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30 March 2010
Author: Giorgos Lazaridis
Simple Linear Fan Controller

In continue to an older set of circuits (Simple Ways to Make Fans Silent), i decided to fulfill the fan controller circuits. Until now, i have present several PWM circuits to control a fan, yet these are rather difficult for a simple PC moder to implement. The latest circuit post was a set of some circuits so easy, that even a kid can do. But They did not provide linear control. Therefore, here i am again, with the simplest linear fan controller circuit.

The circuit

the circuit on a breadboard for test

The circuit is composed by 6 components: 2 resistors, 2 capacitors, the potentiometer and a transistor. The potentiometer is connected as voltage divider. The two resistors will set the highest and lowest value of the voltage divider. The output is driven directly to the base of the power transistor. The two capacitors are to smooth the voltage, as i saw some "waves" in the oscilloscope during the test-runs. C1 can be omitted, as i put it only to straighten the curly base voltage. The circuit was working perfect even without it, but why removing it anyway? We are talking about \$0.05 !

The schematic circuit is as follows:

...It is ridiculously easy! Can be mounted on a tiny pre-drilled PCB, with the potentiometer on one edge. The rest of the circuit will be behind the potentiometer. The circuit is designed to be powered directly from the power supply of the PC (check the Power supply pinouts and get 12V from wherever you can).

Did it work?

Yes of course it did. The output voltage varies from 5 to 12 V. In case that your fan does not revolve with so low voltages, just increase the R3 a little bit. Here are some images from the oscilloscope, reading the tach of the fan:

 In the lowest speed setting, the tacho feedback was 25.9 Hz. Multiplied by 30 (to convert to rpm) is 777rpm In the highest speed setting, the tacho feedback was 41 Hz. Multiplied by 30 (to convert to rpm) is 1230rpm. The fan is rated of course at 1200rpm.

There is a major drawback though. This circuit has a very bad habit. It generates a lot of heat on the transistor. I was rather surprised by the amount of heat. When i was making the PWM circuits, i ran some test with multiple fans connected in parallel, just for fun. The MOSFET was bone cold all the time. Now, with a 300mA fan, the transistor is getting hot. I suppose that for bigger fans, you may consider using a heatsink. Nevertheless, the only "problem" with this is that it does not have a really good efficiency. It will operate without problems.

The reason for all this heat on the transistor is the operation of the circuit itself. When the fan runs in full rpm, the transistor will strangely generate almost no heat at all! This is normal though if the fan is within the current limits of the CE contact of the transistor. The problem begins when the circuit is asked to reduce the speed of the fan. To do so, the transistor will dissipate an amount of power. The more the power the transistor dissipates, the lees the power delivered to the fan! This is how the fan runs slower. As you understand, when the fan runs with the lowest speed possible, the transistor will dissipate the highest amount of power. Thus, the circuit is most efficient when the fan runs at high rpms,

Bill Of Materials
 Resistors R1 5 KOhm potentiometer R2 Resistor 100 Ohm 1/4 Watt 5% Carbon Film R3 Resistor 10 KOhm 1/4 Watt 5% Carbon Film Capacitors C1 10 uF 16 Volts electrolytic capacitor C2 470 uF 16 Volts electrolytic capacitor Transistors Q1 BD243 NPN Silicon Power Transistor

Relative pages
• The power connectors used for PCs since 1981
• How the PC fans work
• The transistor theory of operation
• Learn about the most popular PC Cooling methods
• How the Brushless motors (BLDC) work
• Control a 3-wire Fan with PWM pulses and use the Pulse Stretching method to get clear rpf feedback
• An intelligent self-tunned fan PWM controller
• A 2-speed PWM temperature fan controller
• Dr.Calculus: Voltage divider calculator