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I have a problem. I can read out the tachometer of a PC fan and convert it to frequency with just using a simple microcontroller and labview.

Does anybody here have an idea how to read out the tachometer, without powering the fan? I want to measure the RPM with manually spinning the fan. I want to use the tachometer to measure the frequency of a shaft of a low rpm mini-engine. When I disconnect the power that makes the fan rotate, i do not get a reading anymore from the tacho, even when the blades are still spinning. enter image description here

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Internally the tacho pin is wired as an open collector

enter image description here (source http://www.bearblain.com/fan_speed_control.htm)

With no supply connected to the red wire the base of the transistor can't be turned on so it never pulls down the output (collector). The only way I see is to open the fan and tweak the circuit to power only the tachometer section and not the motor itself.


You can always create an individual circuit to measure the RPMs.
One of the options is with a Reflective Optical Sensor like

enter image description here

Here is a sample project

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  • \$\begingroup\$ Thanks for your help. I have succesfully opened the fan and the accessed the circuitboard. The problem is that I do not know which path to bypass. Also didn't manage to find a scheme of the PCB. I have put a picture in the question area. Are you able to assist me in this? Thanks in advance! \$\endgroup\$ – jemas Dec 30 '13 at 12:50
  • \$\begingroup\$ You need to look deeper, between the PCB and the fan motor or even within the silver blob in the center of the board. The motor and the tach are powered from the same power rails so you need to find and cut one the motor connections (to the left of the Hall Effect switch in your schematic). \$\endgroup\$ – JRobert Dec 30 '13 at 14:10

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