Splitting CPU 4 Pin Header

[PhoenixRestored]

Can you split the 4 pin cpu header to use a 4 pin cpu fan and a 4 pin sytem fan on it?

 

[ehintzy]

Can you elaborate? If I understand you correctly the answer is no because there will not be enough power

 

[PhoenixRestored]

Can you elaborate? If I understand you correctly the answer is no because there will not be enough power

Im wondering if you can split the cpu header to power both a sytem fan and the cpu fan at the same time. You are saying it won't have enough power?

The thing is, my motherboard does not have any 4 pin system headers but I would still like to have PWM control because both my new fans are 4 pin.

 

[ehintzy]

No this will not work. Get a 4 pin spliter

 

[shortstuff_mt]

Yes, you can purchase a splitter cable and power two fans from a single motherboard header. I have a CPU cooler that has two PWM fans. I'm using a splitter to power both fans and it works great. The fans both vary their speed according to the fan profile I set up in the BIOS. The splitter linked earlier will power both 3-pin and 4-pin fans.

 

[PhoenixRestored]

Yes, you can purchase a splitter cable and power two fans from a single motherboard header. I have a CPU cooler that has two PWM fans. I'm using a splitter to power both fans and it works great. The fans both vary their speed according to the fan profile I set up in the BIOS. The splitter linked earlier will power both 3-pin and 4-pin fans.

So the fact that it is a CPU header makes no difference? Everything will be stable? I have heard that it will make the system fan run like it is monitoring the cpu if done like this, is that true?

 

[PhoenixRestored]

No this will not work. Get a 4 pin spliter

But you said it will not work? Are you saying a 4 pin splitter will make a 3 pin header on the motherboard have pwm control?

 

[ehintzy]

With the spiltter you will be able to control them both. With a 3 pin you can control speeds via voltage.

 

[shortstuff_mt]

Yes, you can purchase a splitter cable and power two fans from a single motherboard header. I have a CPU cooler that has two PWM fans. I'm using a splitter to power both fans and it works great. The fans both vary their speed according to the fan profile I set up in the BIOS. The splitter linked earlier will power both 3-pin and 4-pin fans.

So the fact that it is a CPU header makes no difference? Everything will be stable? I have heard that it will make the system fan run like it is monitoring the cpu if done like this, is that true?

Yes, if you are using PWM to control the fan speeds then the fans will be linked to the fan profile for the fan header they are plugged into. My fans are in a push-pull configuration on my CPU cooler, so that's the exact functionality I wanted/expected. I don't see a problem with having a system fan varying its speed according to CPU temp. If that's not the functionality you want then you can look into other options. If you want the system fan to run at full speed you can use a 4Pin Molex to 4Pin PWM adapter.

 

[PhoenixRestored]

Yes, you can purchase a splitter cable and power two fans from a single motherboard header. I have a CPU cooler that has two PWM fans. I'm using a splitter to power both fans and it works great. The fans both vary their speed according to the fan profile I set up in the BIOS. The splitter linked earlier will power both 3-pin and 4-pin fans.

So the fact that it is a CPU header makes no difference? Everything will be stable? I have heard that it will make the system fan run like it is monitoring the cpu if done like this, is that true?

More in terms of power consumption, Im going to have a positive pressure in my case hopefully.

Yes, if you are using PWM to control the fan speeds then the fans will be linked to the fan profile for the fan header they are plugged into. My fans are in a push-pull configuration on my CPU cooler, so that's the exact functionality I wanted/expected. I don't see a problem with having a system fan varying its speed according to CPU temp. If that's not the functionality you want then you can look into other options. If you want the system fan to run at full speed you can use a 4Pin Molex to 4Pin PWM adapter.

Since I can only have 1 PWM system fan, if I split the CPU header, which of my fans should I use? The 120 mm or 140 mm (both are pwm)?

 

[shortstuff_mt]

Bigger fans usually push more air at a lower RPM. If I had to choose between a 140mm and a 120mm system fan I would pick the 140mm every time. You could get creative to run three fans from the CPU header if you want. You could either use a 3-way splitter or use two regular splitters. One splitter would just plug in to the other. I would just use the 3-way splitter and run all three fans from the CPU fan header.

