Where on the mobo do I connect the PSU fan sensor?

[BernardP]

It's probably because the question is so basic that I can't find the answer.

I am getting an Antec Eartwatts 380 replacement PSU for my mATX system and there is a 3-pin fan speed sensor on it. I have a MSI K8NGM2-FID motherboard. Here is the layout:

K8NGM2-FID Layout

And here is what the motherboard manual says about the Fan Power Connectors:

Connectors

I have circled the two fan connectors on the layout. The manual says that the power connectors support system cooling fan with +12V. My understanding is that if I had a 3-pin fan, I could connect it on either SYSFAN1 or NBFAN1 to make it run. (True?)

These connectors are power sources. But I have the 3-pin fan sensor from the power supply that I would like to connect somewhere on the mobo, so that the MSI PC Alert utility that is already running on my system can read the PSU fan speed. But where? There is nothing but these two fan connectors.

The PSU fan doesn't need power from the board. Do SYSFAN1 and NBFAN1 also serve as fan speed sensors? What happens if I connect the 3-pin fan speed sensor from the PSU on SYSFAN1 ?

 

[zagood]

If I remember correctly, the "fan only" header you see on that psu is actually meant for you to plug case fans into...then the psu controls the speeds of those fans. I don't think it has an actual fan speed sensor.

-z

 

[BernardP]

Here is a pic of the Fan Sensor (pop-up on Antec spec page). See the mention "3-pin fan sensor"

That's why I am asking about this. It seems to be a sensor, not a power plug.

3-pin fan sensor

 

[ChefJoe]

Your power supply has an RPM meter/sensor in the fan. If you plug it into the motherboard it will be able to detect the RPM of the fan in the power supply but not control it. You should be able to use either the NBFan (northbridge fan... likely unused) or the system fan header on the motherboard. I would leave the CPU fan header for the processor's fan.

Zagood - you're thinking about the 4 pin molex fan headers for powering 4 pin fans at a speed controlled by the power supply.

 

[Zepper]

If it is a fan speed sensor, there may be one or two wires, should be either just yellow or yellow and black. They should be in a normal three pin female connector and is connected to any system fan connector on the mobo. OTOH, if it is a Molex power connector marked fan only then it is to power fans as mentioned above - don't use it. Use the mobo fan headers or a separate fan controller.

.bh.

 

[BernardP]

Thanks for the replies ChefJoe and Zepper. If I may try to understand better...

Here is again the pic of the fan sensor that is on the PSU:

PSU Fan Sensor

ChefJoe, when you say that the power supply has an RPM meter/sensor in the fan and that if I plug it into the motherboard, it will be able to detect the RPM of the fan in the power supply... What does it stand for? Do you mean plugging the 24-pin ATX connector into the motherboard? Or do you mean plugging the fan sensor pictured above into SYSFAN1 or NBFAN1 on the motherboard?

Zepper, as you see from the above pics, there are two wires leading to the sensor: black and blue. So if I connect this to the SYSFAN1 or NBFAN1 fan connector on the mobo, these would then act as sensors, not as power connectors? Do I undestand that SYSFAN1 or NBFAN1 are at the same time acting as fan speed sensors and fan power plugs? For example, if I plug my 3-pin case fan into NBFAN1, not only will it power the fan, but it will sense its speed for display in PC Alert (True?).

Also Zepper, although there doesn't seem to be a "fan only" connector coming from the PSU in my situation, why do you say not to use it if there is one, and to use a connector on the mobo itself? What is the downside of using a dedicated fan power plug from the PSU?

Thanks for your patience. Lots of questions, but I would like to fully understand this.

 

[Zepper]

Well, your wires are blue and black instead of yellow and black - fan speed wires are inconsistent in color. NBFAN is for northbridge fan and may not have the speed sensing as most fans used on NB coolers have only two wires it would be nice if there is speed sensing too. The SYSFAN connector should have speed sensing. The PSU fan is powered and controlled internally that is why only the blue and black are present (so you have a way to know if the fan is working so you can fix it before your PSU burns up (I know, I almost burned one of my PSUs up because of a loose connection - luckily I have a good sense of smell) - the red wire (usually the color of the fan power wire) is not there. Too bad there are only two fan headers - very chintzy. If the NB header has sensing, use that for your PSU fan, use the sysfan header for your main case fan (normally the exhaust). If only one header has sensing, then leave the PSU sensor disconnected. Another option is to get a separate fan controller that reads the fan speeds too. Those aren't cheap but may be worthwhile for you. I wouldn't be without mine. You can have four or more fan channels on those.

The FAN ONLY power connectors are controlled by the temp of the PSU which makes little sense for fans located elsewhere. The SYSFAN header will be controlled relative to the SYSTEMP which is reported in your monitoring software. IDK what temp controls the NB header, if any. Those are designed to keep temps safe, not optimal. That's why I use a separate fan controller.

.bh.

 

[BernardP]

Originally posted by: Zepper
Too bad there are only two fan headers - very chintzy. If the NB header has sensing, use that for your PSU fan, use the sysfan header for your main case fan (normally the exhaust). If only one header has sensing, then leave the PSU sensor disconnected

The mobo has only 2 fan connectors, but it is after all a basic mATX board. There is no way to know if NBFAN or SYSFAN has sensing: the mobo manual doesn't say. I will have to try connecting them and see what happens.

So I am right to think that the SYSFAN1 connector can be used as sensor-only. And I am right to think that:

--- I can connect the case fan on NBFAN1 and
--- I can connect the PSU sensor on SYSFAN1

Without risking burning-up anything...

In the best scenario:

--- NBFAN1 will be able to spin the case fan AND will have sensing, so I will see case fan speed in PC Alert
---SYSFAN1 will provide sensing info to PC Alert about the PSU fan.

In the worst scenario:

--- NBFAN1 will spin the case fan BUT won't have sensing
--- No PSU fan speed info will show in PC Alert, even if the PSU sensor plug is connected into SYSFAN1, if SYSFAN1 doesn't have sensing.

What I don't want to do is mistakenly send current up into a sensing connector and risk harming the PSU fan itself. Is there a risk of this happening?

 

[Zepper]

There should be a page in your BIOS setup that shows the current readings for any sensors your mobo supports including temps, voltages and fan speeds. The voltage line is the middle pin and the headers and female connectors are keyed so you can't connect them wrong without forcing the connection.

.bh.

 

[BernardP]

Thanks Zepper. I am gonna check the BIOS. So I had understood things correctly in my previous post...

 

[BernardP]

Just a quick follow-up:

I got my new Antec Eartwatts 380 PS and installed it. Contrary to what is shown on the Antec web site, there is no 3-pin fan sensor included, so the PS fan speed doesn't show in PC Alert.

I also installed a new Scythe 120mm 1200 RPM case fan and this comes with 3-pin connector and 4-pin adapter. I plugged the 3-pin directly into SYSFAN1. PC Alert shows the fan is running at 1240RPM.