7V, ground, and fan RPM question

[cobray]

Hi Everybody,

My question: I have a Thermaltake Smart Fan with a 3-pin motherboard connector. I wired the high side to 12V+ and the low side (ground) to the 5V+ to power the fan for 7V directly from the power supply, and plugged the RPM lead directly into the CPU motherboard connector.
While I achieved my goal of powering the fan by 7V I have no RPM signal from to the motherboard. I assume it is from the lack of a ground wire (?)
How can I get back my RPM monitoring function? Can I connect the ground pin from the motherboard connector to ground on the board.

TIA!

BTW: I want to say this is a GREAT FORUM with a good signal-to-noise ratio, as opposed to alot of the other forums I have been trolling.

[cobray]

Can someone help me out? Im a software guy and not a hardware engineer.

[Teus]

some mainboards ignore the RPM speed if it's too low. quite stupid, I don't know why it's got a use

[cobray]

I can still get an RPM reading when the fan is at 5V, so I dont think thats the problem. Thanks for answering anyway Teus.

[cobray]

I have a Thermaltake Smart Case Fan 2 if that matters...

[Ragnarok]

well that's the problem, the lower potential is 5V is not ground

if really want to run 7V and have sensoring, hook up a variable resistor to the 12V rail (make u sure u get one with fairly big at least 5W rating)

[Teus]

Originally posted by cobray
I can still get an RPM reading when the fan is at 5V, so I dont think thats the problem. Thanks for answering anyway Teus.
cool, I didn't know that. to monitor a fan speed, you need both the RPM and ground wire then?