These case fans have 4 pin molex connectors and I have two unused 3-
pin CHA_FAN connectors on the motherboard (Asus P5N-D).
I tried connecting these fans to the motherboard using a 4-pin to 3-
pin adapter cords which powered up the fans but the RPM is not
registering in the BIOS (it hasn't changed from '0'). Am I missing
something? I thought these fans with the adapter cords would work
like my CPU fan. You just plug them in they just work. Do I need
special fans or special adapters? Thanks.
Ignoring the connectors for a moment, fans come with several
different wire configurations.
1) Case fan with two wires. +12V and GND. No RPM signal.
Cannot be monitored. To vary the speed, alter the
voltage level going to +12V. The majority of case
fans are like this (all mine are this kind).
If you want to monitor a fan like this, there are
at least two alternatives.
a) Add the missing circuitry to make an RPM signal.
http://tipperlinne.com/fan-tach
b) Build an optical tachometer, using a phototransistor.
I've done this, and was using my optical gadget
just the other day. Only problem with this concept,
is a 7 blade fan gives 7 pulses per rotation, when
the hardware monitor is expecting 2 pulses per rotation.
Thus, the readout needs to be divided by 3.5 to
get the right answer.
2) Case fan with three wires. +12V, GND, RPM.
Can be monitored. To vary the speed, alter the
voltage level going to +12V. The RPM signal
pulses twice per revolution, and the hardware
monitor measures the period between pulses.
3) Case fan with four wires. +12V, GND, RPM, PWM.
Can be monitored. To vary the speed, keep the
+12V at 12 volts, and apply a pulse width
modulated control signal to the PWM wire.
This fan interface is common now, on retail CPU
fans, but is not common on case fans.
For an example of (3), I can only think of one
right off hand. (Found by looking for "PWM" in
the fan section.)
Rexflo DF128025BH-PWMG 80mm Case Fan - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16835705027
Same, only 120mm.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835705025
This is an example of (2), a three wire fan. The
advert says the third signal is a tachometer output, so
should work with the hardware monitor. This is a
medium speed Panaflo fan.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835705002
There is another kind of three wire fan, where the third
wire is "locked rotor" rather than "tachometer". With
locked rotor, the signal changes state only when the
fan stops turning. A fan with LR really isn't of much
good in a typical computer.
Paul
