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I use a Fan with 4 pins which are 12V, GND, PWM, FG and my circuit it FAN's PWM pin is connected ,through 100R series resistor, to MCU's PWM pin.

today I check the Fan PWM without MCU.(only test for FAN alone) I connect 12V, GND to power supply and probe FAN's PWM with oscilloscope. then it shows that FAN's PWM pin is DC 5.5V.

and when I decrease supply voltage 12 to lower then FAN's PWM is dropped slightly and increase supply over 12 then FAN's PWM pin output is slightly increased.

I have no circuit schematic for this FAN can anyone guess the inner circuit for this FAN PWM?? I think it may cause problem because my MCU is 3.3V but FAN PWM is over 3.3V

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