3pin fan connected to 4pin motherboard header change its speed while heavy load!!

[DDK31]

Hi everybody. I have my doubts about a 3 pin fan speed variation: I have conected a rear fan to a 4pin mobo conector (called "cha_fan", it would be chasis fan - in others mobos it is called sysfan) and watching its speed throw "HW Monitor" it usually goes at 1100rpm, but when I play a game it goes up to 1900rpm. So, my question is how the 3pin fan can change its speed if it has not the fourth cable (pwm)? I also have a 3pin fan into the cpu_fan header and it is always at the same speed (2100rpm); no variation speed!! Don´t understand. Thanks

 

[Someone Somewhere]

3-pin fans can be controlled by using a MOSFET to switch the +12V line frequently (PWM). This is less precise than the control in a 4-pin fan, but adequate.

Some motherboards have hardware to drive both 3-pin and 4-pin fans off the system fan headers. Usually, this expense isn't used for CPU fans because almost every CPU fan is 4-pin anyway.

 

[DDK31]

3-pin fans can be controlled by using a MOSFET to switch the +12V line frequently (PWM). This is less precise than the control in a 4-pin fan, but adequate.

Some motherboards have hardware to drive both 3-pin and 4-pin fans off the system fan headers. Usually, this expense isn't used for CPU fans because almost every CPU fan is 4-pin anyway.

Thanks Someone Somewhere. That´s a good answer and explanation. Learning something new every day!! I read a lot but never had heard something like this. Thank you very much