PWM fan controller

A lot of people like to use the 7V fan mod, this is fine I suppose but it is very inefficient, and it doesn’t allow for RPM monitoring.

This is a fan controller builtĀ out of fairly cheap and readily available parts that is extremely efficient, reliable and doesn’t endanger your computers PSU. I feel very sure that anyone who knows how to solder can build this device, whether they understand the electronic workings behind it or not.

For those of you who don’t know what Pulse Width Modulation is and why it is more efficient than voltage drop speed control read the following paragraph, If you already understand the concept feel free to skip ahead to the construction stage.

Pulse Width Modulation (PWM) is the practice of rapidly turning a motor/device on and off to control its speeds. This is a very efficient speed control method because it runs the motor at its rated voltage, thus more torque, more accurate speed controls and lower starting speeds. Motors can be slowed by lowering their voltage but this in turn reduces the motors torque and in computer fans this could mean less CFM. But how does it work?

A generated frequency from an oscillator is passed into an OP amp. In the op amp is where the actual Pulse width modulation takes place. The OP amps threshold is set. (the threshold is the voltage on the frequency that will trigger a logic High (+5v) on the op amps output. By changing the threshold you change the length of the pulses and thus change the speed of the motor. The output from the Op amp is then sent to a power management device. Usually a power transistor, which operates like a normal transistor but can handle higher current. The result from the power output is the motors rated voltage very rapidly changing from off and on stages.

Where to get the parts?
All of the parts listed below are available from Radio Shack (or The Source as its now known in Canada). The resistors and Capacitor can be ripped off from an old circuit board if you wish. All of these parts should cost you under $8 if you buy them all. Personally I’m cheap so I scavenged for the parts in my junk bin, all I bought was the NE555, the prototype board.

Below are some pictures of my prototype and final design. The schematic is also below.

NOTE:If more than one PWM channel is on the same rail a 1000uF or higher Capacitor should be placed on the power rail to prevent interference and resonance between the channels from the switching that could interfere with the units performance.

Parts:The parts you will need are as follows

  • (1) 390 Ohm resistor (Orange , White , Brown)
  • (1) 330K Ohm resistor (Orange, Orange, Yellow)
  • (1) 10K Ohm resistor (Brown, Black, Orange)
  • (1) 0.1uF (Microfarad) Capacitor. (commonly stamped with the number 104)
  • (1) 10K Ohm Potentiometer (Variable Resistor)
  • (1) TIP31 or similar power transistor (I used a TIP122)
  • (1) NE555 timer.
  • (1) IC PC Board multi-purpose – 417 (by NeXXtech)
  • (x) 2 pin connector. (these are used to plug the fan into the controller. I salvaged mine from an old motherboard. The type that are used for jumpers.) The number of 2 pin connecters depends on the amount of fans you wish to run on the channel. I used 2 sets. (to connect 2 fans) keep in mind that the Tip31 can only supply about 2 Amps without a heat sink.

Copyright 2006 Barret R. de Visser

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