Splitting the rpm signal? Success...

Bluefront · Post by **Bluefront** » Sat Nov 06, 2004 4:54 pm

I'm controlling two fans with an external LCD fan control....Is it possible to tap into the signal wire (yellow) from the fans, and plug it into the MB......so the motherboard can also read the rpm signal?

sthayashi · Post by **sthayashi** » Sat Nov 06, 2004 8:45 pm

Although I've never tried it, electrically speaking, it should be possible.

ultraboy · Post by **ultraboy** » Sat Nov 06, 2004 10:10 pm

It'll work, unless your external fan control is a pwm unit then rpm reading is not very useful there.

Krispy · Post by **Krispy** » Mon Nov 08, 2004 9:30 am

Some time ago, also with a controller, I connected yellow wire only to MB & it didn't give a RPM signal to MBM5. I wondered about spliting the the black wire, but couldn't find any info so I chickened out.

Doubt this will put you off!

sthayashi · Post by **sthayashi** » Mon Nov 08, 2004 10:21 am

IIRC, the Tach line works as a voltage collector circuit. If you want to see the fan pulses on an oscilliscope, you would put a 10k resistor or so between the tach line and your voltage line (12V or whatever you're regulating it to), and then hook up your probes to the Fan tach and ground circuit.

Now generally, it shouldn't make a difference if you split the signal. You may have more resistance, but you should still have the same number of pulses.

silvervarg · Post by **silvervarg** » Wed Nov 10, 2004 12:47 am

The problem I think that you are getting with the temp controlled fan is that the voltage that comes out from the fan to the tach line could be too low to register as high level by the motherboard.
The best way to go here would be to measure the tach signal with a scope and see what comes out. If you are getting a tach signal but it is too low voltage, then it might work to add some bias to the signal.

Perhaps someone with more electronics know-how can help out here? Fancontrol? Cpemma?

nbac · Post by **nbac** » Wed Nov 10, 2004 4:34 am

I think it will work just fine, inputs generally have high impedance
and will not behave like a resistive load at low frequencies (a few
MHz and less)

Bluefront · Post by **Bluefront** » Wed Nov 10, 2004 4:24 pm

Well I had to try it....success. Both the fan controller and Speedfan now read RPMs. And the readings seem to be accurate. The Fan controller reads down to the 100's and Speedfan down to the 10's.....which was the reason for this mod. I wanted a more accurate idea of rpms, plus the controller wouldn't read below 1000rpms. Speed fan seems to be accurate and stable down to the fan's slowest speed...850rpms.

Will it work for you....try it to see.

BTW....I only used the yellow signal wire to the board.

meglamaniac · Post by **meglamaniac** » Thu Nov 11, 2004 1:00 am

Hah, brilliant - I was thinking of doing exactly this when I build my new system. Glad you jumped the gun and guineapigged for me!

One question; how did you connect the split yellow to the board header? Do you have spare plugs? Or does your fan controller use molex connections instead of the 3-pin fan plugs?
The controller I'm getting uses the 3-pins so I guess I'd need spares from somewhere. It's a bit of a pain, but I wanted this one specifically as it doesn't use PWM.

Bluefront · Post by **Bluefront** » Thu Nov 11, 2004 2:56 am

Everything has 3-pin connectors.....I had a bunch of un-used 3-pin wire extensions from a DigitalDoc5 (they come with eight). You could also chop up an old noisy 3-pin fan connector. Got a pile of those...

teejay · Post by **teejay** » Thu Nov 11, 2004 3:04 am

Bluefront, I know you've already tried this and found out it worked, but just to set possibly worried minds of other readers at ease: both my Sunbeam Rheobus and (retired) Coolermaster Aero7 cpu cooler came with a splitter to do just this... so my guess is if they're selling it, it'll probably be safe. At some point you will hit the limit below which the mobo won't read anything of course, but that depends on the board.