The most recent PC motherboards provide four-pin connectors for cooling fans, especially for the CPU fan. The older three pin fans are controlled by varying their DC voltage. The fourth pin on the newer connectors supplies a PWM signal to control fan speed. A three pin fan can be plugged into the four pin connector but with its fixed 12 V supply it runs at full speed all the time the PC is switched on. This is not an ideal situation if only for the noise levels.
During a recent motherboard upgrade the author was reluctant to replace his existing copper-finned CPU cooler which still had plenty of life left in it. An electronic solution was the only way ahead. A circuit was needed to convert the PWM signals from the fourth connector pin into a variable DC supply for the three pin fan. The PWM signal originates from an open collector output which can only be pulled up to a maximum of 5.5 V., For this reason, R1 is connected in series with a zener diode to limit the pull-up voltage to 4.8 V. The PWM signal is integrated by the network formed by R2 and C1. The resulting signal is amplified by an opamp (almost any type that can work at 12 V will do here). The opamp output signal controls an adjustable voltage regulator which supplies sufficient current even for the most powerful fan.
P1 adjusts the fan’s minimum rotational speed (with a cold CPU). Capacitor C1 is connected to VCC so that when the PC switches on it transfers almost the full 12 V to the opamp input to run the fan at full speed momentarily. This ensures the fan gets a small kick to get it going from rest. The regulation sensitivity can be adjusted by changing the value of R4. Incidentally, the plug from an old floppy disk drive power connector can be used (after a little trimming) to connect to the 4-pin motherboard fan plug.