More than you cared to know about PC fans • Hacker News Board

Re: More than you cared to know about PC fans

#41

Post by gnu8 » Wed, Nov 23, 2022, 3:55 PM UTC

axiolite wrote: post #34

> "there is no readily-available 24V source from an ATX power supply," This is false. ATX provides both +12V and -12V: * https://pinoutguide.com/Power/atxpower_pinout.shtml Hooking them together would provide 24VDC. On a 350W PSU, I see -12V rated to supply 0.3A, which would allow for 24V fans drawing up to 7.2W of power, if desirable. I don't see any reason to prefer 24V fans, however. The current losses over such s…

This led me to wonder why we still need the -12V supply - I know the -5V supply was removed a while ago because it was only needed for ISA slots. According to Wikipedia, -12V is for RS-232 ports and PCI slots, both of which are hardly seen on any current motherboards. It should be safe to use it for fans, but 7.2 watts is not very much power. Looking on Mouser, I see 24VDC fans that consume 1 watt or 18 watts depending on speed, so one could easily overload the -12V bus if they are trying to get a lot of CFM. I don’t see a compelling reason to use 24V fans in a PC.

Re: More than you cared to know about PC fans

#42

Post by themoonisachees » Wed, Nov 23, 2022, 4:00 PM UTC

rajup wrote: post #21

Ah reminds me of the time when I was a kid and my PC would randomly crash and would not start for a while. Turns out the processor fan was janky and would not start occasionally requiring a push with a pen or something to get it going. So the POST process in my case had an extra step: "have human check CPU fan"!

Same, but my motherboard actually checked the fan was spinning so post literally involved me checking the fan when told

Re: More than you cared to know about PC fans

#43

Post by robotguy » Wed, Nov 23, 2022, 4:03 PM UTC

amelius wrote: post #40

mrWiz wrote: post #37

Earlier quoted context omitted.

Generally the fans are powered with a steady state DC source and the PWM is just logic signaling. If the fan is powered by a PWM source the tach output will not be a clean square wave (but could still be reconstructed, if the PWM frequency is sufficiently higher frequency than the fan's rotational speed).

Well, PC fans come in 3-wire and 4-wire forms, where the 3-wire type has only pwm,tach and GND connections (no DC power).

3-wire is: TACH, DC, GND

4-wire is: PWM, TACH, DC, GND

https://noctua.at/en/what-pin-configuration-do-noctua-produc...

Re: More than you cared to know about PC fans

#44

Post by deathanatos » Wed, Nov 23, 2022, 4:11 PM UTC

The pinout is wrong, is it not? As it is drawn, if you mate the two connectors, the wires won't match up. (You'll need to mentally rotate it: the hook on the back of the male connector slots into the rails on the front of the female connector to make them impossible to separate without destroying a nail.)

I think the female side is right, it's the male connector that's reversed.

Re: More than you cared to know about PC fans

#45

Post by MivLives » Wed, Nov 23, 2022, 4:15 PM UTC

whalesalad wrote: post #22

What a coincidence, 4x of the 120mm Noctua Redux fans just arrived this morning and are sitting on my living room table waiting to go into my rig. I love in-depth analysis of stuff like this. How do y'all like to manage (chassis) fan speeds? I have been out of the PC world for so long that I forget all pieces of hardware need to be managed.

4 way fan splitter to just let the mobo control it all. I run mini itx so not a lot of fan headers.

Re: More than you cared to know about PC fans

#46

Post by deathanatos » Wed, Nov 23, 2022, 4:17 PM UTC

Also, one thing I've always wondered: why do people want to use hwmon to set fan RPMs? (Or really, why do this from userspace at all?) It seems inherently dangerous to me, as you're asking a process who might not receive CPU time for whatever reasons from the very much not realtime OS to control a fan; if the current RPM is too low, and the system starts generating heat, but the fan controlling process doesn't get CPU time … then what happens?

It seems to me you want fans controlled with something dedicated to it.

