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Old August 29, 2017, 03:44 PM
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Default What effect does mixing different RPM ranges of fans on a PWM controller have?

Bit of a long shot to see if anybody knows the answer.....

Just ordered one of these silverstone PWM controllers ( https://www.amazon.ca/dp/B00VNW556I/...394800_TE_dp_1 ) along with a pair of Corsair ML120 fans.

So far my quick google searches have suggested that the RPM signal is taken off of the fan in position one with the rest of the fans being controlled based on that....

I'm hoping to connect the two 2K RPM ML120s as well as a third 2.4K RPM fan....

Am I correct in my understanding that the PWM signal from the #1 2.4K fan will result in a power profile which would correspond to a slightly lower RPM on the 2K fans but that at the higher end they'll end up maxed out? (Or conversely if I used one of the 2K fans in position one that the 2.4K fan would run slightly faster than the others but still hit the 2.4K upper range when the others are hitting 2K? ).
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Last edited by sswilson; August 29, 2017 at 04:02 PM.
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Old August 29, 2017, 05:00 PM
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This is a good question, I'm curious for the real answer as well.

My understanding is that PWM works by pulsing a signal - the voltage stays 12V the entire time, even at 0%. My total uneducated guess is that PWM signal will be sending a signal of "80%" and regardless of the rpm value of the FAN, each fan's PWM microcontroller will interpret the signal independently and run accordingly (80% signal - 2K fan will spin 1600RPM and a 3K fan would spin at 2400RPM. | 100% signal - 2K fan will spin 2000RPM and a 3K fan would spin at 3000RPM.). Again that just my hunch/assumption
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Old August 29, 2017, 06:40 PM
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Quote:
Originally Posted by Chilly View Post
This is a good question, I'm curious for the real answer as well.

My understanding is that PWM works by pulsing a signal - the voltage stays 12V the entire time, even at 0%. My total uneducated guess is that PWM signal will be sending a signal of "80%" and regardless of the rpm value of the FAN, each fan's PWM microcontroller will interpret the signal independently and run accordingly (80% signal - 2K fan will spin 1600RPM and a 3K fan would spin at 2400RPM. | 100% signal - 2K fan will spin 2000RPM and a 3K fan would spin at 3000RPM.). Again that just my hunch/assumption
That's pretty much what I'm thinking as well.
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Old August 29, 2017, 07:03 PM
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From an electronics standpoint, both of you are correct.

PWM just changes the duty cycle. 100% is "all on", 0% is "off". So if the range of the fan is 10k RPM at 'all on', and you're giving it 50% PWM then it SHOULD run at 5k RPM.

Now, implementation of PWM could vary but the way you both understand it to work is the way it SHOULD work.
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Old August 30, 2017, 12:42 AM
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I've messed around with PWM fans with an oscilloscope and tachometer a fair bit and like the guys say, all fans will have the same Duty cycle (80% or whatever). So, assuming a linear relationship between duty cycle and RPM, the 2k fans would be at 1600 RPM and the 2.4k fan would be at 1920 RPM. That said, most fans do not have a perfect linear relationship between duty cycle and RPM, but they will all be +/- 15% of [max RPM * duty cycle]. If you're planning on running them slow, watch out as they likely have different minimum duty cycles to spin.
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Old August 30, 2017, 03:10 AM
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other factor is line noise with PWM, depends on controller how handle phase generation that why 80% is used the lower 20% is very noisy
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Old September 21, 2017, 07:49 PM
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As others have stated, it works in pulses based on percentage. Regardless of the fans RPM's, the speed they spin will be in line with the pulses. So if you have 2 fans connected to the same PWM header, 1 with a max speed of 1000rpm and one with a max speed of 700rpm and both connected to the same PWM header set to 80%, then the 1000rpm fan will spin at around 800rpm (+/- due to variances between fans) and the 700rpm fan will spin at around 560rpm (+/- due to variances between fans) at the same time.

There is one caveat though. Even if you have multiple fans connected to the same PWM header via a splitter, the motherboard will only be able to read the RPM's of one of the fans. Namely the fan connected to the cable on the splitter with 4 wires. Whereas the rest of the fans (whether they are PWM or not) will be connected to connections on the splitter that only have 3 wires. The connectors themselves will have 4 pin holes, but will only have 3 wires actually connected to them. You can see this for yourself on any PWM splitter. The connection going to the motherboard will have 4 wires on it, but only one of the split connections will actually have 4 wires. The rest will only have 3 wires so as not to confuse the motherboard sensor readouts with multiple RPM signals from multiple fans on the same PWM header.

Last edited by bignick277; September 21, 2017 at 07:57 PM.
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Old September 22, 2017, 07:00 AM
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The original question ended up being moot as the fans in question (ML120s) ended up being 2400 RPM fans (I was misreading the PR of their 2000 RPM range as being their max RPM).
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