The reason for all this heat on the transistor is the operation of the circuit itself. Nevertheless, the only "problem" with this is that it does not have a really good efficiency. I suppose that for bigger fans, you may consider using a heatsink. Now, with a 300mA fan, the transistor is getting hot. When i was making the PWM circuits, i ran some test with multiple fans connected in parallel, just for fun. I was rather surprised by the amount of heat. It generates a lot of heat on the transistor. Multiplied by 30 (to convert to rpm) is 1230rpm. In the highest speed setting, the tacho feedback was 41 Hz. Multiplied by 30 (to convert to rpm) is 777rpm In the lowest speed setting, the tacho feedback was 25.9 Hz. Here are some images from the oscilloscope, reading the tach of the fan: In case that your fan does not revolve with so low voltages, just increase the R3 a little bit. The output voltage varies from 5 to 12 V. 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). The rest of the circuit will be behind the potentiometer. It is ridiculously easy! Can be mounted on a tiny pre-drilled PCB, with the potentiometer on one edge. The circuit was working perfect even without it, but why removing it anyway? We are talking about $0.05 ! C1 can be omitted, as i put it only to straighten the curly base voltage. The two capacitors are to smooth the voltage, as i saw some "waves" in the oscilloscope during the test-runs. The output is driven directly to the base of the power transistor. The two resistors will set the highest and lowest value of the voltage divider. The potentiometer is connected as voltage divider. The circuit is composed by 6 components: 2 resistors, 2 capacitors, the potentiometer and a transistor.
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