Using two different fans from a Y-splitter

[thewhat]

The motherboard only has one 3 pin connector, where the fan speed varies.
So my plan is to use that connector for both the CPU fan and a case fan, with a splitter like this one:

But they are different fans, size wise and everything. The CPU fan is 9cm in diameter and the case fan is 12cm. They are also rated at different RPMs.

So:
1. Could anything go wrong?
2. Will they both operate at the same RPM or not?
3. Assuming not, is there any way to predict the RPM of the fans? Will perhaps both operate at the same percentage of their rated speed?
4. The case fan has an _additional_ speed control from the case itself (with a knob). Would it be ok to use that, too?

 

[EarthwormJim]

Motherboard fan headers are usually pretty underpowered. They're usually sized to supply just enough power for one fan.

I would not run two fans off of one unless you can pull up some current specs for the header.

The fans would only operate at the same RPM if they were identical. You'd have to graph RPM vs. Voltage in Excel to predict what their respective RPMs will be. It's not typically a linear function either, so 50% of voltage will not equal 50% of RPM rating.

I'm confused on the last point. Are you saying there's a little fan controller built into the case that's powered by the PSU?

 

[BoomerD]

Not sure about the difference in fan sizes, but I run two Noctua fans for my CPU on the same header with no problem.

 

[thewhat]

Yeah, I've never heard of people having any issues with running two fans like this. Splitters are rather common. I'm not planning on using any power hungry fans, either.

I'm confused on the last point. Are you saying there's a little fan controller built into the case that's powered by the PSU?

Well, not really 'powered' by the PSU. It's a knob on the side that I guess regulates the voltage of the case fans.
The supplied case fans split their 3 wires into two separate connections: One (yellow+black wire) goes to the case speed controller and the other (red+black wire) goes to either the molex power adapter or to the motherboard's fan pins. So if the motherboard provides some control, I can control the fan in two ways.
I'm not sure how exactly this works, but it does.. I'm assuming the black wire is for voltage control. (link to the case)

 

[EarthwormJim]

Yeah, I've never heard of people having any issues with running two fans like this. Splitters are rather common. I'm not planning on using any power hungry fans, either.



Well, not really 'powered' by the PSU. It's a knob on the side that I guess regulates the voltage of the case fans.
The supplied case fans split their 3 wires into two separate connections: One (yellow+black wire) goes to the case speed controller and the other (red+black wire) goes to either the molex power adapter or to the motherboard's fan pins. So if the motherboard provides some control, I can control the fan in two ways.
I'm not sure how exactly this works, but it does.. I'm assuming the black wire is for voltage control. (link to the case)

It's either a rheostat or a linear voltage regulator. Of course the fan controller is powered by the PSU. Those aren't PWM fans either, so that controller directly supplies power to the fans if you use it.

Black is normally ground, yellow is the RPM sense and red is +12V.

I would still be careful unless you can pull up some current ratings for your motherboard headers. They tend to not be very reliable in my experience and usually top out at 1A. Two fans starting up can exceed 1A.

 

[BoomerD]

Doing some reading, my ASUS P7P55D-E Pro board manual says the CPU fan header supports up to 2 amps (24 watts) of fan power.

The Noctua fans I use are listed at 0.09a, or 1.08 wattts.

http://noctua.at/main.php?show=productview&products_id=12&lng=en&set=1