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Powering and Controlling Fans

I'm in the very early stages of researching components for my first gaming build (researching motherboards, currently) and I'm utterly confused about fans and air cooling—specifically, how the fans are powered and controlled. I've divided my question into several sections.

Let's say I buy a case that has space for 2 front fans, 2 side fans, , 2 top fans, one floor fan, and one rear fan.

1. 8 fans is more than the standard motherboard can handle, am I right? So which fans would I plug into the motherboard, and which fans would I plug directly into the power source?

2. Further, how do I know how many fans my motherboard can handle? I mean, what do the fans even plug into? I've heard something about 3- or 4-pronged plugs, so which should I look for when choosing a motherboard? And how many?

3. For the fans that don't plug into the mobo, but the PSU directly, I guess I need to be making sure that my chosen PSU will have adequate connectors for the fans, no? Or can I be pretty sure that as long as I'm not buying a cheap-o PSU I'll get what I need in terms of PSU fan connections?

4. And then what about LED fans? Do their LED plugs connect to the mobo or the PSU? And again, do I need to be making extra certain that my PSU/mobo will have the appropriate number and kind of connectors for these LED plugs?

5. Then there's the question of controlling the fans. I know I can buy a fan controller panel, or choose a case that has such an option pre-installed, but what should I be looking for in such a controller (assuming I decide to get one at all)? I'm the type of person who would forget to turn the fans up when I start gaming... are there controllers out there that can detect the case temperature and that I could program to increase/decrease fan speed at certain temperature thresholds?I mean, I saw a couple controllers that have warning beeps and stuff, but ideally, I'd want to just set my fans up and forget about them.
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  1. Hey syntax, I'm not an expert on fans but I do have some experience from my last build, this is what I've learnt:
    You mobo will only be able to handle a few fans ( depending on what you get) and I think only the fan for the CPU has a 4 pin connector, for temps and controlling etc. the rest of the headers are 3 pin.

    You can plug what you can into the mobo, the rest can connect to your psu. I had 4 extra 120mm fans with blue led's and one 200mm fan, I was able to plug these all into the psu. The fans I got ( aero cool shark ) came with the 3 pin fan connection and an adaptor to plug I to a molex connector ( on the psu) and the adapters are stackable, so I could have 3 fans connected to one molex connector, so you don't have to worry too much about your psu having too many connectors!

    The only problem I had was noise. Because there is no fan controller the fans just ran full blast the whole time, my system was cool, but noisy (and very dusty :-/)

    These fans had blue LED's built in, and they were powered by the fans power cable, no extra plugs to plug in or anything.

    Hope this helps, like I said I'm no expert, this is just what I have figured out on my own.
  2. Thanks, Ponyface :) I was under the impression that some LED fans have a separate cable for the LED control (I could be wrong). I know that a lot of cases these days have buttons on their front panels that allow you to turn those LEDs on and off... Does anyone know if that control requires a separate plug for the LED? Or can fan speed and LED function be isolated even with just one connector?
  3. No worries! Unfortunately I'm only good for the basics... We a need a serious modder to give more info here, I'll be watching this closely because I'm also interested! I'm getting a new case tomorrow with a fan controller, so I'm looking for the same advice!
  4. Best answer
    Fans can come with one of three basic connection systems.

    1. The common "three-pin fan" has three wires coming out of the motor to a small female connector (with holes); the male connector on the mobo has 3 pins. Pinout diagram here:

    If you look carefully, you can see that, on the male connector, there is a plastic tongue sticking up from the base behind the pins, and it slips into a slot molded in the female connector. This guarantees you can only fit it together one way. Note the color coding of the wires - this is pretty standard.

    Pin 2 (Red) is the +VDC line. The mobo's fan speed control system can vary the fan speed by altering the voltage supplied on this line, ranging from 0 to 12 VDC. Pin 3 (Yellow) carries a pulse signal for fan speed (2 pulses per revolution) generated in the motor and fed back to the mobo.

    2. The newer "4-pin fan" has 4 wires - surprise! - from it and a very similar set of female and male connectors. Wiring pinout here:

    Note that most such fans use the first color coding scheme, but not all - it is not completely standardized. However, the uses of the pins are always the same. Look closely at that. The first 3 pins are virtually identical to those of a 3-pin fan system (one exception I'll get to). Also note that the plastic tongue sticking up from the male connector base is right behind pins 1-3. Thus you can mechanically fit a 3-pin female connector onto a 4-pin male mobo connector, and also the other way around. They will work, but with limitations - will discuss below - this is backwards compatibility.

