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PC fan control Circuit

It is an unfortunate and well-known fact that most PCs of IBM and compatible type make a lot of noise. which is both undesirable lots of noise in the both undesirable (as regards noise in the working environment) and unnecessary (as regards the actual power consumptions. which is often quite low ), many fans in PC power supplies are over rated. Noisy, and run at a constant high-speed. Fortunately. the authors found that such fans can do their protective job just as well at far lower speeds.

The speed controller presented here consists of (1) a temperature monitor based on a LED as the sensor device, and (2) the idle speed regulator. The combination of the temperature monitor and the idle speed regulator result in a linear relation between temperature and fan speed. In other words: the fan will never run faster than strictly necessary. by virtue of the two separate regulator circuits. there is no interaction between idle speed and temperature regulation, as with many other (less sophisticated ) fan controllers. The result is low fan speed (low noise) at low temperature as well as good cooling and a safe startup at all times. When the temperature inside the PC rises. the voltage drop across the red LED. D2 decreases by approximately -2 mV K-1 This result in a higher output voltage of op-amp IC1A. Preset P1 is used to set the start level, while P2 determines the slope of the regulator characteristic. The adjustment range of the idle to full speed regulation is about 2 oC to 30 C.

The idle speed is set to the desired value by adjusting P3. Capacitor C2 ensures a 4-second full speed start-up period while diode D3 restarts the fan after a short interruption on the mains. The output of the temperature monitor and the idle speed regulator are ‘joined’ by two diodes, D4 and D5 at the base of T1. The regulator is stabilized by feeding a small portion of the fan voltage back to the inputs of the opamps. The circuit is built on a small printed circuit board that can be fitted into the PC’s power supply. The LED may have to be moved off the board. and connected with wires. to enable it to be fitted in a position where temperature changes are best noticed.

Parts List.


  • R= 47 kΩ
  • R= 5.6 kΩ
  • R= 56 kΩ
  • R= 10 kΩ
  • R= 1 kΩ
  • R-R9= 100 kΩ
  • R-R8= 1 kΩ
  • R10 = 820 kΩ
  • P= 20 kΩ Multiturn present
  • P= 5 kΩ Multiturn present


  • C1 = 10 μF , 63V radial
  • C2 = 220 μF , 25V radial


  • D1 = 6.2 V, 0.4 W zener diode
  • D2 = LED, red, 5 mm
  • D3 = 1N4148
  • D6; D4; D5 = 1N4004
  • T1 = BC547B
  • T1 = BD242C

Integrated Circuits:

  • IC1 = TLC272


  • K1 ; K1 = 2-way PCB terminal block,pitch 5 mm

PC fan control



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