Simple Temperature Based DC Fan Controller Circuit using Transistors

A simple but powerful temperature controlled DC fan controller circuit that uses transistors is really important for automatically cooling devices when they reach a certain temperature.

This feature helps prevent overheating and improves efficiency.

The circuit uses a thermistor to sense the temperature, which allows it to control the DC fan based on temperature changes.

The circuit can easily be powered by a 12V PP3 battery or a 12V DC power supply, making it a flexible option for different uses.

Circuit Working:

Parts List:

This circuit includes two transistors Q1 BC547 and Q2 BD140.

It also has some resistors, a capacitor, an LED and a direct current 12V fan motor.

The circuit working is mentioned below:

A NTC thermistor R1 is used to detect the temperature.

When the temperature goes up the thermistors R1 resistance decreases.

As the voltage at the base of Q1 increases it starts to conduct electricity which turns on Q2.

When Q2 is activated it sends power to the fan motor, making it spin and cool down the system.

The LED1 lights up to show that the fan is running.

The capacitor C1 helps keep everything running smoothly by reducing sudden changes in voltage.

Potentiometer VR1 works like a feedback resistor that adjusts how smoothly the transistors Q1 and Q2 switch between each other.

This helps the circuit respond better to changes in temperature.

It also stops false triggers by making the transition gradual rather than sudden.

On the other hand pot VR2 manages how bright the LED1 shines making sure it does not get too much current and burn out.

Together with C1 it adds a little delay when the fan turns on which, helps avoid sudden changes in voltage.

Formulas with Calculations:

Below mentioned are the formulas with calculations for Simple Temperature Based DC Fan Controller Circuit using Transistors:

Voltage Divider for Q1 Activation:

The base voltage of transistor Q1 BC547 is determined using the voltage divider formula:

V_B1 = V_CC × (R2 / (R1 + R2))

where,

• V_CC is 12V
• R1 is 15k
• R2 is 1.5k

LED Current Limiting Resistor:

The LED is connected in series with R5 680Ω and VR2 470Ω.

The total resistance is:

R_total = R5 + VR2

where,

• R5 resistor is 680Ω
• VR2 pot is 470Ω

Calculation:
R_total = 680Ω + 470Ω
R_total = 1150Ω

The current through the LED is calculated as:

Formula:

I_LED = (V_CC – V_LED) / R_total

where,

V_LED = 2V (for standard red LED)

Calculation:

I_LED = (12V – 2V) / 1150Ω

I_LED = 10V / 1150Ω

I_LED = 8.7mA

Since a typical LED operates at 10-20mA, this value is safe.

How to Build:

To build a Simple Temperature Based DC Fan Controller Circuit using Transistors follow the below steps for connections and assembling:

• Assemble all the components as mentioned in the above circuit diagram
• Connect the base of transistor Q1 to one end of NTC thermistor R1 and other end of R1 connect to positive supply terminals
• Connect emitter of transistor Q1 to one end of pot VR1 and other end of VR1 connect to collector to transistor Q2
• Connect collector of transistor Q1 to one end of resistor R3 and other end of R3 connect to positive terminal
• Connect resistor R2 between base of transistor Q1 and GND.
• Connect emitter of transistor Q2 to positive supply of +12V DC
• Connect the base of transistor Q2 collector of transistor Q1
• Connect resistor R4 one end to emitter of transistor Q1 and other end of R4 to GND
• Connect the collector of transistor Q2 to one end of 12V fan motor and other end of motor connect to GND
• Connect positive of capacitor C1 between fan motor and one end of resistor R5 and other end of resistor R5 connect to pot VR2 and negative of capacitor C1 connect to GND
• Connect the other end of VR2 pot to GND
• Connect anode of LED1 from one end of VR2 pot and cathode of LED1 connect to GND.

Conclusion:

This Simple Temperature Based DC Fan Controller Circuit using Transistors is designed to turn on a cooling fan when the temperature gets too high.

It is easy to build does not cost much and works great for keeping delicate electronic parts or cases cool.

You can change the resistor values or use a thermistor to set the temperature at which the fan kicks in, making it useful for different situations.

References:

Transistor use – Basic fan speed controller