The other thing I don't get is all the plethora of options my motherboard gives me to set fans only to fixed RPMs. Am I crazy in that I want the fan to be controlled by heat? (More heat => more RPMs. Keep the system cool, but if there isn't much thermal load, spin the fans down and reduce the noise?)

But by fixing an RPM, it seems the only valid input is "100%"; anything else could be too low under stressful conditions.

I could also have a cheap motherboard. (I definitely won't be purchasing from this manufacturer again, and the motherboard does have other severe quality issues…)

Re: More than you cared to know about PC fans

#47

Post by paxswill » Wed, Nov 23, 2022, 4:28 PM UTC

ilyt wrote: post #13

I'm surprised it doesn't mention hysteresis some fans have at low speed. Cheap fans often don't have actual proper speed controller on them which means they might start spinning at say 30% PWM but only stop spinning at 15% PWM. That was probably the thing that surprised me most when I upgraded to nice Noctua, at certain PWM it just started, always, and below that it stopped, always.

Those fans that start at 30% and then allow you to go lower are explicitly allowed by the spec from Intel (linked in the article, sec 3.2 and 3.4).

Re: More than you cared to know about PC fans

#48

Post by MaulingMonkey » Wed, Nov 23, 2022, 4:35 PM UTC

deathanatos wrote: post #46

Also, one thing I've always wondered: why do people want to use hwmon to set fan RPMs? (Or really, why do this from userspace at all?) It seems inherently dangerous to me, as you're asking a process who might not receive CPU time for whatever reasons from the very much not realtime OS to control a fan; if the current RPM is too low, and the system starts generating heat, but the fan controlling process doesn't get CP…

> It seems inherently dangerous to me

In the old days, it kinda was - at least to your hardware.

Then people realized that blowing up components because a fan failed, or became unplugged, or a filter clogged with dust, maybe wasn't a great user experience and/or caused more in-warranty returns that required replacing hardware ($$expensive$$!), and implemented thermal throttling and thermal cutoffs. Nearly two decades ago at this point, I helped a friend diagnose his computer randomly turning off. It turned out to be a CPU fan unplugged itself, causing overheating to trigger a thermal cutoff. No other apparent harm done.

Fans aren't the only means of limiting heat: slowing stuff down and turning stuff off also works. And it turns out users sometimes would rather stuff run slow than run loud, and maybe your crappy motherboard vendor shouldn't be writing a ton of code running in kernel space - with all the potential stability and security issues that might entail - for whatever network-connected bloatware syncs your RGB lighting and fan settings to the cloud. And they will do exactly that, if that's what's required to give their customers what they want.

Just exposing fan RPMs to userspace might be far less dangerous.

Re: More than you cared to know about PC fans

#49

Post by pwg » Wed, Nov 23, 2022, 4:36 PM UTC

deathanatos wrote: post #44

The pinout is wrong, is it not? As it is drawn, if you mate the two connectors, the wires won't match up. (You'll need to mentally rotate it: the hook on the back of the male connector slots into the rails on the front of the female connector to make them impossible to separate without destroying a nail.) I think the female side is right, it's the male connector that's reversed.

> The pinout is wrong, is it not?

Not if both connectors are drawn looking top-down.

Note the ribbing and clip on the female connector, the bottom of the female connector is at the bottom of the drawing. You'd mate these by lifting the female straight up, rotating about 90 degrees along the vertical axis, and pushing down onto the male connector. And when mated that way, the labeled pin-outs line up perfectly.

Re: More than you cared to know about PC fans

#50

Post by zajio1am » Wed, Nov 23, 2022, 4:51 PM UTC

One sad thing with fans in PC is how bad is fan controller / SuperIO driver support in Linux. AFAIK, there is no autodetection for SuperIO chips (like there is for PCI or USB devices) and for many chips there are no drivers anyways.

Seems like using USB connected microcontroller with open-source firmware instead of embedded SuperIO seems like easiest way to have reliable fan control in PC.