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PC Fan Controller Circuit

As computers grow bigger and faster, the power consumption is also increasing (talking about high performance desktop processors and not about low power consumption mobile or laptop processors).

Even the modern electronics have low power efficiency and majority of the power drawn by a computer processor is turned in to heat.

PCFan Controller Circuit

Heat is an enemy to electronics and particularly computers. Hence, several cooling techniques are introduced over the years to achieve suitable temperature and humidity conditions for a computer to run hassle free.

There are several PC cooling techniques like active and passive air cooling, active and passive water cooling, bong cooling, cooling using Peltier thermo elements etc.

Of all the computer cooling techniques, active air cooling is the cheapest and most commonly used technique.

In active air cooling, a small fan will blow cool air in to heatsink or cooling plate that is attached to the processor or any other electronic device that needs cooling.

In this project, a PC fan controller circuit is designed using simple hardware, where the speed of the PC fan is automatically adjusted according to the temperature of the processor.

PC Fan Controller Circuit

Circuit Diagram

PC Fan Controller CircuitComponents Required

  • 555 — 1
  • 2N2222 — 1
  • 10KΩ Negative Temperature Coefficient (NTC) Thermistor — 1
  • 20KΩ POT — 1
  • 1KΩ — 1
  • 100pF — 1
  • 1µF — 1
  • PC Fan — 1

Circuit Design of PC Fan Controller

The circuit is based on good old 555 timer IC. It is configured as an astable multivibrator. Pins 4 and 8 are connected to supply. Pin 1 is connected to ground.

Pins 2 and 6 are shorted. A 10KΩ POT is connected between supply and pin 7 with wiper of the POT connected to pin 7. The other end of POT is left open.

A 10KΩ thermistor is connected between pins 7 and 6. A 1µF capacitor is connected between pins 6 and ground.The output is taken from pin 3 and is given to the base of a transistor.

Advised to use a current limiting resistor for base of the transistor. Hence, a 1KΩ resistor is used in the circuit.

The collector of the transistor is connected to supply and the PC fan is connected between emitter and ground.

Working of PC Fan Controller Circuit

In a normal PC or notebook PC, most of the heat generated is by the processor. This heat must be removed efficiently and quietly. A PC Fan along with a heat sink is an excellent active cooling system for computers.

Most modern motherboards include a provision for on board CPU fan control. With this, the speed of the fan can be controlled by making certain changes in the BIOS settings.

This project is also an automatic PC fan control system but without on board controls.

But why do we need to control the speed of a cooling fan? A PC cooling fan is a source of noise and is an additional component which consumes power.

Speed control of the cooling fan is one way to achieve the following advantages:

  • Less noise when the fan is running slower.
  • Power consumption of the fan is less when it is running at less speed and
  • When the fan is running slower, the lifetime and reliability of the fan increases.

There are many types of PC fans like 2 – wire, 3 – wire and 4 – wire. There are several methods in which we control the speed of the fan like on/off control, linear control, low frequency PWM and high frequency PWM.

In this project, a low frequency PWM control of 2 – wire PC fan is designed. In order to generate the PWM signal, we used the reliable 555 timer IC in astable mode.

We used a 10KΩ thermistor with negative temperature coefficient. Which means its resistance decreases with the increase in temperature.

At room temperature i.e. at 25 0C, the resistance of thermistor is 10KΩ. The pot is set at 10KΩ. At this settings, the percentage of duty cycle of the output PWM signal can be calculated as follows.

ON Time = 0.693 * (R1 + R2) *C1 = 0.693 * (10K + 10K) * 1*10-6 = 13.8mS.

OFF Time = 0.693 * R2 * C1 = 0.693 * 10K * 1*10-6 = 6.93mS.

% of Duty Cycle = (ON Time / (ON Time + OFF Time)) * 100 = 66.6

Frequency of the PWM signal = 1.44 / ((R1 + 2*R2) * C1) = 48 Hz.

When the temperature rises, the resistance of thermistor (R2 in the formulae) will decrease. If the temperature rises to around 50 0C, the resistance of thermistor fall to 3KΩ.

Now, the % of Duty cycle is approximately = 81 and

The frequency of the PWM signal is = 90Hz.


  • A simple and efficient automatic PC Fan Controller is designed in this circuit.
  • This circuit can be used in a PC and the speed of the fan is automatically adjusted according to the temperature of the processor.
  • For better performance, the values of the components can be changed (try with different combinations of components and observe the output).

9 Responses

  1. Hi,
    It’s a cool circuit but : my “power” measurements told me that a buck(or boost) converter is the best solution to avoid heat loss in the transitor (loss of power) (dc-dc converter).
    Between these two solutions, Mosfets canal N are good.

    1. Hi
      I have retired and spend a lot of time on my old hobby, electronics. Fan speed controller is always important in amplifiers, power supplies, etc.
      Is there a PCB layout available?

  2. Hi, looking for a fan cooling circuit to cool 2 x FETs on heatsinks in a 40w guitar amplifier, I have 2 x 12v small bushless cooling fans (2 amplifiers to cool) and 2 small 12v transformers as the amplifier rectified voltage is much higher and I also want the fans and the power for the fans independant from the amplifiers as much as possible so not to introduce noise.
    Questions, will this circuit be usable with my fans? and will this circuit introduce unwanted noise to the amplifier being close to the main boards.

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