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Using a harness that splits into 4 female fan plugs (one primary with a tachometer and the others with no tachometer), a molex power and ground, and a 4 wire male fan plug that connects to the motherboard's CPU fan header, I have four identical delta fans running off my motherboard's CPU fan header's PWM control wire. Two fans are on the CPU heatsink and two fans are on the case. The tach wire that reports to the motherboard comes from the first heatsink fan, the other tachs are not connected to anything. The 12V Power and ground for all the fans come from the power supply's Molex 4 pin so the motherboard header is powering the fans at all, merely providing control and monitoring. Is there a limit I have exceeded as to how many fans I can run on this single PWM control pin? I could make more harnesses and run the two case fans on a separate header, just means more wires and molex plugs and making another fan curve, this is much simpler.
 

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Totally Tubular
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As far as I know, the only official limit on a fan header is power. There is no official limit on splitting the PWM signal itself, though it might get too weak if you use too many splitters. You will just have to try it and see.
 

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As far as I know, the only official limit on a fan header is power. There is no official limit on splitting the PWM signal itself, though it might get too weak if you use too many splitters. You will just have to try it and see.
Alright then I should be good to go. The identical 120mm intake fan, CPU heatsink fan1, CPU heatsink fan 2, and exhaust fan are all more or less in a straight line from the front of the case to the back of the case and the fans are all identical so running the same PWM curve for all of them works pretty well.
 

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What MNMadman said.

I've found normally 8-10 fans on a single PWM control pin are no problem. More than that can be. Some fans like Corsair AF and AP fans only allow 2 or 3 fans on one signal controller.

The limiting factors are that not all fan PWM circuits or PWM control circuits have the same specifications, and this variance in PWM power from fan and controller's abilty to open close circuits means number of fans that can be controlled vary.
 
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