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Motherboard 3 pin fan connector question.

Last response: in Systems
January 31, 2010 5:37:11 PM

I'm not quite sure where to put a question of this nature... And I really don't know what I'd have to search for to find another topic with this question...
Anyways, what I did was, I had went to a store and purchased a 4-pin connector, and connected two fans to it by hard wiring it, and both fans spin at full speed. So this is how it goes...

Motherboard's three pin connector - 4-pin connector, red wire omitted - two fans.

Is this dangerous to my motherboard, seeing it has to feed TWO fans instead of one (which is what it was probably designed for)... It hasn't caused any damage YET, but I'm afraid it might cause some long term circuit damage or something.

I drew something just to make it more clear. Paint for the win!@
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February 1, 2010 1:46:33 AM

Yeah, I got the connectors, well, I kinda made them myself, cut up some molex cables and had the four pin, I'm just worried about the motherboard having to feed two fans. But I assume that's fine as there are actually cables that are meant for that.
February 1, 2010 1:54:05 AM

yes you could damage you mother board with your current set up, currently you are using the speed sensing connector as power. The red is +12 VDC and the black is ground ( 0 VDC ). It will no be a problem if you wire the reds to the red mobo header and the blacks to the black mobo header
a b B Homebuilt system
a b V Motherboard
February 1, 2010 12:55:08 PM

NITROGENarcosis has a useful idea, but not quite right I believe. He / she is right to be concerned that you have connected to the correct pins of the mobo pinout, but using the colors of the wires you hooked up by hand does not do the job. I seriously doubt you have the fan powered by the speed signal pin. Its function on the mobo is to RECEIVE a pulse signal from the fan, and it does not provide DC power to a fan.

Damorian tried to point you to a ready-made adapter to use, but the one suggested takes a 4-pin female Molex power output connector from the PSU and converts it into two outputs - a duplicate of the connector "used" and a 3-pin male that is similar to the mobo pinout connector so you can plug a 3-pin fan into it. BUT OP has a different situation - wants to convert a 3-pin male mobo pinout into 2 female 4-pin Molex connectors to feed fans with Molex male inputs.

What OP has sketched makes some sense, except that he / she has deviated from "standard" color coding in selecting wires. NITROGENarcosis is right - the coding on a 3-pin fan is supposed to be Black for Ground, Red for +12 VDC (or less - controlled by fan speed system) and yellow for pulse speed signal returning from fan to mobo. OP, what you need to do is find your mobo manual and verify exactly which pin is which on the mobo male pinout you have used. For what you are doing it appears you do not have (or plan not to use) the yellow speed signal wires from the fans. So use only the two pins - Ground (should be black wires from the fans) and +12 VDC (should be red fan wires). Oh, and since you started out with a 4-pin female fan connector, be aware that the "standard" colors for those are different from the 3-pin ones, so no wonder you could not conform to the usual ways.

By the way, why? If all you want is to power both fans that have 4-pin Molex male connectors on them, you can do that from 4-pin Molex female Molex output connectors for the PSU. In fact, for exactly this purpose there's a readily available common adapter that converts 1 female 4-pin Molex PSU output into 2, so you can run 2 fans in parallel off it at full speed. Now, IF your plan is to use the mobo's speed control function to vary the speed of these two fans, then I can understand what you have done.

Anyway, to OP's recurring question: most fans consume less than half of the maximum power output available from a mobo's CHA_FANx pinout connector, so even allowing for start-up current it is OK to run 2 fans in parallel off one output pinout.
February 1, 2010 7:15:21 PM

I don't think I've connected the fans to the tachometer... The 4pin connector I have only fits in one orientation on the 3pin slot, and since the red wire is omitted, there is no red wire off my mobo, only a yellow, which I'd assume is the 12vDC because if it wasn't the fans wouldn't spin (I think?). No other pin provides 12 Volts (I think..?). The connector I have just happens to have a yellow wire where it's supposed to be red because it wasn't really meant for this, I guess.

