Fan Controller

I recently (2011) purchased a new desktop computer based on the Asus P8Z68-Vpro motherboard with an Intel quad core I7-2600K CPU running at 4.6GHz. Although the CPU draws only about 7 watts at idle, with all 8 threads running full out the CPU draws about a 100 watts. So I knew my computer would need a good cooling system, especially since my computer is located in the living room of my un-air-conditioned house. So my computer included 4 low noise case fans as well as a large dual fan Noctua CPU cooler.

I also wanted my computer to be quiet, but because of the sophisticated temperature sensing and fan control circuitry on this motherboard, I assumed the fans would be turning slowly except perhaps when doing video processing or other compute intensive tasks during a summer heat wave.

I was disappointed to find that this was not true. The first and most significant problem is that the ASUS fan control software enforced a lower limit of 60% power for the case fans. While the fans were certainly quieter at 60% than 100%, with four of them running is still made a significant amount of noise. It seemed almost too incredible that with all this seemingly well designed hardware and software, they would make it essentially useless to me because of this one unmodifiable parameter. Yet indeed this was true. I considered waiting for ASUS to fix this problem with a bios update, but there didn't seem to be a way of finding out if this was even on their agenda or not.

The second problem was more my fault. The fans on the Noctua CPU are normal voltage controlled fans. Some motherboards can control the rpm of these fans, but this particular motherboard can only control the rpm of a CPU fan that uses the alternate PWM control. I could have, and should have purchased a different CPU cooler that used PWM control. The system configurator/builder should have flagged this incompatibility, but in their defense ASUS has very poor documentation on this aspect of their design. The CPU fans must be PWM controlled and the case fans must be voltage controlled, although this fact is nowhere documented.

Eventually I decided to solve both problems by building my own fan controller. Here is a thumbnail of the schematic of my fan controller. As with all the other pictures on this page, click on the thumbnail to view the full size image. (Use the back button to return to the main page.)

For simplicity, the CPU fan PWM signal from the motherboard is used to control all the fans in the system. This means that the case fans will start spinning up sooner after a compute intensive task begins, but I considered this an advantage. The PWM signal is averaged by the RC circuit, then buffered by the first op-amp. This is followed by two gain stages, one for the CPU fan and the other for the case fans. Each gain of these gain stages is adjusted by a trimmer pot so that the fans are spinning as slowly as possible when the system is cool. The amplified averaged PCM signal is then used to drive the fans thru three high current op-amps (LM675) each of which drives two fans. The sense signal from one of the case fans is fed back to the motherboard (connecter near the bottom of the schematic) so that the various fan monitoring programs can display the speed of the case fans. (Note that the CPU fan sense signal is similarly passed to the motherboard.)

Fan speed test results:

Conveniently, the ASUS AI Suite has a CPU fan test button that slowly cycles the fan power from 20% up to 80%, giving me an easy way to test my circuit, and to record the results which are as follows:

Pwr CPU rpm Case rpm
20% 450 320
30% 690 450
40% 860 600
50% 1000 750
60% 1030 830
70% 1050 1000
100% 1050 1000

The fan speeds at the 20% power level are slow enough that my computer is virtually silent unless a very intensive compute task is in progress.

The ASUS AI Suite also allows one to choose from several fan profiles, including a user programmable profile. I chose the user programmable profile with the following breakpoints: Perhaps the only clever part of the effort was my method of temporarily mounting the circuit board close enough that all the fan connections could be made yet in a position that enabled access to the components for debugging purposes. I wire-wrapped a stiff piece of rubber (a french curve actually) to the board and then clamped the french curve to the top of the computer using a suitably heavy text book. Of course I removed the french curve from the circuit board once the correct operation of the circuit was verified.

Here are two pictures showing the board installed inside the computer. The board is mounted on the back side of one of the unused drive bays. The fan cables are tie wrapped in place so as not to impede air flow.

Here are a few close-up views of the circuit board. Although the header includes four 10k resistors, only three of them are actually used. Remember that you can click on any of these pictures to view a higher resolution version.