Motor Circuit Diagrams

3-Pin Fan in 4-Pin Socket Schematic Circuit Diagram

Modern PC Motherboards and Cooling Fans

In contemporary PC motherboards, there are dedicated four-pin connectors designed for cooling fans, notably for the CPU fan. These newer connectors offer advanced functionality compared to the older three-pin fans. The older three-pin fans operate by adjusting their DC voltage. In contrast, the fourth pin in the recent connectors delivers a Pulse Width Modulation (PWM) signal, allowing precise control over fan speed. While a three-pin fan can be connected to a four-pin connector, it constantly runs at full speed due to its fixed 12 V supply. This setup presents issues, primarily concerning noise levels and is far from an optimal configuration.

3-Pin Fan in 4-Pin Socket Schematic Circuit Diagram3-Pin Fan in 4-Pin Socket Schematic Circuit Diagram

Adapting the Existing CPU Cooler

In a recent motherboard upgrade, the author chose to retain their fully functional copper-finned CPU cooler, which still had a substantial lifespan. To overcome the compatibility issue, an electronic solution was imperative. A circuit was devised to convert the PWM signals from the motherboard’s fourth connector pin into a variable DC supply suitable for the three-pin fan. The PWM signal, originating from an open collector output with a maximum pull-up voltage of 5.5 V, posed a challenge. To address this, R1 was serially connected with a zener diode to limit the pull-up voltage to 4.8 V. The PWM signal was integrated through the network formed by R2 and C1. The resultant signal underwent amplification using an op-amp (any type functional at 12 V sufficed). This op-amp output signal controlled an adjustable voltage regulator, providing ample current even for the most robust fan.

Fine-Tuning Fan Performance

P1 facilitated the adjustment of the fan’s minimum rotational speed, especially when the CPU was cool. Additionally, capacitor C1, connected to VCC, ensured that upon the PC’s startup, it momentarily delivered nearly the full 12 V to the op-amp input, initiating the fan at maximum speed briefly. This kick-started the fan into motion. The sensitivity of regulation could be fine-tuned by altering the value of R4. Interestingly, the plug from an old floppy disk drive power connector could be utilized (with minor modifications) to connect to the 4-pin motherboard fan plug, ensuring a streamlined connection.


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