An easy to construct mod for a Globe 120mm fan(SL1202512L-3M).

This fan usually idles at 1400rpms (12v).....and goes up to 1500rpms with a 10C temp rise. That's about as high as I've ever seen it at max CPU usage.

With this mod I can force the fan to max rpms by temporarily shorting the sensor wire, using a DigitalDoc5 and a 12v automotive relay. I'm using this system to temporarily speed up the CPU fan to 1700rpms, when the DD5 measures a temp rise of about 4C over the CPU's normal max. Should something abnormal happen, and the temp start to rise, the CPU fan maxes out. The fan's own sensor won't normally do this till the temp is way too high.

The DD5 sensor and the fan sensor are put in the same spot...When the DD5 sees a high temp, it sends a 12v signal to this small relay, which then activates and temporarily shorts out the fan sensor wire....and the fan maxes out. When the temps are normal, the DD5 de-activates the relay, and the fan returns to normal operation.

With this relay/dd5 system in place, the CPU fan now has an automatic range from below 1000rpms to 1700rpms. I control the voltage going to the fan with a PWM controller.....which is not really necessary for this mod, but works for me. Speed fan could also do this.

I know it's hard to explain...

Three photos