Mixed RPM on single PWM splitter

xomaxhox · April 13, 2019

I know this question has been asked a fair few times before from researching it, but there doesn't seem to have been a clear, definitive answer.

I've got a PWM fan splitter that's powered via molex, coming from a single CPU_FAN header.

I'm planning on hooking up 3 different types of fan to it, which each have different max RPMs.

As PWM is based on % rather than speed, this should be fine and I won't be running any single fan over its max RPM, right?

E.g. If I have the BIOS set to 50%, the 1200 max RPM fan will run at 600, whilst the 2000 max RPM fan will run at 1000?

Furthermore, does it matter which fan is hooked up as the feedback? As in which RPM fan tells the mobo it's current speed.

Skiiwee29 · April 13, 2019

It will run them all at the same % regardless of the RPM levels. So 30% on 1500 rpm or 30% on 2200rpm will be 30% of the maximum RPM the fan can run.

LogicalDrm · April 14, 2019

Speed of fan is irrelevant overall. Both PWM and DC controlling is done with % of fans max speed. With DC it means that fan connected to rpm sensor header is used as baseline. Controls are set for it, but only in sense where you setup control based on speed of that fan. All connected fans will receive same amount of voltage that would correspond to master fans rpm. Same with PWM, except that fans are already getting full 12V and % just corresponds to how many pulses fan will get per second.

Overall, motherboard heder will only supply 12V max. You can't run fans over their rated rpm unless you give them more than 12V.