I have a problem with reading tach signal from 3-PIN PC fan.
The problem is that as soon as i connect TACH signal from 3-PIN 12V PC FAN to any of MCU input pin or to for example logic state analyzer or i simply connect TACH signal to pullup resistor to 5V the fan speed significantly drops.
The speed drop does not occur when PWM is set to 100%, but it happens at lower PWM, for example, with a TACH signal is unconnected the fan can start up when PWM is around 30% but when TACH signal is connected to anything (even just pullup resistor) it needs at least 60% PWM to start fan.
Same goes when im runing fan at 70% just fine with TACH pin unconnected but when i connect TACH to anything the speed drops so much that fan almost stops.
Fan is powered with 12V while the speed is controlled by 41kHz (also tested with 10kHz and 20kHz) PWM from MCU trought a N-Mosfet transistor (i know that when fan will be powered with PWM =/= 100 the tach signal will return PWM signal instead of actual tach so i was planning to use techinique where before measuring fan speed i would give it 100% PWM for a moment untill i read its speed and then slow it down to desired speed).
Im attaching schematic that i used to connect FAN to MCU
The problem happens:
- Pullup resistor R21 is disconnected from VCC = 5V and i connect TACH signal to any input port of MCU
- As soon as i connect the pullup resistor to TACH signal
The problem does not happen when i run fan from constant low voltage like 5V. In this scenario there is no speed drop when connecting TACH pin.
Also, when the fan speed drops the source voltage remains at 5V (MCU power) and 12V (FAN power).
And i tested three fans, 2 of same brand and type and one taken out of computer just moment ago.
And i just noticed that the moment i connect TACH signal to anything including pullup resistor, the current drawn by FAN increases by 30mA from what it was before where normally the slower fan goes, the less current it draws.