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P7P55D-E EVO, PSU or Motherboard for case fans

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March 15, 2010 3:37:49 AM

Seeking help from Toms Hardware again!

I have a Cooler Master 690 case with 3 (120 25mm) fans; Rear, Front and side, each have 3 pins and a Molex(?) adapter. I have a Corsair 650w power supply with Molex(?) connectors and a P7P55D-E Evo motherboard with 4 fan connectors; 4 pin CPU_FAN, 4 pin CHA_FAN1, 3 pin _FAN2 and 3 pin PWR_FAN.

I connected the CPU fan (Cooler Master Hyper 212) to the CPU_FAN but not sure about the case fans. I have read conflicting information about these fans, eg Do/dont connect case fan to PWR_FAN, do/dont connect a 3 pin fan to a 4 pin etc.

Is it best to just connect all to the PSU using the Molex adaptors?:)  It saves a lot of worry!! Is there an advantage to using Motherboard connectors?

Thanks again, I always ask for advice on this forum, appreciate the support!
a b ) Power supply
a b V Motherboard
March 15, 2010 2:59:11 PM

The PWR_FAN connector on the mobo is ONLY to be used to connect to your PSU, and then ONLY if your PSU has a connector for it. Many do not. Those PSU's that have a 3-pin Fan connector on the end of wires are simply providing a speed signal from the PSU's internal fan to the mobo so it can be monitored and displayed for you. The system does not actually provide any power to the PSU fan, nor does it control its speed. Those functions are done within the PSU itself.

Case fans should be plugged into mobo connectors marked SYS_FANx or CHA_FANx. When you do that they will have their speeds read into the mobo so you can see them. Moreover, you will have some options for how the mobo controls the speed of these case fans, based on temperature measured by a sensor built into the mobo. If you connect a case fan directly to a PSU output like a Molex connector, it will run at full speed all the time. Alternatively, you can buy a fan speed controller that connects between the PSU output and the fan. It will allow you to set the fan to some slower speed manually, but there is no automatic speed control.

By the way, a Molex connector is the type that is used to supply power to IDE hard drives and optical drives. The one on the end of wires from the PSU is female - it contains four tubular brass pieces recessed inside a plastic body, and it mates with four pins fixed into the body of the drive unit. The four contacts are in a straight line and the overall connector is roughly ¾" wide x (less than ¼") high. Each pin is about 2mm diameter. The connector body has two rounded corners so you can only plug in one way.

CPU cooler and case fans now come in two varieties - 3-pin and 4-pin. The 3-pin version has black for Ground, red for + VDC supply (varies from 0 to 12 V), and yellow for the fan speed pulse signal being sent from motor back to the mobo. The 4-pin one uses different colors and signals, adding a square wave signal train sent to the motor. Within the motor for these fans there is a small controller that uses pulse width modulation to provide voltage to the motor so its speed can vary. It is a better motor speed control system. The connectors have been designed cleverly so that any 4-pin mobo output port can accept either 3-pin or 4-pin fan connectors. The connectors only fit one way and ensure the correct signals get to the fan, depending on how it functions. Either way, you end up with a fan whose speed in controlled by the mobo BIOS (and you have some ability to adjust this control system), AND the fan's actual speed it monitored by the BIOS for display.

With 3 fans and 2 mono ports you have a dilemma. One solution is to hook up two of them to your mobo ports and the third directly to a PSU Molex output, either with or without ad added fan speed control module. The other option is to modify two of your 3-pin case fans so they both are powered by one mobo port. To do this you splice together and solder the two black leads from the two fans, then the two red leads. You do NOT splice the yellow leads together, because that would feed a confusing two pulse signals back to the mobo. Leave the yellow lead from one fan just not connected to anything - tape off its end and coil up neatly. That one fan's speed will never be measured by anything. But the two fans wired up in parallel can be powered by one mobo CHA_FAN output.
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Best solution

March 15, 2010 3:13:32 PM

+1 Paperdoc

Personally I plug the fans directly into the psu or into a fan speed controller. Plugging the fans into the psu will make your system run a little louder, but it will always be at max air flow. Connecting it to the mother board or a fan speed controller will keep the noise down when the computer is not under heavy load. If you don’t mind a little extra noise I would just plug them into the psu and call it a day.
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a c 243 ) Power supply
a b V Motherboard
March 15, 2010 3:30:12 PM

