Will pwm splitter between cpu fan and case fans limit max rpm?

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huxley141

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Sep 25, 2017
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Hi,
I have an Asus M5A78L-M/USB3 mobo which has 1 x 4pin CPU fan and 1 x 3 pin case fan connector.
I'd like to run the CPU fan + 2 x pwm case fans (both as intake) using this splitter cable, and use the 3 pin connector to run the exhaust fan (3 pin):

http://gelidsolutions.com/thermal-solutions/accessories-pwm-1-to-4-splitter/

The max rpm of the 3 pwm fans are:

CPU - 2500
Case fan 1 - 1450
Case fan 2 - 1200

My question is: will the rpm of any fan be limited to a speed < 100%, due to their relative differences in max rpm?

I understand that since only one of the splitter cables has all 4 wires, this will connect to the cpu fan, and the other splitters (minus the tach wire) go to the case fans, and the mobo will control the speeds of all 3 based off the feedback from the cpu temp.

In other posts I've read that pwm works in pulses and that if the main fan is operating at 80% then all other fans will be at 80% too. If this is correct, then neither of my case fans would get close to reaching 100% rpm which sounds very counter-intuitive.

Can anyone clear this up in as close to noob / layman terms as possible, please?
 
Solution
clutchc
The "master" fan in that set of splitter cables will provide the only tach feedback. That is why it is best to have like fans so you can assume they are all at the same RPM.

In spite of their rather odd description given on the webpage referencing the CPU fan header, that splitter is usually used to control 4 x PWM fans from one 4-pin SYS FAN header... not the CPU FAN header. Although I suppose it can be used with the CPU FAN if desired. The fans will run at the pulse width modulation (RPM) of the MB header they are connected to, as set in BIOS. If that header is the CPU FAN header, the BIOS will send the same signal to all fans connected to that header as required to keep the CPU at the preset temp. You can't control the fans...
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clutchc

clutchc

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You are basically correct. That splitter (as explained) will allow all the fans to be controlled by the BIOS's regulation of the CPU temp. So as the CPU cooler fan goes, so go the rest of the fans. The Molex connector is simply there to carry the +12V load so that the CPU FAN header does not need to carry the total load.
 
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huxley141

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Sep 25, 2017
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Thanks for the reply, Clutch.
What I'm struggling to get my head around is will all three fans work to the same % of their respective maximum rpm, or will BIOS allow them to keep approx pace with each other? I get that the case fans won't have a tach feedback so BIOS won't be tracking their actual rpm, but if they all spin to the same max % that would mean the cpu fan always spins twice as fast as the case fans.

Ideally, I'd like for the two case fans to hit max rpm by the time cpu fan is at ~60% (which would equate to ~1500). Otherwise, it'll be simpler for me to run both through the psu and be at 100% all the time, leaving BIOS to only worry about the cpu fan.

 
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clutchc

clutchc

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The "master" fan in that set of splitter cables will provide the only tach feedback. That is why it is best to have like fans so you can assume they are all at the same RPM.

In spite of their rather odd description given on the webpage referencing the CPU fan header, that splitter is usually used to control 4 x PWM fans from one 4-pin SYS FAN header... not the CPU FAN header. Although I suppose it can be used with the CPU FAN if desired. The fans will run at the pulse width modulation (RPM) of the MB header they are connected to, as set in BIOS. If that header is the CPU FAN header, the BIOS will send the same signal to all fans connected to that header as required to keep the CPU at the preset temp. You can't control the fans independently.
I hope I am answering your question. If not, I'm not following I guess.
 
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