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Old 05-24-2015, 08:51 AM   #1
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Question How does this fan work?

FIRST
I know this may seem like a very silly question, but from the picture of the fan I can see that the fan is powered through the black wire-shielded connector on the far left, and then an optional fan can be connected through the connector in the middle... but what is the connector, the black and yellow wires coming from the optional connector, on the far right for? I connect 2 fans up just fine without the need for it, I can't see what it does.. though I'm probably being very short-sighted!

SECOND
My 2nd question is how my motherboard fan connectors work. As you can see from the 2nd picture in the motherboard manual I have 3 'Chassis' connectors and 1 'Power' connector. We can ignore the power connector as there's no way to control the speed of it.

As you can see, Chassis 1 is a 4 pin connector, and the other 2 connectors are 3 pin. I have my front 2 case fans (which are 3 pins) connected to Chassis' 2 & 3. I am able to control the speed of these fans through the motherboards software. However, when I connect a fan to Chassis 1 the fan HAS to be a 4 pin connector in order to be able to control the speed of the fan. If it's a 3 pin, the fan will just run at full speed. Now I can see on the manual the 4th pin is 'Fan Speed Control'... but why is it I'm still able to control the speeds of the other Chassis 3 pin connectors, but not this one?

Many thanks in advance for the answers!
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Old 05-24-2015, 09:34 AM   #2
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Default Re: How does this fan work?

I think that works by dropping volts. Less volts less rpm.
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Old 05-24-2015, 10:28 AM   #3
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Default Re: How does this fan work?

The 4 pins are +, ground and 2 control - a 4-pin fan is controllable by PWM.
3-pin fans are only controllable via upping or lowering the voltage going to the header, and only if the motherboard supports this. Not all mobos do, and some have extra headers that have no voltage control while other headers DO have voltage control.
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Old 05-24-2015, 10:30 AM   #4
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Default Re: How does this fan work?

Nope. It works on PWM. The same sort of signal that a flight controller feeds to the electronic speed controllers on flying RC models. The faster the pulses are, the faster the ESC makes the motor spin.
On your fan the blue wire is the PWM feed from the motherboard, telling the fan how fast to spin. The yellow is the tachometer telling the computer how fast it's spinning. A feed back loop of sorts. The red and black are the power to the fan and the fan's ESC.
Quite simple actually.

The other fans actually have no speed control as such. The way the computer adjusts the fan speed on a 3 wire is to read the rpm's from the yellow tachometer line then vary the voltage to adjust the speed.
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Old 05-25-2015, 05:12 AM   #5
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Default Re: How does this fan work?

Thank you both setishock and Darkseeker.

I knew the 4 pin connectors were PWM controlled, I just couldn't understand why I was able to control the 3 pin connectors, but not the 4 pin (with a 3 pin fan). So you're simply saying the motherboard has the feature to adjust the voltage to these 3 pin connectors, but the 4 pin connector actually must have a PWM fan connected to it?

Also, that loose connector on my fan... I'm assuming that can just be connected simply to have a reading of the 2nd fan separately, if I so choose.. would that be right?
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Old 05-25-2015, 10:54 AM   #6
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Default Re: How does this fan work?

A 4 pin fan must have the PWM signal. They're designed that way to have tighter rpm control. When the mother board says go 2600 rpms the feed back from the tach lead tells the MB how fast it's actually going. The MB keeps adjusting the PWM signal until the tachometer feedback says 2600 rpms. A 3 pin just has the tachometer lead telling the MB how fast it's going. The MB adjusts the voltage until the tach lead says it's going this or that rpms.

With voltage adjusted fans, you stand a better chance of a fan stalling as the voltage goes down. If it drops below minimum or stall speed, odds are it will just stop. On a 4 pin, however, the fan gets a constant 12 volts driver voltage and all the PWM does is adjust the phase on the ESC to control the rpms. The tachometer sends back to the fan controller how fast the fan is actually going. If it needs to speed up the PWM pulses are sent out closer together meaning there's more ON time for the fan. It speeds up. If it needs to slow down, the pulses are spread out with more dead time in between them. The fan slows down.

4 pins are more accurate speed-wise. They get used on CPUs more than anywhere else.
They cost more. They require a 4 pin header on the mother board to run them.
3 pins are more common on case fans as a tighter rpm accuracy is not needed. Should run off a 3 or 4 pin header. An adjustment will be needed in the bios to let the fan controller know it's only operating a 3 pin fan off a 4 pin header.

You can daisy chain a 3 pin but you can NOT connect more than one tachometer lead to any one fan header. You can split off the tach leads to different headers but all you're going to see is the speed the fan is going. The only one having any control is the one header that's feeding power to the chain of fans.
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Old 05-26-2015, 02:09 PM   #7
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Default Re: How does this fan work?

Brilliant, thanks Setishock, much appreciated
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