Building a PC: fan hub hookup questions., PWM vs DC etc.

May 7, 2023 6:43 PM

I'm building a PC with Larry David Syndrome, Jr. This is my first build, although I am generally experienced with electronics. We're in the final stages of the build and are hoping for some help to make it over the finish line! The motherboard is a MSI MPG X570. The case is a Fractal Define 7 with a Nexus +2 Fan hub. Here are the pages from the case and motherboard manuals which pertain to fan connections.

I'm confused about how the fans and fan hub should be connected. We've got the case fan hub connected to the power supply, the case fans are connected to the hub. The fan hub also has a sensing wire which, per the case's manual, can go to either the CPU_FAN or SYS_FAN connectors on the motherboard. Presumably this is to allow the mobo to control fan speed.

Questions: should I connect the sensing wire from the fan hub to the CPU_FAN1 connector or the SYS_FAN connector on the motherboard?

Should the actual CPU fan also be connected to the fan hub (there's a designated connector for it) or be connected directly to the mobo?

According to the Mobo manual, the CPU_FAN1 connector is the "auto detection mode fan connector" (I guess to decide if it goes into PWM or DC mode?) The fan hub specifically references PWM, so that's the mode we'd like it to default to.

Thanks in advance for any advice!

posted by Larry David Syndrome to Computers & Internet (6 answers total)

I think whatever you connect to the "CPU_FAN Or SYS-FAN" port on the fan hub controls the speed of all the physical fans connected to it.

It's normally better to have different speeds for the CPU fan and the case fans because the CPU can heat up way faster than the whole case.

So if it were my PC I would probably just have the CPU cooler fan connected directly to the motherboard, and then connect the MB SYS_FAN1 connector to the fan hub so I could control all of the case fans running from the hub using whatever MSI utility.
posted by samj at 1:12 AM on May 8, 2023 [1 favorite]

With 3-pin and 4-pin fan connectors, the third pin (yellow wire) signals the fan speed TO the controller on the motherboard, so that it can detect too slow rotation or total fan failure (dust, cat hair, gerbil vomit, etc). It's not clear from the fan hub manual if the connector that should go to the CPU_FAN or SYS_FAN header is fully wired or just a yellow (and possibly a black) wire.

When you are hooking up the CPU fan via the fan hub I would connect the hub's sense connector to the CPU_FAN header, as that is the one fan any mobo WILL want to see rotating or else it will power down again before the CPU releases its Magic Smoke. But the CPU fan directly to the mobo, the other fans to the fan hub and its sense cable to the SYS_FAN header would be how I'd do it.
posted by Stoneshop at 1:51 AM on May 8, 2023 [1 favorite]

Yeah, the documentation isn't the best written. Made even more confusing because your mainboard basically has a fan hub built-in. Here are the basics.

3-pin fans basically have power / ground / tachometer

4-pin fans basically have power / ground / tachometer / PWM (speed control)

PWM mode = full control over speed and RPM verification

DC mode = limited control over speed and RPM verification

So the hub needs a connection that tell it how fast to turn (Connect to CPU_FAN or SYS_FAN), and a connection on what RPM to report back (RPM value reported to Motherboard).

I dug this up from Fractal's website:

How does the fan hub in the case work?

The Nexus+ 2 fan hub is a PWM fan hub which distributes the signals from the main header to the rest of the fans, making them go at the same percentage of their maximum speed.

The main header is marked in the manual, and on the hub, it's marked as FAN1. This needs to have at least 1 PWM fan connected to it in order for you to control the fans on the hub, considering it's the only header that receives and sends signals to the motherboard. It does not need to be the CPU-cooler connected to the header, but it has to be a 4-pin PWM unit. You don't have to connect the hub to the CPU_FAN header either, but have it on a 4-pin PWM-header(avoid the AIO_PUMP header, since it's sometimes set to run at 100%).

It is not possible to control the fans on the hub separately. They will all be run at the same percentage.

So, I would connect CPU_FAN1 on the mainboard directly to the CPU cooler.

Personally, you can skip the fan hub altogether and connect ALL the fans to the various fan headers on the mainboard itself, as it seems to support 1 CPU cooler and up to 5 more fans. Just remember to go into the BIOS and change the SYS_FANx to PWM if you did connect 4-pin fans to them. This allows motherboard direct access to all of the fans to monitor their health and thus, finer control over the cooling (i.e. optimize between noise and cooling)

You could use the fan hub, which is basically a splitter: instead of controlling ONE fan, you're now controlling 3-4 fans. You lose the individual control over the fans, but you gain a bit of convenience: instead of manage 3-4 fans you now manage them though a single connection.
posted by kschang at 3:42 AM on May 8, 2023 [1 favorite]

Hook the CPU fan to the motherboard connection for it. It's critical and why run it through another set of failure points?

I'd put the hub on sys connection.

Everything will probably just work. But if it doesn't go into the bios and play with the fan settings.

The BIOS for x570 is old enough to be stable and fully featured so I suspect that it won't cause trouble but you may need to mess with it to get a good compromise between temps, sound and especially to prevent the case fans from ramping up and down maddingly.
posted by jclarkin at 9:16 AM on May 9, 2023 [1 favorite]

And I also would want to ditch the hub if not needed. It probably makes the wiring cleaner though.
posted by jclarkin at 9:17 AM on May 9, 2023

Response by poster: Update! I had already connected the CPU fan directly to the mobo and the fan hub to the SysFan connector but that was just a guess / intuition. It was very helpful to hear from others that it was OK to configure it that way. We installed the OS and got the PC running for the first time tonight. Fan activity seems fine (they're all running, not making too much racket) but we'll see how it does when the computer is pushed a little bit. My hunch is that the default fan settings will be fine.
posted by Larry David Syndrome at 7:31 PM on May 11, 2023