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Motherboard bypassing resistor?

AlexiTQ
Level 9

‎09-14-2017 03:51 AM - last edited on ‎03-05-2024 09:52 PM by Community Admin

I recently installed an AIO water cooler on my X99-system (Asus X99-A motherboard with latest BIOS). The pump is connected to the CPU fan header through a resistor supposed to decrease the electric potential to 9V.

At 9V the pump should run at 1800 rpm. Setting full speed in the BIOS Q-fan control, however, gives 2400 rpm. This is the 12V speed of the pump. At full speed it sounds like the pump is actually going beyond that speed (haven't yet checked this in speedfan - will do so when I get home).

Is the motherboard increasing the current and/or electric potential when resistance is applied?
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FaaR
Level 7

‎10-14-2017 08:38 AM

AlexiTQ wrote:
Is the motherboard increasing the current and/or electric potential when resistance is applied?

No, this is not possible. 12V is the max rated output on the fan headers, it's also the max voltage supplied to the mobo itself; fan header power supply pin feeds off of the 12V supply, either direct connection or via the system's fan controller chip, which at least on high-end motherboards is often able to lower voltage by itself through software control without using in-line resistors.
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