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I've built around 5 systems in my entire life. I've attached every chassis fan (case fan) on the 4-pin MOLEX connector ever since.

I've noticed that there is a 3-pin connector there, but assumed that it was for "older" PSUs but now, I've noticed that my board has a 3-pin CHA_FAN header (which, after further reading, can enable "fan control").

So my questions are:

  1. What exactly is the 3-pin used for?
  2. If I plug the 3-pin connector into the board, should I also plug the 4-pin molex?
  3. Why should I plug my case fans to the 3-pin header on the motherboard?
  4. If I have more than 1 case fan, can I daisy-chain the 3-pin connectors and plug it into the single 3-pin header on the board?
  5. If I plug the case fan into the 4-pin molex, what happens to fan-speed control?
  6. Why is fan-speed control important?
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  • I generally use ONLY the 3/4-pin fan header, this is the native connection for most modern fans. The MOLEX is an adapter and removing it in a lot of modern builds allows you to get rid of MOLEX cables entirely (you are using a modular power supply, after all, this is 2011!)
    – Shinrai
    Commented Mar 29, 2011 at 19:04
  • And I'd add that I'm curious what board this is...most new motherboards have at a bare minimum one of these for CPU_FAN and two for CHA_FAN1 CHA_FAN2.
    – Shinrai
    Commented Mar 29, 2011 at 19:05
  • @Shinrai: The board is an Asus P5k-se. I have 1 CPU_FAN, 1 PWR_FAN and 1 CHA_FAN.
    – Ian
    Commented Mar 30, 2011 at 5:01

4 Answers 4

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Actually, the 3-pin connector is newer.

  1. The 3-pin connector provides power to the fan, and optionally (if the fan supports it), monitoring of the speed of the fan.
  2. Does your fan have two wires/connectors‽
  3. If the fan supports monitoring, then you get the added benifit.
  4. Not as such. There may exist special adapters, but it’s better to make use of separate connectors (most boards have three). If the fans support speed-monitoring, then you definitely can’t daisy-chain them (or at least not expect it to work, and possibly damage something).
  5. Nothing. Speed-monitoring and speed-control are separate. The third wire in the fan allows it to “report” its speed to the motherboard. Obviously connecting it to the two wires of a four-wire Molex connector won’t support that. The motherboard controls the speed of the fan by adjusting the voltage it provides to the fan. It can only do that if the fan is directly connected to the board, not if it is connected to the PSU.
  6. Fan speed-monitoring and control allow you to automatically raise or lower the speed of the fans as necessary, either via the BIOS, or a program like SpeedFan. This allows you to keep them running fast for more cooling when the system is hot, and keep them running slow (or off altogether) when the system is cool; providing you with a quieter system (fans can be quite noisy).
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  • @2: Yes. Or I might have mislooked. Is that possible that it has 2?
    – Ian
    Commented Mar 29, 2011 at 18:48
  • I have never seen such a thing (and doubt there is since it would be problematic at best, and dangerous at worst). It’s possible that you are thinking of a Molex-to-3pin adapter that has a “Y-adapter” type look to it.
    – Synetech
    Commented Mar 30, 2011 at 1:09
  • I think I have the "Y-adapter" (pccables.com/images/07057.jpg). Can I still plug this into the board and get rotation signal?
    – Ian
    Commented Mar 30, 2011 at 5:19
  • @Synetech - I have seen plenty of fans with only two wires, but they're generally not heavy duty ventilation fans. A lot of, say, cheap GPU coolers only use two.
    – Shinrai
    Commented Mar 30, 2011 at 14:29
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    @Shinrai, nonsense nothing, it would likely fry something! It reminds me of the phone block in my basement. There is a tag on the Internet-phone “modem” that clearly says to disconnect other phone systems before connecting which makes sense since connecting the VoIP’s modem (which is plugged into the mains) to the phone-block while the phone company’s cables are still connected (and carrying 15V to power the phones) would fry the modem.
    – Synetech
    Commented Mar 30, 2011 at 15:50
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  1. 3-pin used for powering fans and reading the speed of the fans in rpm. Useful for some monitor applications including bios cooling management.
  2. No. The 3pin provides the power for the fan. Only do this if the fan is extra large and the instructions say to do so
  3. To get the RPM speeds of the fan into your computer system for monitoring by applications or bios
  4. No, I dont think there is enough power to power too many fans through this connector. Nevermind the yellow one (the one that does the fan speed) would get confused. Most motherboards have 3 on motherboard connectors for fans.
  5. The fan-speed control is only done through varrying the voltage to the fan. You wont be able to directly control the speed in software, but can still do it with external inline fan speed reducers or front panels.
  6. Fan speed is "important" because the faster the fan is moving the move noise it makes. So reducing the speed makes your computer quieter. It's important not to reduce speed too low on high energy components such as your CPU as it can cause them to overheat.
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4
  • Red wire: 5volt live. Black: negetitive. Yellow: RPM wire
    – Kurru
    Commented Mar 29, 2011 at 18:42
  • Power connectors found on motherboards can be labeled : CPU_FAN, CASE_FAN, PSU_FAN etc
    – Kurru
    Commented Mar 29, 2011 at 18:42
  • I'm using an Asus board and it says in the manual that the CHA_FAN supports Q-fan 2 (a feature of Asus boards to control the fans). If the CHA_FAN is only 3-pin (3rd pin being a rotation signal), how did it manage to control the fan speed?
    – Ian
    Commented Mar 29, 2011 at 18:46
  • @Ian - It varies the voltage output.
    – Shinrai
    Commented Mar 29, 2011 at 19:31
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  1. it's a wire to send back how many RPM the fan is doing. Also with the 3 pin connector if it goes to the motherboard then the motherboard can adjust the voltage or perhaps amps, and thus the speed. If you use an adaptor to plug it into a 4 pin molex then you don't get that.

