Electronic – MOSFET controlling fan speed

mosfetpwm

I have a doubt about an electronic circuit.
Below is the circuit diagram of the fan controller in my 3d printer controller ( Printrboard Rev D ).
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enter image description here
This circuit can control the speed of a fan.

I know that MOSFET can be used as a switch. When sufficient Vgs is applied, MOSFET will turn on and so the load (fan).

This is my understanding of this circuit, please correct me if I’m wrong or missed anything?

5V PWM is a digital signal and by adjusting the duty cycle, I can control the average value.

For 100% duty cycle Average voltage = 5V
For 50% duty cycle Average voltage = 2.5V
For 0% duty cycle Average voltage = 0V

So if I apply a 5V PWM with 50% duty cycle to the gate of MOSFET it will turn the MOSET On and OFF depending upon the Ton and Toff value(Ton= Toff since duty cycle is 50%). Since this ON and OFF happen so fast, if I check the output voltage using a muti-meter it will read 6V (50% of 12V), and in a CRO or DSO I can see the 12 V PWM. Therefore, the fan will operate at 6V and will run at half the speed (if the rated speed is 500RPM, it’ll run at 250RPM).

I tried to explain with a picture also

enter image description here
Is this how this circuit changes the speed of fan?

I'm not an electronics guy so I'm expecting simple answers

Thanks

Best Answer

The analysis is correct. The main period of the duty cycle is higher than the reaction time of the motor mechanic, so the motor reacts only on the average voltage applied by the PWM output signal.

Vo(average)= Vsupply * DutyCycle

Basically it works like a low pass filter.