![]() ![]() powered with PWM, the tacho will not operate correct. If the fan has no constant current - e.g. The tacho is directly powered from the positive and negative wire of the fan. The problem begins when the fan is powered with PWM. From the pulses, someone can calculate the speed of rotation. The third wire will then provide pulses with 50% duty cycle from the internal tacho. So, the third and last way to control these fans - and have also rpm feedback is the "Pulse Stretch method".īut first, let's see the circuit in action!Ī 3-wire PC fan is orientated to work either at full speed, or with a resistor in series to reduce the voltage and thus reduce the speed. I will need these 1800rpm fans to work overtimes. I did not want to change them, as the summer is coming soon, and we have some hell of a hot summer here in Greece. ![]() It was time to start thinking of another way to control these fans. Moreover, the fan stalled once in one month of test-operation, without any particular reason. I had clear readings from the tacho of the fans, yet i could not achieve low speeds without stalling the fan, nor had i an acceptable range of speed steps. ![]() I decided to give another try to these fans with a PWM averaging rpm controller. The inlet fans are 2 Zalman 1800rpm, that unfortunately did not work as expected with the same circuit. The outlet fans, are 2 Coolermaster 1200rpm fans, that are controlled using this PIC 3-Wire Fan RPM Controller. I have 4 120mm fans in my PC cabinet, each one controlled separately from a dedicated PIC. I made this circuit for my PC System Health Monitor project. ![]()
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