Does the speed sensor wire actually give out power or does it take in a reading?
Also, I'd assume the speed sensor wire would be the last one down, like

top-red (power)

I can't use the speed sensor part of it because my fans themselves only have two wires. They don't give out a 3rd wire with the reading (RPMs) for my computer to receive. If I can somehow control the fans through only two wires, please tell me how! I'd love that, never really planned to, but would be awesome.

I set my fans up like this because I needed the molex connectors for other peripherals, and my PSU is silly, it only provides four molex cables, and I couldn't find any of those cables that would convert one molex into two, so I had to settle with this.
February 2, 2010 4:39:45 AM

hmmm, I tend to agree with Paperdoc, the motherboard can probaply cut it but then again why would you want it to?

The good thing about connecting it to the motherboard is that you can see how fast it spins and that the motherboard can adjust it's speed.
You cannot see how fast it spins and you cannot adjust its speed if you connect it trough a molex (or sata) adaptor, however you also cant in the setup you made.

So the adaptor as shown above has no drawback to your current setup since the adaptor goes from one molex to one fan-plug AND one molex, so you dont loose your much needed molex connectors.

If the two setups are equal in pro's and cons then I would go with the setup that is actually made for it by the manufacturers and not something you rigged yourself, anything with ducttape inside a computer gives me the shivers and besides a motherboard-connecter might by able to handle two fans, it still isnt made for it however.

To answer your latest question: yes it is possible to adjust the speed trough two wires but not in your case cuz you need 3 wires.
To explain this a bit more, on a 3 pin connector the third pin is used only to read the rpm's and not to adjust it, the actual adjusting is done by sending more volts trough the first two pins.
So in theory you could adjust the speed (If you figure out how to send more volts trough the pins) but in practice this would not work since you would never know how fast the fan was actually running and thus could not adjust speed accordingly.

I think I just confused myself also with that answer but I hope it helps ;) 

a b B Homebuilt system
a b V Motherboard
February 2, 2010 1:39:15 PM

Actually you CAN control your fans' speeds with the rig you made. A mobo fan speed control system generally does NOT need or use the speed signal being fed back to it from the fan. It only displays it for you, and in some cases it monitors fan speed so it can send out an alarm if it fails.

First, lets' get clear on WHICH mobo fan pinout. Often there are four. The one labeled CPU_FAN can ONLY be used for the CPU cooler. It usually (unless you change the BIOS settings) controls that fan's speed according to a temp sensor built into the CPU, so trying to run another fan off it will base the speed on an irrelevant piece of info, and leaves you trying to figure out how to power and control the CPU fan. Besides, many mobos actually do monitor the CPU fan for failure and shut the system down if it stops (again, unless you defeat this feature in BIOS), so you don't want some other fan plugged in here to confuse the issue!

Your mobo may have a pinout called PWR_FAN. This does NOT supply power or speed control to anything. It is intended to be used for those PSUs that have what looks like a fan connector coming out of it - same do, some don't. All it does is feed a fan speed signal from PSU fan to mobo to be monitored. Do not use this for any other fan connection, either.

Most mobos have one or two pinouts called SYS_FANx, which IS where you plug in the case fans. At least one of these (sometimes both) is part of a feedback control loop configured in BIOS. The BIOS measures inside-the-case temperature via a sensor mounted on the mobo (NOT inside the CPU). Based on that it varies the voltage sent out to the SYS_FAN pinout on the + VDC (red wire) from 0 to 12 VDC, with Ground being the black wire on a 3-pin connector. Although it measures fan speed, it does not use that information to control the speed. This is a simple classic negative feedback control loop with action (adjust fan speed, via fan supply voltage) in proportion to the deviation of measured process variable (mobo sensor temp) from manually-entered setpoint. In OP's case, this control loop will work to control the voltage to the fans he /she has rigged in parallel off the mobo fan connector, just as long as he / she has connected the fans' black wires to the pinout's Ground pin #1, and the red wires to the variable + VDC pin #2.

