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case fans and their usage

Posted on 2005-03-14
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Last Modified: 2013-11-10
i have 2 12cm fans that i have in the front and back of my case... these fans came from the thermaltake case i use... they are listed to spin at 21dBa at 1400 rpm.  however... they spin at 2000 rpm.  is this normal>???  i was under the impression that the listed speed was the max speed they can go at!  is my system overworking them?
  the other issue i have with them is that they have a 3 pin connector to the motherboard... and a 4 pin molex connector.  IF I just connect to the motherboard... they dont turn on.  if i just connect the molex... they turn on... but... cannot track them in the system (system reads 0 rpm).... i have to connect BOTH 3pin on mobo and molex 4pin... to make them run and read their spin rate.  is this normal?  is there any harm in connecting both power sources to one fan????

  is there any way to slow these fans down?  they are going way to fast and are kind of noisy....  my system is much cooler than it needs to be... around 25 degrees celcius.
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Question by:alenknight
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Expert Comment

by:tmj883
ID: 13540565
The fans are rated for 1400rpm solo...running in a push pull arrangement (fr in, rr out) the rpms will be higher...normal operation. Your mainboard does not have enough power in the 3 pin connector for fans this large. Normally to use rpm monitor and molex 4 pin, only the rpm wire of the 3 pin connector need be connected to the mainboard. I would not apply power from both sources.
http://www.directron.com/3splitter.html
The above device is the proper way to do this but can be hand fabricated easily.
http://www.directron.com/12to7.html
This adapter allows you to run fans at 7V for quieter operation but less airflow.
It can also be fabricated...
Or you can even fabricate one that does both.
T
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Author Comment

by:alenknight
ID: 13540598
unfortunately... i have no idea how to "fabricate" that... any advice would be useful.... i cannot order as i am abroad....
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Expert Comment

by:Callandor
ID: 13541207
See the link nearby: http://www.directron.com/3to4adapter.html 

Note what wire is used to connect both ends.
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Author Comment

by:alenknight
ID: 13541268
ummm.... i can connect to the power supply.  i dont have a problem with that... i need to know why its spinning so fast when i connect it to the power.  the company... thermaltake states its a 1400 rpm fan... so why is it spinning at nearly 2000????  is that normal?
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Expert Comment

by:Callandor
ID: 13541369
It doesn't sound normal.  They would spin faster if you gave more voltage than was expected to them, so check the voltage with a multimeter - if these are 12v fans (most likely), there should be 12v across them when they are spinning (unless they were meant to be connected to a lower voltage source).  You can cut the voltage in half if you wire them in series, but then you would lose your fan speed sensor.
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Expert Comment

by:tmj883
ID: 13541379
It is very simple and NO mystery, that two fans running in series will run faster...what is so hard to understand????
T
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Accepted Solution

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tmj883 earned 2000 total points
ID: 13541742
Fan 3 pin connector:
yellow...rpm monitor
red...12 volt
black...ground

Molex 4 pin connector:
yellow...12 volt
black...ground
black...ground
red...5 volt

To reduce fan voltage to 7 volts(rpm reducer) and maintain rpm monitoring:
Cut the red 12v and black ground wires near the fans 3 pin power connector, leaving the yellow rpm wire intact. This will connect to the three pin outlet of the mainboard.
Next, connect the Red 12 volt fan wire to the Yellow 12 volt on the molex 4 pin connector. Connect the Black ground wire from the fan to the Red 5 volt of the Molex connector. This reduces the voltage potential to the fan to 7 volts.
I give a diagram but EE unable to paste it.
Hope this helps but for most, it is easier to buy, perhaps a simple fan speed controller...US$20...T
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