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Thread: A Rant About PWM (in general) & Z97 MBs

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    Angry A Rant About PWM (in general) & Z97 MBs

    This is a prime example of bad advertising:

    http://forum.corsair.com/v3/showthread.php?p=710824

    The headers on these latest motherboards are not PWM despite having four pins. They actually use voltage (as I am sure most of you are already aware) to control RPM instead. Most fans have a lower bound voltage tolerance on spin up of somewhere between 30%~40%

    With default settings they certainly won't start up from a cold boot as the temps would not be high enough to warrant a 30%~40% ramp up. Only after some time operating would temps get high enough to signal an increase thus starting the fans.

    This is a configuration issue at the end of the day. Rectified by setting the base tolerance to just enough to kick the fan off in both BIOS & any software that supersedes at the OS level.

    I don't think there's nearly enough documentation on modern fan control methodology to be honest. Certainly the advertising glosses over the fact that MB headers are actually voltage regulated. There's only two true PWM signal headers on the boards; the main CPU fan header and the CPU OPT header.

    Indeed one could well plug in a splitter and daisy chain the PWM signal around their case. However doing so only gives you PWM in the context of the CPU and NOT the individual temperature sensors located around the board. So in that respect it ends up being totally pointless and even detrimental for case fans; imagine the scenario where the GPU or the chip set gets hot ahead of the CPU. Basically your context is not globally considerate.

    Yes fan speed control is argued to not matter with case fans. I disagree, we have a feature-set there, why not take advantage of it. Personally I can't be bothered to use a control unit as I'd prefer for the software to do what it's been designed for therefore saving me flicking switches and groping knobs all the time.
    Last edited by Nuluvius; 16-06-2014 at 08:35 PM.

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    Re: A Rant About PWM (in general) & Z97 MBs

    I'm not sure what you intended to link to, but it goes to the Corsair support forums.
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    Re: A Rant About PWM (in general) & Z97 MBs

    Well considering that the obsidian 750d stock fans are all 3 pin non-pwm fans no motherboard you plug them into will be able to control them via pwm.

    Now you can control a 3pin fan via pwm by pulsing the voltage (and a very few fan controllers do this and no motherboard that I know of do.) however this isn't how a 4pin pwm fan works, they work by sending a pulsed on/off signal to a controller chip in the fan, the fan still uses a constant 12v input but it is interrupted by the control chip to cut the voltage to the motor.

    I cannot speak for specific motherboards but I know quite a few of the newer asus ones can work in PWM or voltage control depending on what fan is plugged in, it automatically detects if a PWM connection is there, if not and a tac signal is there (ie a 3pin fan is plugged in) then it swaps to voltage control.
    Although sometimes fan headers are grouped together, often all headers labelled "sys" share a single common controller but it's all dependent on the motherboard, in general the lower end the motherboard is the fewer separate controller groups it has.
    (although I don't know about their Z97's)

    A simple way to test if a fan is on pwm is to stop it, just grab hold of it and feel it, if it "twitches", also when you let go there will be a slight pause before the fan starts up, then it's under pwm control.
    Remembering that a fan has to be a 4pin pwm fan in the first place.

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    Re: A Rant About PWM (in general) & Z97 MBs

    Quote Originally Posted by Pob255 View Post
    A simple way to test if a fan is on pwm is to stop it, just grab hold of it and feel it, if it "twitches", also when you let go there will be a slight pause before the fan starts up, then it's under pwm control.
    Remembering that a fan has to be a 4pin pwm fan in the first place.
    I notice if I dial them down to zero and then slowly increase again, there is a point where they will spin a bit and then stop until I turn them up enough to keep them going. They also twitch if I grab them (but so do most things, fnar fnar!).

    So... when my 3-pin fans do this, the fan controller (Lamptron FC6) they're plugged into is actually using PWM to pulse the voltage?

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    Re: A Rant About PWM (in general) & Z97 MBs

    Cannot confirm if the Lamptron FC6 is modulating the 12v or just working by resistance to reduce the voltage.

    If it's resistance then there's an additional thing going on to spike the voltage (thus creating the twitch), it's probably monitoring the tach signal (rpm) and if it's 0 then it spikes the voltage to get the blades turning.

    The thing about fans is that they have a minimum starting voltage, something to give the motor enough oomph to overcome the inertia of the mass of the blade head at rest, however once spinning far less kinetic energy is required to keep the blades spinning (just the friction of the air and the bearings)
    So by creating a 12v spike to start the blades spinning the voltage can be safely lowered below the minimum starting voltage.

    Does the rate of "twitching" increase and decrease as you change the speed dial? if so it's sounding more likely to be a pulsed 12v

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    Re: A Rant About PWM (in general) & Z97 MBs

    Quote Originally Posted by Pob255 View Post
    Does the rate of "twitching" increase and decrease as you change the speed dial? if so it's sounding more likely to be a pulsed 12v
    Not the rate, no.
    Increasing from zero does increase the amount of oomph, as fans spin more before they slow to a stop, until there's enough to get the revs up permanently. Lowering from full also gets them to a point where they'll slow to a stop.

    FC6 only has 3-pin headers anyway, though, so I doubt it's using proper PWM.

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    Re: A Rant About PWM (in general) & Z97 MBs

    Depends what you mean by proper. You can control any C motor using a PWM method - a PWM fan has the PWM controller built in, but there is no reason why the PW control can't be done on the motherboard.

    The Mobo in question is ambiguous, but it is possible that the Chassis connectors have PW outputs for controlling a conventional fan.
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