combining two 3 lead fans to one mobo fan connection
 

pl advise.
i paralleled the color coded leads of two compusa three lead fans to one three pin female fan connector.
but i am uncertain if the tachometer circuit of the two paralleled fans and the mobo tachometer reception will be damaged from paralleling both fan tachometers to a single mobo tachometer channel.
my understanding of the internal fan tachometer is a hall effect fet firing a pulse per revolution.
bussing the two fan's firing pulses to a single mobo fan channel could possibly harm fan and/or mobo circuitry.
can anyone please explain whether this is harmful to hardware or whether no ill effects will result?
my mobo supports only two three lead fans, but the case has four three lead fans, two forward, two aft.
i want to combine the forward fan pair and combine the aft fan pair, allowing the two individual mobo fan channels to accomodate all four three lead case fans.
any help will be sincerely appreciated. thanks

If you want two fans on one connector, you'll have to leave one tachometer cable unconnected. The problem is, the two fans won't be running at the same speed and won't allways give pulses at the same time. Because of this, your motherboard will read completely wrong values.

and, if your fans draw to much wattage you can burn out the mobo fan header (or worse) would not recommend it

the 80mm two ball bearing compusa fan pair is .24amp apiece when running on 12vdc, so that's one half amp combined, or 6 watts of electrical power flowing in the motherboard fan channel. what does the motherboard fan channel normally support in watts typically? the aopen motherboard literature does not disclose wattage ceiling limits. any response would greatly be appreciated. board is i945GA-PLF

i bench tested the fan pair using the motorola cellphone charger (5vdc) and measured fluctuating Hertz (Fluke 76), fluctuating AC volts, and fluctuating DC volts across the tachometer and ground pins. but the fans ran very smoothly. there was no harm caused to the fans. question is, whether the mobo reception of same will wreak havoc or just be tolerated. if i insert an inline capacitor per each tachometer lead before unioning into the mobo, would the resultant tach signal be more acceptable? or would the coupling caps prohibit dc bias necessary for tach fet pulsing operation?

You don't have to use the sensor wire if reading the RPM is not an important factor to you. Connect the red wire from the fan to the yellow wire of a 4-pin molex from the power supply; black to black and ignore the yellow wire from the fan you could both fans in 12VDC with no worry.

hey Hewo, 80mm fans are indeed no issue; if you were to hook up high speed 120mm fans there is reason to worry:)

thanks jmke
i discovered yesterday that by inserting an inline schotty fast diode to each tach lead before combining their signals, the delivered fluke 76 readings are stable. that makes more sense if you compare it against what a single tach output reads like. i am trying to accurately benefit the protection services offered by the mobo bios features by representing air movement from two identical fans being monitored by only a single mobo fan monitoring channel. i believe what this amounts to is integrating the signals and feeding the integrated signal to the mobo. this signal must represent combined flows such that should baseline flows depreciate, the mobo will be able to intercede automatically.

normally, a slow blow fuse could be inserted into the common ground feed, whose value would be steadystate current of two fan motors (.48amp). this protects the mobo fan channel from overcurrent condition permanent damage.