My goal is to use the PWM signal of a raspberry pi to control the speed of a 12V 0,24A DC fan.

The drawing below shows how I've wired everything up.

https://preview.redd.it/q3hajn2igegd1.png?width=696&format=png&auto=webp&s=b7f9c7a46d9a186b513c4aae2346e2c07711fb6d

I purchased a cheap mosfet module off of aliexpress:

https://de.aliexpress.com/item/1005005625110568.html

Amazon carries them too:

https://www.amazon.de/gp/product/B07HBQZ9BK

The module uses a FR120N mosfet.

Link to datasheet: https://pdf1.alldatasheet.com/datasheet-pdf/view/1223399/VBSEMI/FR120N.html

I use the pigpio python library to control the duty cycle of the pwm signal at 20kHz. When using a multimeter the readings (RPi PWM - RPi GND) perfectly correspond to the duty cycle I set in the code.

The problem: The voltage readings across the fan do not match the duty cycle at all.

At 50% duty cycle I would expect 6V but I get almost the max voltage already. (See table below)

https://preview.redd.it/gkeewuljgegd1.png?width=193&format=png&auto=webp&s=e0943f62f52e10331250e1eb1cc709ac4650a733

In summary I am able to control the duty cycle of the PWM signal but the output of my mosfet module is not what I expected.

The subjective fan speed matches the measurements of VFan (meaning there is no observable change between a duty cycle of 50-100%)

I understand that a minimum average voltage to turn on the fan is needed but why does the fan get about 6 V at 20% duty cycle and not at 50%?

Why is there minimal voltage change across the fan from 50% to 100% duty cycle?

Why does the voltage across the fan not correspond to the duty cycle?

I guess it has sth to do with the mosfet module but I am unable to pinpoint the issue.

My goal is to use the PWM signal of a raspberry pi to control the speed of a 12V 0,24A DC fan.

The drawing below shows how I've wired everything up.

https://preview.redd.it/q3hajn2igegd1.png?width=696&format=png&auto=webp&s=b7f9c7a46d9a186b513c4aae2346e2c07711fb6d

I purchased a cheap mosfet module off of aliexpress:

https://de.aliexpress.com/item/1005005625110568.html

Amazon carries them too:

https://www.amazon.de/gp/product/B07HBQZ9BK

The module uses a FR120N mosfet.

Link to datasheet: https://pdf1.alldatasheet.com/datasheet-pdf/view/1223399/VBSEMI/FR120N.html

I use the pigpio python library to control the duty cycle of the pwm signal at 20kHz. When using a multimeter the readings (RPi PWM - RPi GND) perfectly correspond to the duty cycle I set in the code.

The problem: The voltage readings across the fan do not match the duty cycle at all.

At 50% duty cycle I would expect 6V but I get almost the max voltage already. (See table below)

https://preview.redd.it/gkeewuljgegd1.png?width=193&format=png&auto=webp&s=e0943f62f52e10331250e1eb1cc709ac4650a733

In summary I am able to control the duty cycle of the PWM signal but the output of my mosfet module is not what I expected.

The subjective fan speed matches the measurements of VFan (meaning there is no observable change between a duty cycle of 50-100%)

I understand that a minimum average voltage to turn on the fan is needed but why does the fan get about 6 V at 20% duty cycle and not at 50%?

Why is there minimal voltage change across the fan from 50% to 100% duty cycle?

Why does the voltage across the fan not correspond to the duty cycle?

I guess it has sth to do with the mosfet module but I am unable to pinpoint the issue.