Paperdoc said:
The PWR_FAN connector on the mobo is ONLY to be used to connect to your PSU

Why ?
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a b ) Power supply
a b V Motherboard
March 15, 2010 6:33:25 PM

The mobo PWR_FAN connector has 3 pins, and the wires coming from the PSU are red, black and yellow in most cases and do connect to the correct pins on the mobo connector. However, that port is NOT designed to put out power on the red line to the fan. It is designed only so that the black line connects mobo signal ground with fan signal ground, and the yellow line brings the pulse output stream from the fan tach generator to the mobo pin. So the mobo gets a proper speed signal to monitor. But it can't provide any power for a device plugged in there. Any regular 3-pin fan plugged in there probably won't work. Worst case I've heard of in these forums is people who managed to plug a PSU power output connector intended for a floppy drive onto the PWR_FAN pins, forcing power into that mobo port and potentially damaging it.
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a c 243 ) Power supply
a b V Motherboard
March 15, 2010 6:46:03 PM

Paperdoc said:
The mobo PWR_FAN connector has 3 pins, and the wires coming from the PSU are red, black and yellow in most cases and do connect to the correct pins on the mobo connector. However, that port is NOT designed to put out power on the red line to the fan. It is designed only so that the black line connects mobo signal ground with fan signal ground, and the yellow line brings the pulse output stream from the fan tach generator to the mobo pin. So the mobo gets a proper speed signal to monitor. But it can't provide any power for a device plugged in there. Any regular 3-pin fan plugged in there probably won't work. Worst case I've heard of in these forums is people who managed to plug a PSU power output connector intended for a floppy drive onto the PWR_FAN pins, forcing power into that mobo port and potentially damaging it.

You should check out some motherboard manuals.
The PWR_FAN header is the same as any other fan header on the board [ 12v, ground and fan sense ] obviously plugging a FDD power connector there will cause problems, connecting a fan won't.
I use the one on my P7P55D to power the pump on my H50.
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a b ) Power supply
a b V Motherboard
March 15, 2010 7:30:08 PM

That's interesting info. I assume the +12 VDC on the red line is not variable so it does not achieve speed control, but at least the power IS there. I had not read that. Now I wonder whether that it true for all such PWR_FAN ports, or just some.
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a c 243 ) Power supply
a b V Motherboard
March 15, 2010 7:36:19 PM

The same is true for my Asus Rampage formula board.
I've got a Gigabyte board in one of the office systems , I'll check it out later.
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a c 248 ) Power supply
a b V Motherboard
March 15, 2010 8:11:23 PM

paperdoc - I knew the original purpose of the pwr_fan connection. Some manufacturers still include it as a courtesy. I have a new Asus Sabertooth 55i motherboard that has a fan connector labelled pwr_fan. What surprised me was that it could be used to power a case fan without any problem. I can check the fan in BIOS and in utilities. Seems to work just fine.
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March 17, 2010 5:37:43 AM

Thanks for all the replies!

It seems I was not the only one unsure about the PWR_FAN port:) 

I decided to connect them to the PSU as the cables from the fans reach the motherboard but only just (too tight for my liking!). I also figured that there is nothing bad about having fans running at full all the time (except maybe noise!)

If the noise is too much I will try to connect to the motherboard but think it will be fine!
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May 25, 2010 1:08:11 PM

i have no idea want kind of fan i need for the same motherboard everytime i believe i have the right one its not any ideas
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June 4, 2010 3:41:48 AM

mgaddis0809

I never actually purchased a fan for the motherboard. I bought the Cooler Master Hyper 212+ heatsink/cpu cooler which comes with a fan and the Cooler Master 690 chassis which has 3 built in fans. I connected all fans directly to the power supply, a 550w corsair.

As I have connected them all directly they are always on full, actually, when I sit next to my PC I feel the cold air! I live in Tokyo and the temp is now in the high 20's. Summer will be high 30's. I guessed it is best to have them running full power. I also plan on overclocking and was told the 4 fans should be ok!

I got great advice from everyone here and know that you will too. I am not an expert and this is simply what I chose to do after getting plenty of advice! Good luck!
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June 4, 2010 3:44:46 AM

Best answer selected by will2power.
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