  2. No. The motherboard provides the power, I don't think you can even plug it in both!

  3. From motherboards i've used, there's only one 3 pin header, it's usually meant for CPU fan. It's labelled so in the motherboard manual. But you can check your motherboard manual and see how many there are and how they're labelled

  4. I doubt it they'd probably lose power or try to draw too much power

  5. no control , no RPM sent back to view fan speed.

  6. you may want it to spin lower to have less noise, but you may want it to spin more to reduce cpu heat or for a case fan, heat in the case.. so it's something people keep an eye on.

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  • @5: So the fan runs at MAX SPEED when I connect it to the 4pin molex?
    – Ian
    Commented Mar 29, 2011 at 18:50
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    @Ian Yes, it does.
    – AndrejaKo
    Commented Mar 29, 2011 at 19:21
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For number two: If and only if the 3-pin connector only has yellow wire both connectors should be used.

3-pin connector

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  • Can you provide a picture of this as I can't visualize how a 3-pin connector can only have a single wire. Thanks!
    – Ian
    Commented Mar 29, 2011 at 19:04
  • 1
    @Ian Here you go.
    – AndrejaKo
    Commented Mar 29, 2011 at 19:20
  • Wow, I have never seen this before. I can't fathom why you'd make a fan like this...?
    – Shinrai
    Commented Mar 30, 2011 at 14:30
  • @Shinrai So you could have fan with high consumption which can report its speed, maybe? Fan speeds can be controlled with various system monitoring front panels or just a simple potentiometer, but getting its speed is a bit more complicated, so it makes sense to have separate speed output.
    – AndrejaKo
    Commented Mar 30, 2011 at 14:39
  • @AndrejaKo - I guess, but I have never seen a fan that could realistically go in a case that could possibly draw that much power. If you can get a 200mm fan going at full speed I dunno what more you'd want!
    – Shinrai
    Commented Mar 30, 2011 at 15:01

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