Original article can be found here: http://pcbheaven.com/circuitpages/Simple_Linear_Fan_Controller/
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 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:
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