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Molex and motherboard fan headers. What?

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  • Fan
  • Motherboards
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Last response: in Components
June 18, 2012 8:37:46 AM

My motherboard has a ton of fan headers built in, 4 for case fans and 1 cpu fan. It's a Gigabyte GA-Z77X-UD5H, for the reference.

Now, my case (Antec DF-30) has 4 fans in it, each of them with a molex, my PSU has more than enough molex connectors to power it all.

My question is; what are the advantages and disadvantages of Molex power as opposed to being powered via the motherboard? I understand that when attached to a motherboard, I can see fan temperature and RPM in my UEFI, but what else?

Also, my XFX 850W Non-Modular PSU has a cable containing 4 molex connectors leaving that main cable at different points, but at the end there is a 4-pin motherboard fan connector (It's hard to explain and I don't know if this is commonplace but I'll provide a pic if necessary). Would this enable me to monitor my fan temps/speeds via the computer?

Thanks.

More about : molex motherboard fan headers

a c 244 ) Power supply
a b V Motherboard
June 18, 2012 11:07:24 AM

tprezzle said:
My motherboard has a ton of fan headers built in, 4 for case fans and 1 cpu fan. It's a Gigabyte GA-Z77X-UD5H, for the reference.

Now, my case (Antec DF-30) has 4 fans in it, each of them with a molex, my PSU has more than enough molex connectors to power it all.

My question is; what are the advantages and disadvantages of Molex power as opposed to being powered via the motherboard? I understand that when attached to a motherboard, I can see fan temperature and RPM in my UEFI, but what else?

Also, my XFX 850W Non-Modular PSU has a cable containing 4 molex connectors leaving that main cable at different points, but at the end there is a 4-pin motherboard fan connector (It's hard to explain and I don't know if this is commonplace but I'll provide a pic if necessary). Would this enable me to monitor my fan temps/speeds via the computer?

Thanks.

Connecting the fans to the psu will run them at full speed and will not allow you to monitor RPM's
Never heard of fan temperature monitoring

It's for floppy drive power , do not connect it to your motherboard
http://www.playtool.com/pages/psuconnectors/connectors....
June 18, 2012 12:10:26 PM

Ah my bad! I may buy a floppy drive just for a reason to use it...

Anyway yes, I was thinking of temperature monitoring that is available on various fan controllers, that connect to 4 pins. Example:

http://www.aerocool.us/peripheral/v12xt.htm

Would using adapters so I can connect them to the motherboard mean that I get reduced fan performance?
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a c 77 V Motherboard
a b ) Power supply
June 18, 2012 4:20:09 PM

From your case manual it appears that you cannot connect ANY of your four case fans to the mobo for automated control. Instead, you would use the manual controls built into your case design.

A mobo can monitor and control a fan either of two ways. For a "3-pin fan", the wires from it are Red for the + VDC and Black for the Ground of its power supply, and Yellow for a fan speed signal. The speed signal is a pulse train generated by the fan motor (2 pulses per revolution) that is sent via the Yellow wire back to the mobo for monitoring and displaying speed. Fan speed is controlled as the mobo varies the +VDC line from 0 to +12 VDC. For a "4-pin fan", the wire colors are different, but the first three lines are the same, with one exception. The +VDC line is ALWAYS at +12 VDC. The fourth wire carries a PWM signal from the mobo. It is used within the fan motor to control the actual flow of current from the +12 VDC supply line through the motor, thus achieving speed control.

It appears from your case's manual, however, that the fans included in your case are neither of those two designs. The appear to be, if you will, a "2-pin fan" design - there is not such thing, really. What I mean is, it looks like the fans you have only have two wires going to the motors - the Ground and the +VDC lines. I suspect they do NOT have the third wire carrying the speed pulse signal back, nor do they use the PWM signal design. So you can't expect to connect any of these fans to the mobo and get automatic control of fan speeds by the mobo.

Instead, the design includes features to give you manual control of all the fans. For the top and rear fans, there is a set of two switches on the back panel. One turns their LED lights on or off. The other allows you to select either full or slow speed of these fans, for maximum air flow or reduced sound. For the two fans built into the front cover plates, each has a small knob mounted next to it that allows continuously variable speed settings for each respective fan, again set manually by you.

