Hi guys, it's my first time using an aftermarket CPU cooler (Scythe Mugen 3) and also the first time I have a case fan that's pretty big (180mm on the Silverstone Temjin TJ08-E).

The first time I booted up the system (Asus P8Z77-M Pro) it said CPU fan warning.
I'm guessing the fan runs too slowly for the motherboard's liking? I went into the BIOS and it shows red, running at about 520rpm.

The case fan goes red when set to the low position and goes into the black when it's on high. Case fan is a "dumb" fan, so not BIOS controlled.

Can I set this to ignore the CPU fan warning in the BIOS? I set the "Low Limit" to 200rpm, so I assume the warning will go away. Obviously I don't want to fry a CPU. Temps seem fine at idle (29C/84F CPU).

Yes set both the CPU Fan Speed Low Limit and the Chassis Fan Speed Low Limit to 'Ignore'. This is an Asus BIOS quirk, there is no real advantage in having these limits sets, they are just warnings. It does mean that the BIOS will not show the actual fan speeds. On the other hand it does not interfere with Windows utilities such as Asus Probe or Fan Xpert showing the correct CPU and chassis fan speeds for fans plugged into the motherboard fan headers.

I just installed my ASUS P8Z77-M PRO with a Mugen 3 (Scythe 120mm PWM fan) and I'm seeing the same thing. The warning is easy enough to figure out, but why is it spinning the fan at just 430-500 RPM?

The case fans, two Fractal Design 120mm, which I believe are 1000 RPM fans, are spinning at about 940 RPM. Another Scythe 120mm PWM 1300 RPM case fan is at 1030 RPM, which also seems low. I assume the two FD fans are being run at a full 12V, so I'd expect the Scythe to be also.

I didn't see anything in the BIOS referring to PWM settings, or even RPM targets at all, just percentages. Haven't played with Fan Xpert+ yet.

Because with a cooler like the Mugen 3 the fan does not need to run any faster than that to keep the processor temperature stable. Like all recent systems your motherboard is automatically downclocking and undervolting the CPU under idle/low load conditions so there is no need for higher fan speeds. The Monitor menu in the BIOS offers the choice of Silent, Standard and Turbo profiles. Generally if you want a quiet system it should be enough just to set the Silent profile. This profile typically holds the CPU fan speed down to the lowest possible setting and only at around 50C does it start to elevate the rpm. So less noise but a degree of protection if for any reason the CPU gets hotter than normal.

Fan Xpert can over-ride the BIOS setting if desired, and there is a user controlled profile if none of the preset profiles is sufficient. There is also a manual option in the BIOS, these are the percentages you refer to and it is possible to trial a manual profile using Fan Xpert before transferring it to the BIOS. If the CPU is 40C or less the fan will run at the minimum setting set by the manual profile. Once it reaches 75C or higher it will automatically go to 100%. The percentage figures refer to PWM duty cycle, there is a fan profile facility in Fan Xpert which will show the approximate fan speed for each increment in the duty cycle.

The chassis fans are covered by a similar system. Again try setting the Silent profile in the BIOS for the chassis fans as a first step. As far as I can judge, setting the Silent profile for 3 pin fans is equivalent to setting the voltage to 7V. But if system temperatures increase more voltage will be applied. So in practice you get motherboard based thermal control in the 7V to 12V range.

OK, I've played around a bit with Fan Xpert+ now.


While that may be true, I don't believe it's the reason that Fan Xpert+ operates the CPU fan at 20% when idling. I think the reason has more to do with it assuming that the stock fan (which is what, 2000 or 3000 RPM?) is installed. That may be a reasonable assumption.

I'm still wondering about PWM control, though. When Fan Xpert+ is first run, it knows nothing about a fan's max RPM until the user manually runs a test at 100%. If that's true, then 20%, 50%, etc. are percentages of what exactly? I can only assume that it's actually doing voltage control of the CPU fan rather than using PWM, at least until it knows what RPM 100% is.

I'm a little surprised that in Fan Xpert+ the fan profiles can't be spec'd in RPM, instead of percentages.

The PWM % refers to the duty cycle, http://en.wikipedia.org/wiki/Duty_cycle, as lodestar pointed out; essentially 100% is constant 12 V and then it gets "chopped up" to achieve a drive similar to a lower constant voltage. The system doesn't need to be aware of fan RPM to adjust the duty cycle.

It is likely the pre-made profiles - profiles being the "reason" why fans get driven at their given RPM - are made for or at least tested on a specific type of fan, so YMMV if you don't use a fan with matching characteristics. You had the right hunch, guessing they probably used a high-RPM fan model as reference, as that would explain the low RPM on your low-RPM model using the hi-RPM model's profile.

Ah, I see. I thought PWM fan control actually monitored RPM and adjusted according. Sounds like there's not really a whole lot of difference between PWM and voltage controlled fans if the motherboard is capable of controlling either directly through the fan headers.

It's all still very "mechanical", both methods simply meting out the amount of drive given to the fan motor. The PWM method is supposedly more efficient, but when we're talking less than 1 W of power, I doubt the thermal impact or power loss is significant - PWM is probably just more convenient or cheaper to implement.

The smarts still have to come in chip form (or via software like Speedfan). On Asus Fan Xpert 2 boards there exists such a smart chip (and to a lesser extent on the Xpert+ boards), which we've been sorely missing since Abit's µGuru. I think there's an even older predecessor, but only the ancients know that one.

PS. I personally have a feeling the 12 V pulses would be better for starting up fans and keeping high-range models spinning at low speeds, but I'm not sure if I've read anything to back this up.