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Question regarding motherboard fan slots/speed control.

Last response: in Components
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August 31, 2012 8:25:12 PM

So I have an ASRock Z77 Pro4 (http://www.asrock.com/mb/Intel/Z77%20Pro4/) motherboard and I'm trying to hook up my fans. I have the CPU in CPU_FAN1. Here are my remain slots:

CPU_FAN2 - 3-pin Empty.
CHA_FAN1 - 4-pin connector, all of my fans are 3-pin.
CHA_FAN2 - 3-pin connector. I have a splitter so I can at least plug two of my four fans into here.
PWR_FAN1 - 3-pin Empty.

Here are my questions:

1. I'm trying to let the motherboard control the fan speed automatically, not with PWM as I don't have PWM fans, but just with the standard motherboard auto controls using 3-pins which according to my research will work. I understand that CHA_FAN will have this functionality, but even though they're case fans and not CPU fans, can PWR_FAN and CPU_FAN have automatically controlled speeds by the motherboard and work correctly?

2. If I need to use the 4-pin CHA_FAN1 can I plug a 3-pin into it without any negative impact?

3. I accidentally bought a PWM splitter that I could presumably use if #2 is OK?

Any other advice? I want to get my PC up and running tonight and want the fans controlled, but I need to find a slot for my last two fans somewhere between CHA_FAN2, PWR_FAN1, and CPU_FAN2. I don't have another adapter and they don't appear to be sold locally so I'd prefer to work with what I've got. Thank you!
August 31, 2012 8:42:34 PM

the only fans you can control are the 4 pin fans as they are usually used for the cpu but the 3 pins run at 100% all the time unless you connect them to a fan controller.

thats from my own experience
August 31, 2012 9:02:29 PM

jamesg2571 said:
the only fans you can control are the 4 pin fans as they are usually used for the cpu but the 3 pins run at 100% all the time unless you connect them to a fan controller.

thats from my own experience

Reading up it seems to say 3-pin fans can be dropped between three different voltages to alter fan speed but that's all I need. PWM from what I understand offers way more speeds/options. Only fans with Molex (power) connectors cannot be controlled at all and are always at 100%.
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September 2, 2012 4:31:35 PM

You can plug a 3 pin fan into a 4 pin header and it will work fine , it you look at the connector closely you will notice that the connector has a flat piece sticking up beside the pins and this is positioned so that a three pin will slide onto that spot and the fourth pin will be left out. This is done by design and that flat piece is put there for three pin connectors.
Usually a fan will run on a two pin connector and will run at 100% and when you add the third pin you then add controll of the fan speed , the fourth pin adds temp control. The reason the MB has three pin connectors is so that the bios can controll the speed of the fans , the third pin reports the rpm that the fan is running at.
I never used the bios to control the case fans I always prefered a fan controller mounted in the front 5.25 bay because it gave me more direct fan control. Those front bay fan controler can be very diverse and the more expensive ones will automaticly control fan speed acordind to the temps since you can make various sttings.

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a b ) Power supply
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September 3, 2012 2:30:42 AM
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CPU fan plugged into CPU_FAN1 look correct. As long as both fan and port are 3-pin, or both are 4-pin, it will work perfectly. With this you CAN (and will by default) control the CPU cooler automatically according to the internal temp inside the CPU case. That temp is measured by a sensor built into the CPU chip and read by the mobo.

inzone misunderstands the 3rd and 4th wires on fans. The first two wires are Ground (Black on a 3-pin) on Pin 1 and +12 VDC (Red on 3-pin) on Pin 2 for power to the fan. Pin 3 is the fan speed signal line (Yellow on a 3-pin). I carries a pulse train signal generated inside the fan motor (2 pulses per revolution) back to the mobo for monitoring. However, interestingly, it is NOT used to control fan speed - only to read it (and, in some cases, to trigger a warning if it stops). On a 4-pin fan, the 4th pin carries the PWM signal from the mobo to a special chip inside the fan case that uses it to control the flow of current from the +12 VDC line through the motor.

Actual control of speed is base entirely on measured temperature at some appropriate point, and a setpoint. Whether 3-pin or 4-pin, the basic control strategy is the same. For the CPU, the temp sensor is built into the CPU chip. For the case or SYS_FAN or CHA_FAN, the temp sensor is built into the mobo at some point. In each fan control loop, the measured temp from its sensor is compared to a setpoint and used to slow down or speed up that fan.

HOW the speed of the fan is changed is what differs. On a 3-pin system, the mobo reduces the +12 VDC line voltage to slow the fan down. On a 4-pin system, that +12 VDC line is kept at that voltage always, but the special PWM signal is used inside the motor casing to control how much of the time that voltage is actually turned on to the motor. The chip using this signal is a simple on /off switch, turning on the feed of 12 VDC to the motor and turning it off again, very rapidly at the frequency of the PWM signal. That signal looks like a "square wave", except that is "% ON" is varied (not constantly 50%) to control fan speed.

