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Case fan power & control

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June 24, 2010 5:25:22 PM

In looking at the Antec 902 case,the four fans have standard 4 pin molex power connectors with only two pins wired to the fans. It looks to me that the only option is to plug them into PSU connetors and give up any motherboard regulation of fan speed as even if there is an adapter ?? to plug them into the mobo (Gigabyte X58-UD3R) headers two wires would not provide any sense or control signals. Is there any way around this short of replacing the stock case fans? Thanks.

More about : case fan power control

a b ) Power supply
June 24, 2010 6:02:31 PM

You can replace these fans that are fitted with 3 wires for motherboard control. Even Antec sell fans that can replace the standard Antec 902 fans.

The answer is yes. There are available after market 120mm fans that will meet your needs.
a b ) Power supply
June 24, 2010 8:38:16 PM

Replacement is an option. But adapters also are an option. If you can't find them ready-made, you need the pinout details for 4-pin mobo fan connectors here:

http://www.allpinouts.org/index.php/Motherboard_%28CPU%...
NOTE that Ground is on one end, and +12 VDC next to it

and the 4-pin Molex power connector from the PSU:

http://en.wikipedia.org/wiki/Molex_connector
NOTE +12 VDC on one end, Ground next to it; BUT get the right end - other end is +5 VDC

A 3-pin fan output on a mobo that actually provides fan speed control varies the +VDC voltage to achieve control. It is a feedback control loop based on a temperature sensor built into the mobo. It does NOT use measured fan speed for control. Measured speed is the signal on the third lead from the end. It is monitored and displayed by the mobo BIOS, and most systems allow you to specify that this should NOT be done (Ignore the fan speed) if you don't have it available.

A 4-pin fan output works differently. Some actually can be configured with a BIOS setting to behave exactly like a 3-pin output if that's what you have. Others are locked into 4-pin mode. In that mode, the +12 VDC line is ALWAYS at that voltage (fan speed is controlled differently) so there is no fan speed control possible unless you actually use a real 4-pin fan - a plain 3-pin fan on this type of port will just run full speed.

So read what your mobo manual says your mobo will do with 3-pin fans. IF it will control them you can use (or make) adapters to power your 4-pin fans as if they were 3-pin units just missing the fan speed feedback line.
Related resources
June 25, 2010 12:17:23 AM

Paperdoc said:
Replacement is an option. But adapters also are an option. If you can't find them ready-made, you need the pinout details for 4-pin mobo fan connectors here:

http://www.allpinouts.org/index.php/Motherboard_%28CPU%...
NOTE that Ground is on one end, and +12 VDC next to it

and the 4-pin Molex power connector from the PSU:

http://en.wikipedia.org/wiki/Molex_connector
NOTE +12 VDC on one end, Ground next to it; BUT get the right end - other end is +5 VDC

A 3-pin fan output on a mobo that actually provides fan speed control varies the +VDC voltage to achieve control. It is a feedback control loop based on a temperature sensor built into the mobo. It does NOT use measured fan speed for control. Measured speed is the signal on the third lead from the end. It is monitored and displayed by the mobo BIOS, and most systems allow you to specify that this should NOT be done (Ignore the fan speed) if you don't have it available.

A 4-pin fan output works differently. Some actually can be configured with a BIOS setting to behave exactly like a 3-pin output if that's what you have. Others are locked into 4-pin mode. In that mode, the +12 VDC line is ALWAYS at that voltage (fan speed is controlled differently) so there is no fan speed control possible unless you actually use a real 4-pin fan - a plain 3-pin fan on this type of port will just run full speed.

So read what your mobo manual says your mobo will do with 3-pin fans. IF it will control them you can use (or make) adapters to power your 4-pin fans as if they were 3-pin units just missing the fan speed feedback line.


I may be confused because looking at the actual case in a retail store, I noticed that there are only two wires running from each molex connector back to its fan; the other two pins on the connector are not wired. From reading your post it appears that I need at least three leads to the fan for the mobo to control it so connetcing to a 3 or 4 pin header via an adapter would provide physical connectivity but no sense/control signals. Am I interpreting this correctly?

Best solution

a b ) Power supply
June 25, 2010 9:25:38 PM
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On each fan there are only two leads, probably black and red, or black and yellow. Black (will be one of the middle two positions on the connector) is Ground. The other is the +12 VDC supply to the fan. The fan will run fine if those two power supply lines are connected. If the +12 VDC line is ALWAYS at 12 volts, the fan will always run at full speed. But if that line's voltage is reduced, it will run slower.

On the 4-pin fan pinout on the mobo, the #1 pin on the end is Ground, and next to it is the +12 VDC pin. If the mobo controls this port as a 3-pin fan, all it does is vary the actual voltage on that #2 pin, from +12 max to less for slower fan speed. So simply connecting those two mobo pins to the two lines to one fan will power the fan and control its speed IF the mobo is actually varying the voltage. If it is not (for 4-pin fan control that #2 pin is always at +12 VDC), then the fan will always run full speed. That is ALL that is involved in fan speed control. The third line (yellow on a 3-pin fan, and pin #3 on the mobo pinout) is for a pulse signal generated by the fan motor and sent back to the mobo for monitoring the fan speed. But it does NOT need to know the fan speed in order to control it. It will display the speed, and maybe send you a warning if it stops, but it does NOT use the speed in its control scheme.

So, at very minimum, you CAN feed power to your case fans from mobo 4-pin ports. However, if the mobo cannot adjust its system to control a 3-pin fan's speed on that port (that is, it insists that it only does 4-pin PWM control), there's little advantage over connecting instead to a PSU output that also can supply a constant +12 VDC to the fan's leads.
a c 144 ) Power supply
June 27, 2010 3:19:42 PM

The Antec fans also have a little three speed switch. I have found on my three 900's that running the fans faster than Low does not improve cooling.
June 28, 2010 5:55:31 PM

Best answer selected by oleahy.
June 28, 2010 5:57:11 PM

I got it now, thank you.
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