fans wont run at max rpm

nick209 · July 18, 2017

I just got 2 corsair af140's mounted them in the front of my nzxt s340elite and plugged them into my asus prime z270a. when I went into the bios to mess with the fan curve I decided to set them to max just to see how loud they could get and they were only going to 950 to 980 rpm. I thought they were suppose to top off at about 1200 rpm

Windows7ge · July 18, 2017

From my experience motherboards don't really put out the 12V the fans need even when you set the fan speed to "max" now I haven't put a multi-meter to the fan pins on a motherboard (although I now wish to try) so I could be mistaking voltage limiting for current limiting and it may be limiting the current, not the voltage but from what I've seen most motherboards actually run the fans between 25%~30% up to 80%~90%. It may have something to do with all the components in the vicinity drawing power. When the controller sends 12V to the fan pins there may be a voltage drop as it travels across the board or other components eat some of the power as it travels and current limiting prevents the PSU from compensating.

This is all theory. I have no evidence to back up any of it.

WoodenMarker · July 18, 2017

Can you post a photo of the settings you're using? Are you sure you're using the settings for the correct header?

Windows7ge · July 18, 2017

20 hours ago, nick209 said:

I just got 2 corsair af140's mounted them in the front of my nzxt s340elite and plugged them into my asus prime z270a. when I went into the bios to mess with the fan curve I decided to set them to max just to see how loud they could get and they were only going to 950 to 980 rpm. I thought they were suppose to top off at about 1200 rpm

I decided to follow through with the tests I said I wanted to conduct.

I put a multi-meter to the fan header pins on an ASUS Sabertooth X79 with the BIOS set to run it at 100%

With no fan connected the voltage measured was: 12.18V

With the fan plugged in the voltage measured was: 7.58V~7.91V

I then tested a direct connection to the PSU.

The 12V rail from the PSU measured: 12.3V

I then put a fan in parallel with the probes to measure the power. Results were: 12.27V

This supports my theory. A direct connection to the PSU allows the fan to pull as much current as it needs. However when attached to the motherboard there appear to be limiters that prevent the motherboard from compensating with the load increases on the fan header. It IS sending a full 12+V out the header but once the load is placed on the circuit it pulls the voltage down and something isn't letting the PSU compensate. I think it is some type of built in protection to prevent the motherboard from drawing too much current. On my MSI X99A SLI PLUS motherboard I know power is being shared between the PCI_e lanes and the fan headers on the motherboard because when I load up the graphics cards there is an audible decrease in the connected fans. When I release the power hungry cards from their load the fans on the motherboard audibly increase again.

In conclusion I don't think there's much you can do about it. Unless you want to obtain a modded version of your BIOS that allows you to over-volt your fans (Which may not even be possible due to the 12V limit). The only way to get 100% out of them is to plug them directly into your PSU.