PWM control of two fans through splitter?

[jliu1052]

I have an ASUS z270-a, which has two PWM pins for controlling case fans. Currently, this is controlling two 3 pin fans using voltage control for speed through Asus' QFan.

However, I plan to switch over to four PWM fans using two PWM splitters (each splitter connects to two PWM fans). Will the ASUS BIOS still be able to control the fan speed based on core temp?

 

[Paperdoc]

Yes. The mobo web page says every fan header on the board supports BOTH fan designs and includes an auto-detect function to detect which fan type you have plugged in and adjust itself. IF that does not appear to work, there is a way for you to set it manually in BIOS Setup See your manual, p. 3-7, Section 3.2.3. It shows the QFAN control screen. There you can select the particular fan header you have plugged your fans into. Then at upper right you can change the Mode setting from "DC" (used for 3-pin fans) to "PWM" for your 4-pin fans. Remember to SAVE and EXIT when you have finished your changes.

 

[jliu1052]

Yes. The mobo web page says every fan header on the board supports BOTH fan designs and includes an auto-detect function to detect which fan type you have plugged in and adjust itself. IF that does not appear to work, there is a way for you to set it manually in BIOS Setup See your manual, p. 3-7, Section 3.2.3. It shows the QFAN control screen. There you can select the particular fan header you have plugged your fans into. Then at upper right you can change the Mode setting from "DC" (used for 3-pin fans) to "PWM" for your 4-pin fans. Remember to SAVE and EXIT when you have finished your changes.

And this works even through a splitter? Will the bios detect that there are four fans through two heads? Or will it just be able to power and control them properly?

 

[Paperdoc]

Yes, it works. For each mobo header with one Splitter and two fans, the header will detect only ONE of those two. The header really has no way to detect how many fans are connected. Its speed measuring system counts a series of pulses sent back to it from the fan motor on Pin #3. But that system cannot deal with two pulse series that overlap, so the Splitter will send back to the header only ONE fan's signal. The other's signal will be ignored entirely. The has no impact on control of the fans. On each header / Slitter combo, the two fans merely share the signals provided by the mobo and do the same thing.

The control system really is a TEMPERATURE control system. It gets a temperature measurement from an appropriate sensor (for the case fan CHA_FAN headers, this is a sensor built into the mobo), compares that to a target the BIOS knows, and adjusts the signals to the fan as appropriate. Its does NOT aim for a fan speed. It does manipulate the fan speed, but its target is the temperature at the sensor, and it will change fan speed however it needs in order to achieve that target. It actually does NOT even use the speed signal for control purposes. The speed signal IS used for another purpose - failure detection. If the header fails to get a fan speed signal, it sends out a message about fan failure. In the specific case of the CPU_FAN header, some mobos actually will shut down the system for protection if that CPU fan fails. But that is not done for case fans.

In a 4-pin fan system, the signals from mobo header to fan are the Ground (Pin #1), +12 VDC supply (Pin #2) and PWM signal (Pin #4). Each fan uses the PWM signal with a small chip in the fan case to alter the flow of current from the +12 VDC supply line through the motor windings, this altering its speed. A Splitter merely shares all three of those signals to both its fans, so both can do exactly the same thing. A HUB is a different device that arranges to get the +12 VDC supply from a different source that can provide more current to many fans (not needed for 2 fans). It can be identified because it has an extra "arm" that must plug into a power output connector from the PSU. Unfortunately, many sellers use the label "Splitter" also for Hubs. You do NOT need the Hub style device (with the extra arm to the PSU) to connect two common fans to a single mobo header.

 

[Paperdoc]

Thanks for Best Solution.