How to control case fans


I have a GA-880GMA-UD2H motherboard in an HTPC case with 3 120mm fans. Right now all the fans are running all the time. Shouldn't I be able to have the motherboard control them so they come on only when needed? All the case fans are 4 pin. Do I just get some 4 pin splitters and daisy chain them and then plug it into the motherboard. Do I need a 4 pin power plug to supply power from the PSU or will plugging it into the motherboard. Will the motherboard control them? Do I need to install software to make it work? How do I go about doing this (if it is even possible)?

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  1. Easiest way is to buy a couple of these:
    Rosewill PWM fan splitter (need two as they only handle a pair of case fans each).

    I use these on client systems all the time; I always spec/buy a pair of case fans (1 intake, 1 exhaust), and I mod 'em to fit, and to provide feedback to the fan monitor channels. Big advantage of these is it doesn't matter how many you daisy-chain, as the 12V power is supplied by the MOLEX, not the CPU_FAN board connector. Prettier, tidier, and able to provide the feedback as wired are these:
    ARCTIC COOLING AF12PWM 120mm Case Fan - intake
    ARCTIC COOLING AF12PWM 120mm Case Fan - exhaust
    They do a wiring trick they call PST (PWM Sharing Technology) which supplies the 12V from the CPU_FAN connector - good for roughly five fans total.

    If you are able/willing to splice/dice/solder, I can provide a schematic to roll your own splitter, but I don't have a four fan drawing already made - let me know if you want one...

  2. Thanks for the reply Bill, does it matter if one of the "split" y cables has only 3 out of the 4 pins? I've purchused several PWM fan splitter cables for my 4 pin fans, but only one of the "split cables has 4 pins. I've plugged them into the mother board, no fans, plugged them into the PSU, fans all the time. I'd love to have the drawing, I will probably get the Rosewell PWM fan splitter but the drawing would help me understand how this works correctly.

  3. The cheap/poorly wired ones usually just disregard the 'tachometer' signal that comes back to a board header to inform the BIOS as to whether your fans are running or not... How the whole deal works:

    Pin 1 is ground...

    Pin 2 is the +12V supply to the fan...

    Pin 3 is the 'tachometer' signal back to the board to sense speed - this is generally done by hall-effect (magnetically actuated) switches on the stator, typically produces two pulses per revolution of the fan - lets the board 'watch' the speed, mainly so you can set an alarm (in the BIOS, typically on the "PC Heath Status" page) should it fail to run - this is the wire typically 'missing' from splitters...

    Pin 4 is the key to the whole shebang! It's a 5V signal that's switched, if I'm not mistaken about the spec, at 25KHz - twenty-five thousand times per second. This means it's switched on and off approx once every 40 μsec (microseconds). It's connected to a gate on the fan that switches on and off the 12V supply to the fan itself at the same rate, and the speed is dependent on the 'duty cycle' of the signal - the English, I believe, have the most understandable name for this - they cal it the "mark to space ratio"... How it functions is thus: the signal is switching, again, once every 40 μsec; if it's on for twenty, and off for twenty, the 'duty cycle' (or mark to space) is said to be 50%, and the fan runs at half speed; if it's on for ten, and off for thirty, the duty cycle is 25%, and the fan runs at a quarter speed; conversely, if it's on for thirty, and off for ten, duty cycle is 75%, fan runs at three-quarters speed; and, quite obviously, if it's just off - so is the fan; and at full speed it's 'just on'!

    If this still fails to make sense, post back - somewhere, in the clutter on this thing, I have a set of waveform drawings to try to make this all a little clearer - and I will give you a set of drawings in a bit - I have to do a little cutting & pasting in AutoCAD from the sets for my workstation...
  4. Well, we're semi-screwed, here... I didn't know, offhand, that it was an M-ATX board - only has two fan headers, so can only monitor two fans! Also, note - the color codes are the 'generally accepted' ones; Microsoft, being a law unto themselves, have a different one; and Arctic, whose fans I mentioned, has yet another!
  5. Greetings,

    Thanks for the info. I think I'm just going to get three 120 case fans with low flow, as the demands of my rig just aren't that great. I didn't know that m-atx boards couldn't control three fans.

    Thanks, Mark
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