    The 4th pin carries a new signal, the PWM (Pulse Width Modulation) signal from mobo to fan. In this system, the voltage on Pin 2 is always +12 VDC (unlike the 3-pin system where the voltage varies). Inside the motor casing of a 4-pin fan is a small chip that uses the PWM signal to control whether the voltage available is actually sent on to the fan motor. The PWM signal is like a "square wave" of sorts, but not exactly. Its "% On" value changes. The signal frequency is about 25 kHz. when the PWM signal is "On", the chip inhibits current flow to the motor; when it is "Off", the full supply voltage is sent to the motor. Within one cycle of the signal, the longer the signal is "Off", the longer the motor gets current flowing though it, and the faster the motor will turn with this signal. Note that, in the absence of any PWM signal (such as if you plug a 4-pin fan into a 3-pin mobo connector), there is no current inhibition and the chip will feed the full supply voltage to the fan at all times. That's the backwards compatibility part. But in this situation, the 3-pin fan port varies the voltage on Pin 2, so the fan's speed CAN be controlled this way. On the other hand, if you plug a 3-pin fan into a 4-pin mobo port, the fan cannot use (and does not even receive) the PWM signal, so it runs at full speed at all times because the 4-pin mobo port keeps the voltage on Pin 2 at +12 VDC.

    Here's a trick that adds confusion, and is poorly documented in mobo manuals. SOME mobo makers mount 4-pin fan output ports on their boards and make SOME of them (not necessarily all) adjustable in BIOS Setup. You may be able to set manually whether a port behaves as a true 4-pin port (fixed 12 VDC and PWM signal) or as a true 3-pin port (varying VDC and no PWM signal). Some makers even have automated this process on some of their 4-pin ports so that the port itself detects which of the two fan types is actually connected, and sets itself for that.

    3. Fans that plug into a "4-pin Molex" connector directly from the PSU are more limited in what can be done. (The "4-pin Molex" connector, or more properly the Molex 8981 Disk Drive connector in the lower part of this article:

    is common on most PSU's - there are usually several of them.) Originally they were used to power IDE hard drives, optical drives, etc. But the Yellow and Black leads on Pins 1 and 2 provide 12 VDC that can be used for fans.

    Note, however, that this connector system has no wiring to take a speed pulse signal from a fan's Pin 3 back to anywhere - and certainly not to a mobo port. So there are the two limitations of these fans. They can only operate at full speed because their power supply is fixed, and they cannot report their speed to the mobo.

    Sometimes a fan supplier will sell a 3-pin fan along with an adapter to allow you to connect it to a 4-pin Molex connector from the PSU, so it can be used in either manner.

    Now to questions of control of fan speed. Almost all mobos can monitor and control the speed of some fans. At least there will be male output ports on the mobo for one cooling fan for the CPU plus one case cooling fan. Often there are two case fan or SYS_FAN ports, sometimes more. Sometimes there is a port called PSU_FAN.

    In all cases, the basis of control is a separate feedback loop for each fan output port. The mobo uses temperature measured by a sensor to guide each loop. For the CPU, the sensor is actually built into the CPU chip itself by its manufacturer, and the mobo simply uses that signal. It compares the measured temp to a target temp set in BIOS, and alters the fan control signal (either voltage on Pin 2 for a 3-pin fan, or the PWM signal on a 4-pin fan) to speed it up or slow it down as needed to keep the measured temp close to the target. For the SYS_FAN (case cooling) the mobo has its own sensor built into itself that it uses to measure temp and control those fans.

    NOTE that in this control system, the actual fan speed is NOT used to control the fan. It is measured and displayed by the BIOS for your information. But the speed control is based solely on the temperatures measured by the respective sensors.

    The PSU_FAN port is different, though. Its original design was solely to be used in conjunction with SOME (not all) PSU's that have a special set of three wires coming out of them that look exactly like those of a 3-pin fan. Their only function, really, is to send the speed signal from the fan inside the PSU to the mobo so it could be monitored and displayed. This 3-pin port does NOT actually provide any power or control of fan speed for the PSU's internal fan. All that work is done by the PSU itself. The intent is that, if you have no such connection wires from your PSU, you plug into this mobo port nothing! HOWEVER, it turns out that many mobo makers also provide the Ground and +12 VDC signals on this port, so some people use it to power a case cooling fan, although it can only run at full speed - there is no reduction of the +12VDC signal to control fan speed.

    Within the BIOS (or sometimes, only by using a separate utility that runs under Windows) you can examine the settings for each fan speed controller - CPU_FAN, SYS_FAN1, SYS_FAN2 - and adjust them if necessary. These also display for you the measured temperatures and the fan speeds. In the special case of the CPU_FAN system, two additional uses are made of the data. The first is that, if the CPU internal measured temp gets too high, the BIOS will reduce the power to the CPU and its operating speed; if it gets a bit higher, it will shut down the whole system. These are to prevent disastrous overheating that would destroy the CPU. Both of these protections work no matter no you set up the CPU cooling fan. The second extra is that many mobos will check the CPU fan speed frequently. If it ever gets no pulse signal it assumes the fan has failed and shuts the whole system down immediately, without even waiting for the CPU temp measurement to show overheating.

    With that background, we can get to how and where to plug in fans.

    1. It is always a good idea to plug the CPU cooling fan into the CPU_FAN port of the mobo and allow it to do all its functions. It will vary the fan speed according to CPU heat load; it will protect the CPU from fan failure. If you opt NOT to do this and plug that fan into another power source, and you have a mobo that monitors the CPU fan speed for failure, you have to go into BIOS Setup and tell it NOT to monitor that speed signal it is not getting through the CPU_FAN port.