 

[PhoenixRestored]

Bigger fans usually push more air at a lower RPM. If I had to choose between a 140mm and a 120mm system fan I would pick the 140mm every time. You could get creative to run three fans from the CPU header if you want. You could either use a 3-way splitter or use two regular splitters. One splitter would just plug in to the other. I would just use the 3-way splitter and run all three fans from the CPU fan header.

Ah thank you. But the other person who answered this thread said at first that a single splitter would not have enough power, would having a 3 way splitter not cause any power issues?

 

[shortstuff_mt]

No, it will work fine to power three fans from a single header. Companies wouldn't sell the adapters if they didn't work.

 

[PhoenixRestored]

No, it will work fine to power three fans from a single header. Companies wouldn't sell the adapters if they didn't work.

Oh ok. Do you know of any 3 way pwm splitters then? I can't seem to find any

 

[Paperdoc]

There's a much easier way to do this, and it uses the backwards compatibility designed into 4-pin fans. You do not need splitters, etc.

First you need to recognize that the pin arrangement and mechanical design of 3- and 4-pin fans and ports means you can plug any fan into any port, and it will work at least to some extent. So you CAN plug a 4-pin fan into a mobo 3-pin SYS_FAN port, and it will work.

How well? Quite well, actually. A 4-pin fan connected to a 3-pin mobo case fan (SYS_FANn) port will receive a DC supply voltage that varies according to the mobo's automatic case fan control system; thus, its speed WILL change under proper control. It also will send its own fan speed signal back to the mobo to be monitored. The fan will NOT receive the PWM signal from pin #4 because there is no pin #4 on the port. But the function of a PWM signal is to REDUCE the fan's speed from full speed in a true 4-pin control system. With no PWM signal, the fan simply will not reduce its speed and will run at the speed determined by the (variable) DC voltage, which is the real way the fan's speed will be controlled.

There is only one thing you "lose" doing this. The fan speed control will be accomplished by varying the voltage supplied to it (the same way a 3-pin fan is controlled), rather than by using PWM control. The PWM system is slightly better at running a fan at low speed, and at STARTING a fan at low speed. (A typical 3-pin fan control system will START such a fan at full speed and then reduce voltage to set the low speed.)

A mobo bases its SYS_FAN speed control on an actual temperature measured by a sensor built into the mobo. So it make sense to let this system control the speed of the case ventilation fan, rather than using the CPU's system which is based on a temperature sensor INSIDE the CPU chip.

 

[PhoenixRestored]

There's a much easier way to do this, and it uses the backwards compatibility designed into 4-pin fans. You do not need splitters, etc.

First you need to recognize that the pin arrangement and mechanical design of 3- and 4-pin fans and ports means you can plug any fan into any port, and it will work at least to some extent. So you CAN plug a 4-pin fan into a mobo 3-pin SYS_FAN port, and it will work.

How well? Quite well, actually. A 4-pin fan connected to a 3-pin mobo case fan (SYS_FANn) port will receive a DC supply voltage that varies according to the mobo's automatic case fan control system; thus, its speed WILL change under proper control. It also will send its own fan speed signal back to the mobo to be monitored. The fan will NOT receive the PWM signal from pin #4 because there is no pin #4 on the port. But the function of a PWM signal is to REDUCE the fan's speed from full speed in a true 4-pin control system. With no PWM signal, the fan simply will not reduce its speed and will run at the speed determined by the (variable) DC voltage, which is the real way the fan's speed will be controlled.

There is only one thing you "lose" doing this. The fan speed control will be accomplished by varying the voltage supplied to it (the same way a 3-pin fan is controlled), rather than by using PWM control. The PWM system is slightly better at running a fan at low speed, and at STARTING a fan at low speed. (A typical 3-pin fan control system will START such a fan at full speed and then reduce voltage to set the low speed.)

A mobo bases its SYS_FAN speed control on an actual temperature measured by a sensor built into the mobo. So it make sense to let this system control the speed of the case ventilation fan, rather than using the CPU's system which is based on a temperature sensor INSIDE the CPU chip.

Oh wow, I did not know most of that, thanks. And are these voltage changes manual or automatic since the actual fan is PWM?

 

[shortstuff_mt]

No, it will work fine to power three fans from a single header. Companies wouldn't sell the adapters if they didn't work.