Thus, a 3-pin fan plugged into a "normal" 4-pin output port will always run at full speed because it has no way to use the PWM signal (and it does not even get it - no 4th wire!). Now, some mobos allow you to set whether the fan you plugged into a 4-pin port was 3- or 4- pin type, and if you say 3, it will change the way the port is managed to make it a 3-pin port with the +VDC line voltage varying to achieve control. In fact, some mobos are so smart they sense this themselves and set their mode automatically. I don't know about your mobo, but read its manual for clues.

The PWR_FAN port is different. It was originally designed to work with some PSU's that have coming out of them what looks like a normal 3-pin fan set of wires. But the real function was ONLY to carry the speed pulse signal from the PSU fan's motor to the mobo to be measured and displayed. It does not actually control the speed of the fan inside the PSU. Now, some PSU's actually do control their own fan's speed, but they don't let the mobo do it. So, the original plan was that, if you don't have those special connection lines coming out of your PSU, you plug nothing into the PWR_FAN port. HOWEVER, apparently on many mobos that PWR_FAN port also has the normal Ground and +12 VDC on Pins 1 and 2. Thus, if you plug into it a 3- or a 4-pin fan, it will provide the full 12 VDC power supply to the fan, running it at full speed, and your BIOS will report that the PSU's fan speed is (whatever that fan speed is), NOT the PSU's fan speed.
September 6, 2012 12:53:23 AM

Paperdoc said:
CPU fan plugged into CPU_FAN1 look correct. As long as both fan and port are 3-pin, or both are 4-pin, it will work perfectly. With this you CAN (and will by default) control the CPU cooler automatically according to the internal temp inside the CPU case. That temp is measured by a sensor built into the CPU chip and read by the mobo.

inzone misunderstands the 3rd and 4th wires on fans. The first two wires are Ground (Black on a 3-pin) on Pin 1 and +12 VDC (Red on 3-pin) on Pin 2 for power to the fan. Pin 3 is the fan speed signal line (Yellow on a 3-pin). I carries a pulse train signal generated inside the fan motor (2 pulses per revolution) back to the mobo for monitoring. However, interestingly, it is NOT used to control fan speed - only to read it (and, in some cases, to trigger a warning if it stops). On a 4-pin fan, the 4th pin carries the PWM signal from the mobo to a special chip inside the fan case that uses it to control the flow of current from the +12 VDC line through the motor.

Actual control of speed is base entirely on measured temperature at some appropriate point, and a setpoint. Whether 3-pin or 4-pin, the basic control strategy is the same. For the CPU, the temp sensor is built into the CPU chip. For the case or SYS_FAN or CHA_FAN, the temp sensor is built into the mobo at some point. In each fan control loop, the measured temp from its sensor is compared to a setpoint and used to slow down or speed up that fan.

HOW the speed of the fan is changed is what differs. On a 3-pin system, the mobo reduces the +12 VDC line voltage to slow the fan down. On a 4-pin system, that +12 VDC line is kept at that voltage always, but the special PWM signal is used inside the motor casing to control how much of the time that voltage is actually turned on to the motor. The chip using this signal is a simple on /off switch, turning on the feed of 12 VDC to the motor and turning it off again, very rapidly at the frequency of the PWM signal. That signal looks like a "square wave", except that is "% ON" is varied (not constantly 50%) to control fan speed.

Thus, a 3-pin fan plugged into a "normal" 4-pin output port will always run at full speed because it has no way to use the PWM signal (and it does not even get it - no 4th wire!). Now, some mobos allow you to set whether the fan you plugged into a 4-pin port was 3- or 4- pin type, and if you say 3, it will change the way the port is managed to make it a 3-pin port with the +VDC line voltage varying to achieve control. In fact, some mobos are so smart they sense this themselves and set their mode automatically. I don't know about your mobo, but read its manual for clues.

The PWR_FAN port is different. It was originally designed to work with some PSU's that have coming out of them what looks like a normal 3-pin fan set of wires. But the real function was ONLY to carry the speed pulse signal from the PSU fan's motor to the mobo to be measured and displayed. It does not actually control the speed of the fan inside the PSU. Now, some PSU's actually do control their own fan's speed, but they don't let the mobo do it. So, the original plan was that, if you don't have those special connection lines coming out of your PSU, you plug nothing into the PWR_FAN port. HOWEVER, apparently on many mobos that PWR_FAN port also has the normal Ground and +12 VDC on Pins 1 and 2. Thus, if you plug into it a 3- or a 4-pin fan, it will provide the full 12 VDC power supply to the fan, running it at full speed, and your BIOS will report that the PSU's fan speed is (whatever that fan speed is), NOT the PSU's fan speed.


Just wanted to thank you for this very well written response. I was browsing the web with similar questions and stumbled across this thread. Found this very helpful!

-Finn
September 7, 2012 12:12:15 AM

Best answer selected by shadowxor.
a b ) Power supply
a b à CPUs
a b V Motherboard
September 7, 2012 7:11:39 AM

This topic has been closed by Mousemonkey
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