Details of the 3-pin pinout on a mobo and the matching cable are shown here:

For the 4-pin system, see here:

Note that the connection system is backwards-compatible. If you look at the male mobo pinout with the barrier tab sticking up BEHIND the pins (and hence to the right), the pins (numbering from right to left) are #1 - Ground, #2 - + VDC, #3 - Speed signal, #4 - PWM power to fan. If you're using a 3-pin female connector from a fan, the first three pins are exactly the same, and the connector only allows you to fit on these three pins. Note also that the color coding on the fan's wires is different. Ground on Pin #1 is still black. +VDC on Pin #2 is red for 3-pin, but yellow for 4-pin. Speed signal return on Pin #3 is yellow for 3-pin, but green for 4-pin. And on the 4-pin connector only, the PWM power signal (Pin #4) is on a blue wire. For an explanation of the various ways fan speeds can be controlled, including the 4-pin fans now in computers that use 25 kHz PWM signals, see this link:

It helped me correct an error in my previous understanding. When using 4-pin PWM control, a mobo pinout supplies both the PWM signal and the full +12 VDC signal to the fan. Withing the fan itself a small switching circuit using a FET uses the PWM signal to control how much time the fan actually usess power from the +12 VDC supply, thus changing its speed. The circuit also ensures that the speed pulse signal is available to send back to the mobo, and that the fan can start up smoothly and run reliably at low speeds.

However, if you have a 3-pin fan to power, it is important to tell your mobo that. In that mode, the signals on a 4-pin output connector must change so that the PWM signal is irrelevant, but the +12 VDC pin now is no longer always at +12 VDC. It is varied to achieve fan speed control, the same way any other 3-pin fan speed control output would do.

OP, your sketch causes some confusion, and probably the root is that the 4-pin connector you got from the store does not have the "standard" colors for a 4-pin fan. Actually, it looks more like a power connector for a 3½" floppy drive. Anyway, you may have your fan connections reversed. On the 3-pin output on the mobo, Pin #1 is ground, and that should go to the fans' black leads. Pin #2 is +12 VDC (variable) and should be wired to the fans' red leads. The other two pins are not to be used for your case. Check on your fans. They usually have an arrow showing the fan's proper direction of rotation. I suspect yours are turning backwards, and you should reverse the way you've connected fan leads to connector pins. (Of course, that will change the direction the fan blows. If you mounted your fans for one particular direction of air flow as wired, changing the wires means you have to re-mount your fans the other way, too.)

Now, since you're connecting to a 3-pin mobo header you don't have any settings to adjust in the BIOS for 3- or 4-pin controller mode. However, you should ensure you have plugged this whole thing into the correct mobo pinout. Plug your rig into a SYS_FANx pinout, but read the mobo manual carefully. My mobo, for example, has two SYS_FANx pinouts, but only one of them actually does control of the fan speeds on it based on mobo temperature sensors. The other just feeds full voltage at all times to its fan, so that one will run at full speed.
February 8, 2010 11:00:45 PM

One of the connectors on my mobo (Asus P5QL PRO) is pwr_fan and the other one is just "case".
I can't find any of those fan connectors in my area and the shipping cost makes it really not worth it... I'll just stick to this, or I might try to make my own connectors like the ones mentioned in the 2nd post.

Oh and I use electrical tape :D 
a b B Homebuilt system
a b V Motherboard
February 9, 2010 2:13:31 PM

"Oh and I use electrical tape :D "

Ah, a more sophisticated user than those who make do with duct tape. Git 'er done!

Again I suggest you check the fan rotation direction. Truthfuly, if it is running backwards and you have mounted it backwards so that it blows the direction you want, that MAY be OK. A simple DC fan motor really does not care which way it is connected to power, except that it determines the direction of rotation. However, the fan blades may well have been designed for a particular rotation direction to optimize its air flow and reduce its noise. For that reason you might want to make sure it is running as designed.