Oh ok. Do you know of any 3 way pwm splitters then? I can't seem to find any

It's true that you can plug 4-pin fans into a 3-pin header and the fan will vary speed according to the voltage provided to the fan. That is certainly an option. I linked to a 3-way PWM splitter in an earlier post, but you might have missed the hyperlink. Here's the splitter I linked to above:

Silverstone CPF02 11.81" 300mm (100mm/100mm/100mm) Black Sleeved One-to-Three PWM Fan Splitter

 

[PhoenixRestored]

No, it will work fine to power three fans from a single header. Companies wouldn't sell the adapters if they didn't work.

Oh ok. Do you know of any 3 way pwm splitters then? I can't seem to find any

It's true that you can plug 4-pin fans into a 3-pin header and the fan will vary speed according to the voltage provided to the fan. That is certainly an option. I linked to a 3-way PWM splitter in an earlier post, but you might have missed the hyperlink. Here's the splitter I linked to above:

Silverstone CPF02 11.81" 300mm (100mm/100mm/100mm) Black Sleeved One-to-Three PWM Fan Splitter

I guess I did miss it, but thanks. Would you happen to know if the other guy's solution would be manual or automatic with the voltage control? That solution would technically be easier with the fan placement I want if the voltage change is automatic, but I don't understand how that would be.
Also, with the Silverstone header that was linked, and with one other splitter I saw, it says that on two of the female heads have 3 pins including the PWM pin (it is missing one of the middle ones); what does that mean or do to the fan?

 

[shortstuff_mt]

If the motherboard supports fan speed control on the system fan headers then you can set up profiles in the BIOS for the fan speed. I haven't seen where you have posted what motherboard you have. I believe the missing pin in the splitters is the RPM signal back to the motherboard. You only want that signal coming from one fan since the motherboard thinks a single fan is plugged in to the header.

 

[Paperdoc]

More info to help you understand the fans.

Most mobos have at least three fan ports - 1 for the CPU_FAN, and two marked SYS_FANn. A few even have a second CPU_FAN port for systems that use 2 fans on the CPU cooler. A few have more than 2 SYS_FAN ports. All of these are capable of automatic fan speed control. Most are set to do this by default, but allow you to choose to disable that. They all work on the same basic feedback control system. For each fan there is a temperature sensor and a target temperature. Actually, there are three temperatures: the target, a lower limit below which the fan will shut off or at least slow to minimum speed, and an upper limit at which the fan should be running at full speed. Anywhere between the lower and upper limits, the fan speed will be automatically adjusted to try to achieve the target. The main differences between the ports are where the temperature sensors are, and how they change the fan speed. NOTE that the control loop in each case is trying to achieve a measured TEMPERATURE, not a fan speed. So, although fan speed is measured and displayed for you, it is not used in the control system. The actual three temperatures are different for different CPU chips and mobo chips, and the BIOS is pre-programmed to recognize those items and set those three temp settings for you. But you can change them if you wish in most cases.

The CPU_FAN speed control loop uses a temp sensor built into the CPU chip itself, so it is important to connect to this port ONLY the cooler for the CPU. This cooling system also has a couple of extra functions not directly part of the cooling system, but dependent on the temperature sensor. The first is two additional high temp limits. If the first is reached, the BIOS will slow down the CPU frequency substantially to keep it running but generating much less heat, hoping it will cool down again. At a higher temp, the BIOS will shut the system down completely to prevent damage to the CPU. The other protection in SOME mobos is based on monitoring the CPU fan's speed. If the fan speed signal fails, the BIOS assumes the fan has failed and is not working. In that case it will shut down the entire system very quickly, not even bothering to wait for the CPU temp sensor to indicate a high temp condition. Again, this is a good reason to connect ONLY the CPU cooling system to the CPU_FAN port. If a user opts to use a different cooling system and does not connect the CPU fan here, it is best to go into BIOS and set that fan control system's option to IGNORE the speed of the CPU fan so it does not create false alarms from a lack of speed signal.

The two SYS_FAN ports are intended to power and control case ventilation fans. Each uses a temp sensor built into the mobo, sometimes in or very close to two critical chips, the North Bridge and South Bridge. Cooling of all mobo components is heavily dependent on case ventilation.

Mobos generally do NOT participate in cooling of components on an added video card. That is left to the vid card maker to design and control cooling of their GPU chip, and possibly provide software for you to adjust that or at least to see the results.

