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Connecting fans to mobo/psu question

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March 22, 2012 9:52:01 PM

So I'm building a pc for the first time.

I have a HAF 922 case and a PC Power and Cooling Silencer Mk II 750w PSU. My psu has two cords with molex connections on them. At the end of one cord is a 4-pin connector.

My question is this: Do I attach my fans to the molex connectors and the 4-pin to the mobo to control the speed? If I connect a fan to the other molex cord (without a 4-pin connector) will it run at full speed or still be controlled? How do I tell which orientation to plug the 4-pin connector in?

Any and all help is greatly appreciated.
a c 243 ) Power supply
March 22, 2012 10:32:38 PM

zakh508 said:
So I'm building a pc for the first time.

I have a HAF 922 case and a PC Power and Cooling Silencer Mk II 750w PSU. My psu has two cords with molex connections on them. At the end of one cord is a 4-pin connector.

My question is this: Do I attach my fans to the molex connectors and the 4-pin to the mobo to control the speed? If I connect a fan to the other molex cord (without a 4-pin connector) will it run at full speed or still be controlled? How do I tell which orientation to plug the 4-pin connector in?

Any and all help is greatly appreciated.

That is a Berg connector ( floppy drive power ), do not connect it to your motherboard.

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a c 157 ) Power supply
March 22, 2012 11:55:37 PM

+1^
If your fans have molex connectors, connect them to the PSU's molex connectors. They will receive the full +12V and run at full speed.
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March 23, 2012 1:03:34 AM

Okay thanks, i turns out the fans I have are ridiculously quiet even at full speed so I'll just keep 'em plugged in.
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Best solution

a b ) Power supply
March 23, 2012 2:54:15 AM

The ONLY way for a mobo to control fan speeds is if the fan is plugged into one of the mobo's FAN ports. The options for that usually are the CPU_FAN port (use ONLY for the CPU cooler), and one, two or more SYS_FANx ports intended for case cooling fans. (See last paragraph for one other port type.)

Fans CAN be powered instead from 4-pin Molex power output connectors from the PSU. Those are the ones about 1" wide with 4 round holes in them in a straight line. These provide the Ground and +12 VDC supplies needed to run a fan at full speed, BUT there is NO fan speed control this way. The +12 VDC line is ALWAYS at that voltage.

You will find a smaller 4-pin connector on the same set of wires from the PSU that has the 4-pin Molex larger connector. That smaller one is the power supply connector for a 3½" floppy disk drive. You must NEVER plug this into any port of the mobo. I recall one forum in which a poster told of doing this (plugged into one of the SYS_FAN ports), mistakenly believing this is how fan speed control could be done. This applied power from the PSU to the port, and guess what? It burned out part of his mobo!!

There are ways to provide power to fans from a 4-pin Molex connector and also to achieve some fan speed control. That can be done by buying and installing third-party fan controller units. The master unit gets power from the Molex; you plug the fans into the output ports of the controller unit. There are two basic types of these. The simpler one just has speed control knobs on the front and allows you to manually set each fan's speed to your liking. It may or may not display the fan speed on a front panel. The more complex ones try to duplicate what a mobo's automatic controller can do. They include temperature sensors you must mount at specified points on your mobo, and they try to control respective fan speeds according to the temps measured that way.

There is one other type of "fan port" on many mobos, labelled PWR_FAN. Its intended purpose it simply to accept the connector on a special set of wires coming out of the PSU, and they (and the connector) look just like a 3-pin fan connection. BUT what this really does is send the fan speed signal from the PSU's internal fan to the mobo via this port so your mob can monitor it and show it to you. The PSU fan speed is NOT actually controlled by this port. IF the PSU speed is controlled, that is done within the PSU itself. NOTE that some PSU's do NOT have one of these signal output connectors, so you can't do this. However, many people have found that the mobo PWR_FAN connector does actually have Ground and +12 VDC on its first two pins, so if you're not using it to connect the speed signal wires from the PSU, you can plug in there a 3-pin fan which will always run at full speed. There is no speed control on this port because it never was designed to control the fan in the PSU.
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March 24, 2012 4:53:08 AM

I'm very glad I didn't plug that in. I found out all the fans had 3-pin to molex adapters on the so I took off all of them and plugged the fans into the cha_fan headers on my motherboard. Thanks for your help
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March 24, 2012 4:53:40 AM

Best answer selected by zakh508.
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