Drive three case fans from one 3-pin headers?

[Timothy Miller]

I replaced a failed ATX motherboard with a new micro ATX. The old board had 3 case fan headers, while the new one only has 1. I can run the extra fans via adaptors from molex to 3-pin, but I'd like them to be variable speed.

So I have a stupid question. What if I were to try to drive three fans from one 2-pin header?

Too much current demand?

What's the third pin for? Hall effect rotation speed sensor or something? If so, then I'm guessing that even two fans on the same connector would confuse the hell out of the measurement circuitry.

The new board has a Haswell on it. That runs really cool, but I like the intake fans to blow across the hard drives. I suppose another option is to use an in-line resistor to manually run the intake fans at low speed.

Any other clever ideas?

Thanks.

 

[Tech Tweaker]

The third pin is for RPM sensing.

As for whether or not it is too much voltage or amperage, it depends largely on what kind of fans you are using.

I'd recommend a fan controller personally, if you want to control their speeds. It's how I got around the same problem with my mATX that only has three onboard fan headers (I run 11 fans and a watercooling pump (Corsair H60) though).

Yes, putting two fans on the same header typically confuses the sensor, making it useless for RPM sensing. Even with fans of the same make and model this can cause problems.

Or you could buy a bunch of 3-pin fan splitters and connect them to the power supply directly via 4-pin molex to 3-pin fan power adapters, but you would have no control over their speeds.

 

[dostov]

I usually stay under 10 watts for one header and I never had any problems.

As for speed monitoring, you can plug all the fans with splitters but disconnect all yellow wires but one. This will report the speed correctly for that one fan that is still connected, and you can assume the speed is the same for all others of you use all the same model.

 

[Marc79]

Get a splitter with a power connector, I wouldn't recommend running 3 fans off one header you don't want to overload it. I use Akasa 3 way splitter with power connector to run 2 of my radiator fans off one cpu_header. You can connect up to 3 fans with this splitter, there's also a 5 way splitter available if you need to connect more fans. The power for the fans will come from your PSU sata connector.

Here's the link to a 3 way splitter that I have.

http://www.frozencpu.com/products/1...y_Connector_AK-CBFA06-30.html?tl=g47c251s1325

 

[NiHaoMike]

What's the variable speed based on? Variable speed for a case fan can do more harm than good if implemented incorrectly. If it's based on CPU temperature, your GPU could overheat if it was loaded while the CPU is only under a light load. I have also seen really dumb "smart" fan implementations, like one that merely cycled the fan on and off - bad for solder joints!

In my PC, the side panel case fan is variable speed ("PWM" type with a digital inverter, but the concept is the same) from a signal from the motherboard (based on voltage regulator temperature), but that signal first goes through the PMIC board, where the PMIC can override that and enforce a minimum speed if it detects the GPU drawing more power or if the battery charger module is outputting a lot of current.

http://www.frozencpu.com/products/16...l=g47c251s1325

That would not work unless it's PWM type variable speed. That means a 4 pin connector while the OP's are only 3.

 

[Timothy Miller]

Good suggestions

I like the idea of using a splitter with the sense wire disconnected. Good point about the GPU. In this case, I'm using the GPU built into the Haswell (it's a server, and the GPU is under only enough load to be a console). So I think that CPU temperature will be a good thing to measure. I'll set a min speed that's quiet just to keep the drives cool.

I'm probably using the "thanks" feature too much.

 

[NiHaoMike]

Are you sure it's only a 3 pin connector? Something as new as a Haswell would almost certainly have PWM.

 

[Timothy Miller]

3-pin, 4-pin

Are you sure it's only a 3 pin connector? Something as new as a Haswell would almost certainly have PWM.

Yes, there is one 4-pin for the CPU fan and one 3-pin for a case fan.

 

[dostov]

Yes, there is one 4-pin for the CPU fan and one 3-pin for a case fan.

Your MB might very well do voltage control on the CPU header so you can plug in a 3-pin fan even though its a 4-pin header. Some (most?) motherboards can do either voltage or PWM control. This way you can link your fan speed to your CPU temps.

 

[RnRollie]

fan splitters for voltage control use 3 pin + 2pin + 2pin + ...
+ , - & rpm sense, only ONE fan reports RPM, all other fans get the same voltage from the header. You should NOT mix different types of fans

fan splitters for PWM control use 4 pin + 3pin + 3pin + ...
+ , - & rpm sense & pwm signal. only ONE fan reports RPM, all other fans get the same PWM signal. You should NOT mix different types of fans

As for the MB header: most are only rated upto 1 Amp (and a lot not even that). So, if fan specs (Watts/Amps) are known, you can calculate the max number of fans you can split off the header. Note you can NOT use most Delta fans on a MB header, as they tend to pull +1 Amp

A better solution is PWM fans, where the power is delivered by ways of a molex or sata connector from the PSU and the "control" is done by the MB circuitery using only the PWM signal and the RPM feedback.

Most MB are "simple": One mixed PWM (CPU_FAN -4pin) / Voltage (CPU_FAN_2 -3pin) header.
+ 1, 2 or 3 Voltage headers (CASE_FAN_1, AUX_FAN, PWR_FAN,...), all 3pins, allowing some basic control on the CPU_FAN & the CASE_FAN from BIOS (or speedfan). Good MBs throw in a 2nd PWM header + circuitery somewhere.