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-   -   Case fan % seems off (https://forum.overclock3d.net/showthread.php?t=94264)

trawetSluaP 08-04-20 02:51 PM

Case fan % seems off
Hi all.

I control my case fans using a fan controller all works well, however, the fans seem to behave strangely based on the % they are running at.

So for example if the fan is a 2000rpm fan will run at around 600rpm 50%, 1200rpm at 90% then the curve ramps up in the last 10% where it reachs 2000 rpm at 100%.

I have the min and max % set at 0 and 100 respectively.

I guess it's possible this behaviour is normal but just seems strange to me!

I use Silent Wings 3 fans, a mix of 120s and 140s.

Any info would be appreciated.

AlienALX 08-04-20 05:10 PM

That sounds like PWM behaviour to me. Are they 4 pin?

trawetSluaP 08-04-20 06:22 PM


Originally Posted by AlienALX (Post 1018274)
That sounds like PWM behaviour to me. Are they 4 pin?

Yeah, controlled through PWM. Would it be better to set control differently?

AlienALX 08-04-20 06:29 PM

I don't know if you can. Basically pulse width will send pulses to the fan. So it's not like a 3 pin. With a 3 pin you control the speed with the voltage, but a PWM is a signal basically telling the fan to go on, then off. Well, not off, but just not on....

So for low speed the pulses are slow. On.......on.......on etc. As the speed increases so do the speeds of the pulses but IMO it's nowhere near as accurate. That may be why you are seeing the RPM change in the last 10%.

With any water cooled rig I basically set the fans once (I use 3 pin) and usually around the 7v mark. If they are not capable of cooling the rig sufficiently I up the voltage (depends on the RPM of the fans) until they are either barely or still not audible, then I leave it there.

If I had to have them spinning any more than they do always it would be too noisy. If it's too hot? you don't have enough surface area (or rads, basically).

trawetSluaP 08-04-20 07:04 PM

It's not too hot and I'm sure I have plenty of surface area with a 360mm and 2 x 420mm rads, however, I live in a very stuffy house and it gets very hot in the room where my PC is so I have a fan curve set-up depending on the water temps.

AlienALX 08-04-20 07:05 PM

Hmm. All I can advise in that case is to find your "almost worse case scenario" and just leave them there.

tgrech 08-04-20 08:16 PM

It doesn't sound particularly typical imo, that's somewhat inverse to what you'd usually expect, where because of the effects of inertia and resistance, particularly air resistance, which can be an x^2 relationship with speed, usually you need notably more power to gain RPM towards the top end.

A worst case scenario for a motor is like this:

But because PC fans are quite light(And usually have an IC within them to regulate the form of the pulses) it's usually closer to an ideal linear relationship like this:

So I'd assume something isn't spot on, whether that's the tach(You should be able to ear test that one), the PWM output not being linear with the %age input you set (Maybe it's pre-biased to try and cancel out the above effect), maybe the input voltage dropping from 12V (Possibly because too much current is being drawn through the fan controller) or something else limiting the current.

Of course, maybe the PWM IC within these fans are just a bit weird, so may be worth checking one on its own or something if you can.

Warchild 09-04-20 01:33 PM


Originally Posted by trawetSluaP (Post 1018284)
Yeah, controlled through PWM. Would it be better to set control differently?

Well with controllers you can do several things with PWM.

not only can you set a range for 0-100% PWM which in turn means 0-100% RPM to an extent,

but you can also set a range for the power going to the fan.

e.g. I set a max power for the fan of 9v so when its at 100% PWM its only supplied with 9V, that means your true max fan speed isnt achieved, even though your settings say max PWM.

In addition to that, some controllers can behave like this by default, for example to keep low noise, they might not supply 9v to your fan when its at 80% PWM, but if you set PWM to 100% the controller realises you want full juice so sends 12V through.

trawetSluaP 09-04-20 05:30 PM

I'm sure I've set the fans up to use the whole PWM range being PWM controlled.

Here are how I have them set up:


These are 2000rpm fans and as you can see, at 49% they are only at around 500rpm.

tgrech 09-04-20 06:24 PM

Should you set the minimum power to 12V? Is it not reducing both duty cycle and voltage if not? Not too familiar with that software.

Also does that "speed control" setting offer closed-loop loop functionality using the tach?

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