Hi All

I'm looking into a circuit for controlling a 4 pin PWM fan from a 3 pin motherboard header.

Typically the 3 pin motherboard header (ignoring the sense pin) will vary the voltage supplied to a 3 pin fan from 7V to 12V DC.

The PWM fans on the other hand (Again, ignoring the sense pin) have a constant 12V supply, with a 5V PWM signal applied to the 4th pin at a frequency of 25kHz. 0-100% duty cycle will vary the fan speed from 30-100%.

So what I'm looking for is a circuit that will give a 5V 25kHz PWM output with an analog input varying the duty cycle from 0% at 7V input to 100% at 12V input

Has anyone done this before? Or does anyone have any suggestions?

 

Doable. I'll post a circuit in due course.

Edit:-
Welcome to the forum!
Here you go

 

Good one, thank you!