    2. Do NOT plug ANY fan other than a CPU cooling fan into the CPU_FAN port. If you do so, its speed will be controlled by the CPU internal temp, and the CPU itself will get cooling from some uncontrolled fan. Moreover, the BIOS will be getting a wrong speed signal to check.

    3. You can connect case cooling fans to the SYS_FAN port(s) of the mobo, and their speed will be varied according to the case's internal temperature. In this way you can avoid having them run full speed when not needed.

    4. If your PSU has those special temp sensor leads coming out of it, connect them to the PSU_FAN port of the mobo. If not, you can connect a case cooling fan here IF you want it to run at full speed all the time.

    5. You can connect any number of case cooling fans to a 4-pin Molex output from the PSU, and all so connected will run at full speed.

    6. What about connecting many fans to one port? Well, the conventional wisdom is that you CAN do up to TWO similar fans connected to ONE mobo port (by soldering wires together) because the ports can supply enough power for that, but not more per port. (The limit is really the heavy current needed briefly to start the fans up when they are stopped.) Of course, you would only connect CPU cooling fans to the CPU_FAN port, but some CPU cooling systems do use 2 fans in a "push-pull" arrangement. IF you do this for a pair of 3-pin fans on one port, do NOT connect together both Yellow wires from the fans. Connect only ONE of them to the connector and mobo port, and leave the other Yellow wire not connected to anything. Connecting both wires would feed two pulse signals into the mobo port and confuse it badly. IF you do this for a pair of 4-pin fans, similarly do NOT connect both wires for pin 3 together - these are the speed pulse signals. But DO connect together the wires for pin 4 - the PWM signal.

    7. What about using the 4-pin Molex connectors from the PSU for multiple fans? Well, these sources can provide power for MANY fans from one PSU output connector. For that reason, many fans designed to run off the 4-pin Molex connectors come with stacking connectors so that you can easily plug several into one PSU output. Of course, all of these fans will run only at full speed.

    So you can see that, using these possibilities and some soldering skills, you can arrange to power AND control the speed of several fans from the mobo, within limits. You can run 1 or 2 CPU cooling fans from the CPU_FAN port. You can run up to 2 fans off each of your mobo's SYS_FAN ports. You might even run a fan or two off the PSU_FAN port; however, that port can only run its fans at full speed, the same as ones powered directly from the PSU via the 4-pin Molex connectors. so you're better off to do the latter. If you have more fans than that, some will have to be from 4-pin Molexes from the PSU.

    I have not discussed above separate fan controllers you can mount in the case. Some of these are supplied with a case, and some can be bought as third-party accessories. In all cases, these units derive their power from 4-pin Molex connectors from the PSU. They have NO connection to the mobo ports, so any fans connected to them cannot feed speed signals to the mobo for BIOS monitoring.

    Most such controllers operate like 3-pin fan controllers in the sense that they achieve fan speed control by reducing the voltage provided to the fan. HOWEVER, very few of them have any reliable way to monitor or use temperature probes to achieve automatic control the way the BIOS does. Fan speed usually is set entirely manually by a knob or similar control operated only by the user. Some such controllers actually use standard 3-pin fans and can accept the fan speed signal and display it on their own front panels. However, they cannot relay that info to the BIOS on the mobo. Other controllers are designed to use fans intended for connection to 4-pin Molex connectors.

    So, when you plan out your fans control system and fan purchases, plan these things carefully:

    1. What types of fan connectors are built into your mobo? 3-pin or 4-pin, and this may be different for the different ports. It is common now to use 4-pin fans for the CPU, and sometimes for the case fans, but sometimes you find BOTH 3-pin and 4-pin ports for case fans. Then there's the question of whether the 4-pin port(s) are adjustable for 3-pin fans. Anyway, buy fans suited for the ports you have and what they are cooling. For example, you will probably need a 4-pin fan for your CPU, and could need some of each for the case cooling. After that, any additional case fans will have to be for 4-pin Molex connectors.

    2. If you are going to use a fan controller (either 3rd party or supplied as part of your case) get the right fans for this. One problem some people have had is this: the case comes with a simple controller and fans, but all the fans are of the 4-pin Molex connection type to be used with the included controller. None of these fans can be connected to any mobo port. So it you want to use the mobo ports and their automatic control, you have to buy additional suitable fans.

    3. LED's? LED's need a relatively constant voltage to run properly, but the fans are designed to run with different voltages. So there are compromises involved, like LED's whose brightness varies a lot as the fan speed changes. Some such fans MAY have separate connections for fan and LED power, but that leads to nonstandard connection systems. If LED fans are supplied with a case, however, sometimes the makers have set this up right. There are leads from each fan to a LED control switch on the case, plus other leads for the fan motors. You have to read the details carefully.
  5. Best answer selected by SyntaxSocialist.
  6. ^^

    Wow. Thank you, Paperdoc! Immensely helpful! And I'll bet Ponyface appreciated that response, too!
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