Now to fan types and how speed control is accomplished. The common 3-pin fan and port provides a Ground connection on Pin 1 (Black wire), +VDC on Pin 2 (Red wire) and a speed pulse signal on Pin 3 (Yellow wire). The +VDC line voltage is changed by the mobo automatic system between 0 and 12 VDC to achieve speed changes. The speed pulse signal (2 pulses per revolution) is generated in the fan motor and fed back to the mobo via Pin 3, where the pulses can be counted to display the fan speed and verify that it is working. This is why adapters that allow you to connect more than one fan to a single port must NOT connect all the fans' Yellow wires together. Feeding more than one string of pulses into the mobo counting system would produce erroneous speed readings.

4-pin fans have exactly the same connections on Pins 1, 2 and 3, with one exception. The +VDC line on Pin 2 is ALWAYS +12 VDC, and is not changed by the automatic speed controller. Pin 4 supplies the PWM (Pulse Width Modulation) signal at a frequency of about 25 kHz. It looks almost like a square wave, except that it is not just 50% on and 50% off. Its "% On" value is changed to achieve fan speed control. Inside the fan case there is a small controller chip that uses this signal to control when the +12 VDC supply from Pin 2 is actually passed on to the fan, so the fan does not receive power at all times. In fact, the PWM signal is used as a suppressor: when it is "On", the voltage is NOT supplied to the fan motor, so the more the PWM signal is on, the slower the fan will run. This is part of the backward compatibility feature: if you plug a 4-pin fan into a 3-pin port it will receive no PWM signal, and the fan motor will always receive whatever voltage the 3-pin port is providing on Pin 2, allowing speed control of that fan.

On the other hand, if you plug a 3-pin fan into a 4-pin fan port, the fan cannot use the PWM signal and it will always run at full speed, because the 4-pin port always keeps the Pin 2 signal at +12 VDC.

There are some mobos with very clever 4-pin fan ports. Some allow you to set in BIOS Setup whether the port behaves as a 4-pin or 3-pin port. Some even automate this process and detect the fan type connected, and adjust their own behaviour. In both cases you can achieve automatic control of any fan plugged into this clever port, 3-pin or 4-pin.

Some mobos have another fan port called PWR_FAN with 3 pins. It is NOT a normal fan port. The design intent for this was to allow use of Power Supply Units (PSU's) that have a special set of three wires coming out that look exactly like a 3-pin fan wire set, and can plug into this special port. All it actually does is feed to the mobo the speed signal from the cooling fan inside the PSU. The port does NOT make any attempt to control that PSU fan speed - if it is variable, that control is done internally inside the PSU. In general, unless you have a PSU with such a set of tires coming out, do NOT connect anything to the mobo PWR_FAN port. HOWEVER, many mobo makers have put the Ground and +12 VDC signals on Pins 1 and 2 of this port. The result is that you CAN connect a fan (3-pin or 4-) here and it will run at full speed all the time. If you do this you must remember that any display of the "PSU Fan Speed" is not, because you connected the wrong fan to the port.

 

[PhoenixRestored]

More info to help you understand the fans.

Most mobos have at least three fan ports - 1 for the CPU_FAN, and two marked SYS_FANn. A few even have a second CPU_FAN port for systems that use 2 fans on the CPU cooler. A few have more than 2 SYS_FAN ports. All of these are capable of automatic fan speed control. Most are set to do this by default, but allow you to choose to disable that. They all work on the same basic feedback control system. For each fan there is a temperature sensor and a target temperature. Actually, there are three temperatures: the target, a lower limit below which the fan will shut off or at least slow to minimum speed, and an upper limit at which the fan should be running at full speed. Anywhere between the lower and upper limits, the fan speed will be automatically adjusted to try to achieve the target. The main differences between the ports are where the temperature sensors are, and how they change the fan speed. NOTE that the control loop in each case is trying to achieve a measured TEMPERATURE, not a fan speed. So, although fan speed is measured and displayed for you, it is not used in the control system. The actual three temperatures are different for different CPU chips and mobo chips, and the BIOS is pre-programmed to recognize those items and set those three temp settings for you. But you can change them if you wish in most cases.

