Wiring a fan to read speed

[SvdSinner]

I'm building a watercooling system and have two large 120mm fans that are wired for a 3-wire fan header. For a couple of reasons, I want to power them with 4-pin power header, but I still want to read the fan speed from the 3rd wire. If I simply run the 3rd wire to the 3rd pin on the motherboard while running the +12 and GND to the 4-pin header, will the motherboard detect the speed? Or do I need to connect something to the +12 and GND also? (If so, what?)

[dobbz]

Yeah, that'll work fine.

[Nazo]

Someone had trouble where that trick wouldn't work for them before on here. I think it probably depends on the motherboard. I don't know if they ever found a solution, but not while I was looking. Anyway, there's no harm in trying it.

[emorphien]

Originally posted by Nazo
Someone had trouble where that trick wouldn't work for them before on here. I think it probably depends on the motherboard. I don't know if they ever found a solution, but not while I was looking. Anyway, there's no harm in trying it.

Interesting, perhaps the motherboard didn't want to read it because it wasn't providing power to a fan on that connector. If your motherboard fights with you then you could probably use a front panel device.

[Nazo]

That's precicely what I told him.

Anyway, try it and see. I seriously doubt it's physically possible for that to do any harm to the mb or anything after all. If all else fails, there's those $40 or so panels that show all that nice info and look kind of neat to boot.

[wayne]

Originally posted by Nazo
That's precicely what I told him.

Anyway, try it and see. I seriously doubt it's physically possible for that to do any harm to the mb or anything after all. If all else fails, there's those $40 or so panels that show all that nice info and look kind of neat to boot.
thats how the tornado fans were designed... to have the rpm detector on the motherboard and run power off the molex so it doesnt overload the circuit on the motherboard

[NewBlackDak]

Some motherboards want ground to read the RPM sense as well. If you're worried about it run the sense and ground to the 3 pin in the mobo, and only run the +12 to the molex.

[Sniper_Merc]

Originally posted by NewBlackDak
Some motherboards want ground to read the RPM sense as well. If you're worried about it run the sense and ground to the 3 pin in the mobo, and only run the +12 to the molex.

Thats what I was just about to mention.

My motherboard doesn't read unless it uses both the third wire and the 12v ground, at least on the CPU motherboard specified connector, the secondary connector next to the vid card actually does read it with only the single wire.

Go figure:rolleyes:

[thewhiteguy]

Originally posted by NewBlackDak
Some motherboards want ground to read the RPM sense as well. If you're worried about it run the sense and ground to the 3 pin in the mobo, and only run the +12 to the molex.

Make sure you run ground to BOTH the motherboard and molex connectors.

[Nazo]

Geez, now would you just tell that other person all that stuff... I did and I don't think he ever tried it...

[NewBlackDak]

Originally posted by thewhiteguy
Make sure you run ground to BOTH the motherboard and molex connectors.

Why would you do this?
I was under the understanding that ground is ground. I wou;dn't want to run both to ground unless I had a couple diodes in there.

[Nazo]

The reason doing that is so the motherboard doesn't get the entire load. Even over the ground it can only handle so much.

[thewhiteguy]

Yeah using the ground and/or 12V off the motherboard will cause the same amount of load on the motherboard as if it were hooked up right to the motherboard.