Pwm fan with power connector thermalright silver arrow

[gunslingor]

Found this odd preparing to install:
http://www.thermalright.com/installation/Silver%20Arrow%20SB-E%20Extreme/Silver%20Arrow%20SB-E%20Extreme%20installation.pdf

4 pins, 2 to the mobo and 2 to directly to the psu? Shouldn't all wires go to the mobo?

 

[adycopilu]

You have one CPU fan header on the mobo, but in reality you have 2 CPU fans. So you could connect one of the fans to the mobo, and the other one to the PSU. This way, you would command the rpm of only one fan (the one connected to the mobo, by PWM), the other one would be at full speed all the time.
So the solution is to use an Y-cable and connect both fans. Mobo will send the PWM signal to both fans and both fans will operate at the same speed all the time. However, it's not possible to get rpm readings from both fans (pin no.3) due to electrical reasons, so actually only one of the fans will report back the speed. If you check this picture (http://www.xbitlabs.com/picture/?src=/images/coolers/thermalright-silver-arrow-sb-e/02_tr-sbe_acc_big.jpg), you can notice that one header has the 3-rd wire cut out. The fans are identical and ordered by the same PWM signal, meaning that they will rotate roughly at the same speed, so you can read only one of them, the other will not be far off. Speed sensor pin and PWM pin are hence connected to the mobo.
The other two pins are +12V and ground and could be connected to the motherboard, like you asked. However, that would put maybe too much stress on the fan header, which was designed to provide power to only one fan. The PSU connector on the other hand can provide much more power and should cover the needs for 2,3,4 or even more fans.
So this solution is a workaround, it doesn't matter where do you take the power from. Actually, the fans can be connected to any +12V source, as long as it can provide enough Watts (a few Watts per each fan). The connection is correct and it will work flawlessly.

 

[gunslingor]

Interesting, actually my mobo has a "CPU fan" and "CPU opt" (option) so I think I could plug em both in and see the readings, maybe. I guess it'll take some testing.... I do have a big ass mother of a heats sink and fans not sure but suspect its correct to power them off the power supply, they go to 2600 rpm.

 

[adycopilu]

I think CPU fan header is a standard 4-pin header (PWM enabled), and the CPU-out header has just a PWM-pin and maybe a speed sensor pin (if there are any other pins on this header, maybe they are just dummy pins).

 

[ko888]

Interesting, actually my mobo has a "CPU fan" and "CPU opt" (option) so I think I could plug em both in and see the readings, maybe. I guess it'll take some testing.... I do have a big ass mother of a heats sink and fans not sure but suspect its correct to power them off the power supply, they go to 2600 rpm.

THE CPU_OPT header is also PWM controlled and is tied to CPU_FAN according to ASUS. The CPU Fan headers are designed to handle +12V @ 1 Amp.