[SOLVED] Can PWM Fan Control mode control DC Fan?

[PeacefulTech]

Hi guys,

Although my mobo can switch between PWM/DC fan control mode, it cannot do so between fans connected to the CPU_FAN and CPU_OPT header. In other words, either both of these fans are in DC mode, or both are in PWM mode.

I have a PWM fan connected to CPU_FAN and a DC fan connected to CPU_OPT. I was wondering whether any fan control to the DC can would occur if I select the fan control mode to be PWM.

I also wonder what would be the optimal fan control profile I should set here? Should I should DC or PWM?

Thank you!

 

[Darkbreeze]

What is your motherboard model?

 

[PeacefulTech]

What is your motherboard model?

ROG Strix B550-F Gaming

 

[Paperdoc]

To your question, no. Any 3-pin fan connected to a header using the PWM Mode will always run full speed because of the differences in design.

There is another option. All 4-pin fans have a backward compatibility feature - their speed CAN be controlled by a header that uses the older 3-pin DC Mode, although that is not the ideal way. So you can control both fans if you set the two headers to DC Mode.

By the way, you confuse Mode and Profile. Profile is the strategy the header uses to decide what speed the fan should be running, depending on the temperature measured by a sensor. It has options like Standard or Normal, Turbo (constant max speed), Quiet (constant low speed), and Manual or Custom that allows YOU to set the "fan curve" of speed vs. temp. After that decision is made, the MODE sets the type of electrical signals sent out to the fan from the header to achieve the speed that is decided - PWM signals, or varying DC voltage signal.

Question: are both of these fans somehow dedicated to cooling the CPU chip? It's unusual to have two different fan designs for that, but possible. But IF one of those fans is doing a different job like case cooling, then it should NOT be connected to that header - the CPU_FAN and CPU_OPT headers are guided by the temperature sensor INSIDE the CPU chip.

 

[Darkbreeze]

To your question, no. Any 3-pin fan connected to a header using the PWM Mode will always run full speed because of the differences in design.

There is another option. All 4-pin fans have a backward compatibility feature - their speed CAN be controlled by a header that uses the older 3-pin DC Mode, although that is not the ideal way. So you can control both fans if you set the two headers to DC Mode.

By the way, you confuse Mode and Profile. Profile is the strategy the header uses to decide what speed the fan should be running, depending on the temperature measured by a sensor. It has options like Standard or Normal, Turbo (constant max speed), Quiet (constant low speed), and Manual or Custom that allows YOU to set the "fan curve" of speed vs. temp. After that decision is made, the MODE sets the type of electrical signals sent out to the fan from the header to achieve the speed that is decided - PWM signals, or varying DC voltage signal.

Question: are both of these fans somehow dedicated to cooling the CPU chip? It's unusual to have two different fan designs for that, but possible. But IF one of those fans is doing a different job like case cooling, then it should NOT be connected to that header - the CPU_FAN and CPU_OPT headers are guided by the temperature sensor INSIDE the CPU chip.

This.

And I'll add a little and say that IF the fan connected to the CPU OPT header is NOT being used specifically for CPU cooling then not only is it not advisable for it to be connected there but it will also likely drive you nuts because unlike the other fan headers when set to the motherboard "source" anything set to use the CPU thermal sensor is going to continuously and rapidly ramp up and down any time there is any kind of load on the CPU.

If you really need to use the CPU OPT header because you are using a dual fan air cooler or something along those lines you would be a lot better off to simply get a second fan identical to the primary CPU cooler fan that is also a PWM model and resolve many problems all at once.

 

[iPeekYou]

Hi guys,

Although my mobo can switch between PWM/DC fan control mode, it cannot do so between fans connected to the CPU_FAN and CPU_OPT header. In other words, either both of these fans are in DC mode, or both are in PWM mode.

I have a PWM fan connected to CPU_FAN and a DC fan connected to CPU_OPT. I was wondering whether any fan control to the DC can would occur if I select the fan control mode to be PWM.

I also wonder what would be the optimal fan control profile I should set here? Should I should DC or PWM?

Thank you!

Basically, if you have only 4-pin fans on a header (can apply either to a single fan connected directly or a fan hub with multiple fans with all 4-pin units), use PWM.

If there is a 3-pin fan present, ideally you'd want to put the 3-pin fan(s) on a separate header from the 4-pin ones. Use DC control for that.

PWM control on DC fans? Fans run at 100% all the time
DC control on PWM fans? Works for a while, it'll eventually kill the PWM control unit inside the fans.

In theory, DC is better since it saves power, however minuscule, since it applies varying voltage to control speed. However, PWM fans tend to have the ability to spin lower at their minimums compared to 3-pin units. Both models of fan usually have starting voltage of 5V (7V if there's lighting on the fan), and it takes about 40% of so at the BIOS fan curve to start the fan. PWM fans don't care about that, they receive full 12V all the time, and modulate their speed according to PWM signal.

I do find it strange that your board cannot separate control modes for different fan headers. My board is older model, and a bottom of the barrel B350 board. I use DC on SYS_FAN since the hub calls for it, and PWM for CPU_FAN.

 

[Paperdoc]

The oddity of having two fan headers tied to the SAME configuration settings is understandable in OP's case because these two are BOTH CPU cooling headers. CPU_OPT is just a twin of CPU_FAN.

 

[Darkbreeze]

And the behavior is different from brand to brand and board to board model. Some of them simply duplicate whatever configuration the CPU_FAN header has, while other boards allow for independent controls on that header.

My much older Hero VIII for example WILL allow you to change both the type, DC or PWM, and the source, so that that header can be used with case fans or whatever, based on feedback that is not from the CPU. I've seen other boards like this too and also a great many where whatever is going on with the CPU_FAN header is exactly what you'll get for any fan connected to the CPU_OPT header.

 

[iPeekYou]

Ah I see. Haven't had many ASUS boards coming my way --at least not the modern UEFI boards. Last one was an LGA775.

Thanks for the info @Darkbreeze @Paperdoc

 

[Darkbreeze]

If ever you have specific questions, most of us, to the extent we are ABLE to answer them, are always willing to so as info is always free. Well, maybe just a little time involved occasionally, but what the heck.

 

[Paperdoc]

Agreed. As a small bonus, here you'll get reactions form several people with different experiences, so the total picture is more complete.