Now, it is possible with a bit of work to convert all these fans to deliver speed control by the mobo, because fundamentally their speeds are controlled by the voltage delivered to them from a supply that is +12 VDC max, or lower. In each case their basic connection system is via a 4-pin Molex male connector intended to be plugged into a 4-pin female connector from the PSU. If you get adapters, you probably can convert each of these Molex males to a standard female 3-pin connector that could plug into a mobo fan port. Thus the +VDC and Ground connections would be from the mobo ports, rather than from the PSU.

There are some unknowns here, though. As I outlined above, the way that a mobo can control a fan's speed is different for 3-pin and 4-pin fans, and your case fans are like 3-pin fans, except that they are missing the speed signal line. Your mobo manual shows all 4 case fan ports have 4 pins, as does the CPU fan port. For the CPU fan, it makes it clear that the way this port operates can be set in BIOS to either a 3-pin or 4-pin mode, depending on which fan type you have on your CPU cooler. But for the 4 case fan ports, it does NOT say this, and appears to imply that all of those will operate a 4-pin fan controller ports. If that is the case truly, they would do this: the 4-pin port supplies a constant +12 VDC to the 3-pin fan which thus runs constantly at full speed. This is exactly the same as if you had connected your case fan to a PSU 4-pin Molex connector in the first place. In other words, you have gained nothing.

Now, although your mobo's manual does not say so, it is possible that it allows the case fan ports to be configured as 3-pin fan ports, just as it does for the CPU cooler. IF that turns out to be the situation, then using adapters to connect the fans to the mobo ports CAN yield fan speed control by the mobo.

IF you can connect your case fans to the mobo via adapters and achieve speed control that way, be aware of three additional factors:

1. You will NOT have any means to measure and display the speeds of these fans, because they all lack the speed pulse generator system and cannot send such a signal back to the mobo. In terms of speed control, this is not a problem. The speed control system uses actual temperature measured inside your case (via a sensor built into the mobo itself) to guide it, and does NOT need to know the fan speed.

2. However, many fan speed controller systems in mobos ALSO check the fan speed to verify that it is operating and providing some cooling. If it gets no speed signal, it will set off an alarm to warn you the fan has failed. So, if you're using a fan that cannot provide a speed signal, you need to configure a setting for each case fan port that it should ignore this port's fan speed and not try to use it for anything. Similarly, even if you do NOT connect your fans to the mobo (that is, you use the Molex connections as the case was designed for), you probably will need to tell your mobo to ignore ALL of the case fan ports' speed monitoring systems, since none of them will get any signal to monitor.

3. If you're powering the case fans from the mobo and it is reducing the fans' voltage supplies to accomplish this, make sure to set your case's manual speed controls (the switches on the back, and the knobs on the front) to full speed at all times so that they do not interfere and further reduce the fans' supply voltages. If you don't the voltages could get so low the fan could stall and fail to start up again.

As a final note, you mentioned monitoring temperatures. Third-party fan speed controller modules sometimes also provide temperature monitoring. They do this by providing temperature sensors that you must mount in your case in various places. Unlike mobo fan control systems, they do not typically use those temperatures for control purposes - they just display them for you. But those modules do NOT tie into temperature sensors built into your mobo, so they won't show you those.

These days all mobos have at least two temperature sensors built into them. One actually is inside the CPU module itself, and the mobo uses it to measure CPU internal temperature and control CPU cooling. The other is built into the mobo close to some important components, and is used to control the case fan speeds via the mobo ports, as outlined above. In a few cases, mobos have more than one of their own temp sensors and can use them separately. In all of these situations, the mobo can do three things with these signals. They can display them for you, either in BIOS Setup screens or via some small app that runs under Windows and is included on the CD of utilities and drivers that comes with the mobo. They can use the actual measured temperatures to control the cooling provided by the fans connected to the mobo ports. And they can use the to generate emergency condition signals if any temperature rises too high, so that the mobo can issue warnings to you or even shut down the system to prevent disastrous overheating that could cause component damage. Mobos can only do this, however, with their own sensors. They cannot tap into additional signals from external sensors such as those that come with some of the third-part fan control modules.