The tachometer signal, also known as “tach signal”, and “FG signal”, conveys rpm information in the form of square waves. The frequency of this square wave output is proportional to the rotation of the shaft.
Usually, a high sensitivity hall-effect sensor detects the rotation of the shaft providing two pulses per rotation, where the North Pole generates a positive pulse and the South Pole a negative pulse. The output of the hall sensor is usually weak and not a square wave; hence, a differential amplifier amplifies it, and a Schmitt trigger provides the switching hysteresis to reshape it into a square wave.
The final tach signal is usually an open collector design that provides a square waveform output to the motherboard. The voltage levels are usually at TTL level (+5 V) through a pull up resistor and an open collector transistor, however, this is by no means a rule and there will be exceptions.
To determine which standard a motherboard uses for the tach signal, check the voltage across the pins on the motherboard fan socket with an oscilloscope; there should be 0 V (ground - black), +12 V (power supply - red).
For the tach signal, the wire is usually yellow or white, and an oscilloscope will show a +5 V square wave output. The physical location of the pin within a three-pin connector is usually at the centre; however, it may vary for other motherboards such as Dell.
CPU Cooling Fan
The tach signal not only provides rpm information but also indicates the operational state of the fan. If the rpm value is low or absent, then the computer will fail the power-on self-test (POST), and will not start. However, manufacturers sometimes leave a by-pass option.
Engineers usually become aware of the tach signal and different standards when looking for a replacement CPU cooling fan. If you have a fan that has failed, or are replacing it with a quieter one, then the CPU Cooling Fan Replacement article also has information of interest.
This Article Continues...CPU Cooling Fan Tachometer Standard
Fan Tachometer Schematic / Circuit Diagram
Fan Tachometer Block Diagram
Fan Tachometer Open Collector Circuit
Tachometer Square Wave output Signal