The CPU_FAN speed control loop uses a temp sensor built into the CPU chip itself, so it is important to connect to this port ONLY the cooler for the CPU. This cooling system also has a couple of extra functions not directly part of the cooling system, but dependent on the temperature sensor. The first is two additional high temp limits. If the first is reached, the BIOS will slow down the CPU frequency substantially to keep it running but generating much less heat, hoping it will cool down again. At a higher temp, the BIOS will shut the system down completely to prevent damage to the CPU. The other protection in SOME mobos is based on monitoring the CPU fan's speed. If the fan speed signal fails, the BIOS assumes the fan has failed and is not working. In that case it will shut down the entire system very quickly, not even bothering to wait for the CPU temp sensor to indicate a high temp condition. Again, this is a good reason to connect ONLY the CPU cooling system to the CPU_FAN port. If a user opts to use a different cooling system and does not connect the CPU fan here, it is best to go into BIOS and set that fan control system's option to IGNORE the speed of the CPU fan so it does not create false alarms from a lack of speed signal.

The two SYS_FAN ports are intended to power and control case ventilation fans. Each uses a temp sensor built into the mobo, sometimes in or very close to two critical chips, the North Bridge and South Bridge. Cooling of all mobo components is heavily dependent on case ventilation.

Mobos generally do NOT participate in cooling of components on an added video card. That is left to the vid card maker to design and control cooling of their GPU chip, and possibly provide software for you to adjust that or at least to see the results.

Now to fan types and how speed control is accomplished. The common 3-pin fan and port provides a Ground connection on Pin 1 (Black wire), +VDC on Pin 2 (Red wire) and a speed pulse signal on Pin 3 (Yellow wire). The +VDC line voltage is changed by the mobo automatic system between 0 and 12 VDC to achieve speed changes. The speed pulse signal (2 pulses per revolution) is generated in the fan motor and fed back to the mobo via Pin 3, where the pulses can be counted to display the fan speed and verify that it is working. This is why adapters that allow you to connect more than one fan to a single port must NOT connect all the fans' Yellow wires together. Feeding more than one string of pulses into the mobo counting system would produce erroneous speed readings.

4-pin fans have exactly the same connections on Pins 1, 2 and 3, with one exception. The +VDC line on Pin 2 is ALWAYS +12 VDC, and is not changed by the automatic speed controller. Pin 4 supplies the PWM (Pulse Width Modulation) signal at a frequency of about 25 kHz. It looks almost like a square wave, except that it is not just 50% on and 50% off. Its "% On" value is changed to achieve fan speed control. Inside the fan case there is a small controller chip that uses this signal to control when the +12 VDC supply from Pin 2 is actually passed on to the fan, so the fan does not receive power at all times. In fact, the PWM signal is used as a suppressor: when it is "On", the voltage is NOT supplied to the fan motor, so the more the PWM signal is on, the slower the fan will run. This is part of the backward compatibility feature: if you plug a 4-pin fan into a 3-pin port it will receive no PWM signal, and the fan motor will always receive whatever voltage the 3-pin port is providing on Pin 2, allowing speed control of that fan.

On the other hand, if you plug a 3-pin fan into a 4-pin fan port, the fan cannot use the PWM signal and it will always run at full speed, because the 4-pin port always keeps the Pin 2 signal at +12 VDC.

There are some mobos with very clever 4-pin fan ports. Some allow you to set in BIOS Setup whether the port behaves as a 4-pin or 3-pin port. Some even automate this process and detect the fan type connected, and adjust their own behaviour. In both cases you can achieve automatic control of any fan plugged into this clever port, 3-pin or 4-pin.

Some mobos have another fan port called PWR_FAN with 3 pins. It is NOT a normal fan port. The design intent for this was to allow use of Power Supply Units (PSU's) that have a special set of three wires coming out that look exactly like a 3-pin fan wire set, and can plug into this special port. All it actually does is feed to the mobo the speed signal from the cooling fan inside the PSU. The port does NOT make any attempt to control that PSU fan speed - if it is variable, that control is done internally inside the PSU. In general, unless you have a PSU with such a set of tires coming out, do NOT connect anything to the mobo PWR_FAN port. HOWEVER, many mobo makers have put the Ground and +12 VDC signals on Pins 1 and 2 of this port. The result is that you CAN connect a fan (3-pin or 4-) here and it will run at full speed all the time. If you do this you must remember that any display of the "PSU Fan Speed" is not, because you connected the wrong fan to the port.

I can't find anything in my BIOS having to do with system fans except in the hardware monitor where it displays the RPM speed; nothing to do with profiles except for CPU "Smart" Fan

 

[Paperdoc]

Tell us what mobo you have - exact make and model. Then we can look up your manual and help find stuff if it's there.