan.

Note:

• Ensure all connections are secure and correct before powering up the circuit.
• Adjust the VR to set the temperature threshold for fan activation.
• When the temperature exceeds this threshold, the fan should turn on automatically.

Conclusion:

To conclude, a Temperature Controlled DC Fan Circuit is a useful electronic circuit that automatically adjusts the speed or operation of a DC fan based on the surrounding temperature.

It helps maintain an optimal temperature for electronic devices, computers and other applications preventing overheating and ensuring efficient operation.

References:

Temperature Controlled DC Fan Using Thermistor