To calculate the resistor you need (to put in series with the +12V wire), first determine the fan's voltage and power draw (usually listed on the fan itself). (I say "pulses" because the tachometer pin is pulled high by the motherboard, and every time it is "pulsed", the fan pulls the pin to ground, or 0V - and this is why you can't put a resistor on the ground wire if you want to slow the fan down). The motherboard (and your hardware monitoring software) can then infer the speed of the fan from the rate of these voltage "pulses". According to the fan specifications I linked to above, the standard is to provide two "pulses" per revolution. Since it is tied to the same power rail as the motherboard, there is no need for an additional ground wire. This is the classical noise versus heat debate all over.įor those wondering why a fan with no speed control even has three wires, the third wire is used as a tachometer output signal. Slowing down your fan will lower the efficiency of your heatsink's ability to dissipate heat. If you do choose to slow your fan down (usually for noise purposes), do ensure that your load temperatures do not get too hot. Alternatively, you could use a potentiometer (and use these calculations to provide a rough estimate of the range of resistance needed). If you're interested in determining how to slow your fan down with a resistor (it's honestly pretty easy), I have provided calculations at the bottom of this answer. You could also install a hardware fan controller (which is also essentially a resistor, albeit a variable one called a potentiometer). Due to the tachometer sensor discussed in the next section, you cannot put the resistor on the ground wire. Alternatively, you can purchase a LNA (low-noise adapter), which is essentially the same thing (although it saves you soldering in a resistor yourself). If you wish to slow your CPU fan down, you can install a resistor along the the power wire (+12V) in the fan. PWM fans require explicit support from both the motherboard and fan itself. A three-pin fan connector does not include the ability to control speed dynamically, since it lacks the appropriate PWM control logic in the fan itself.
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