Fan Headers / 3 pin / 4 pin

Mortster · 27 May 2007 at 16:11

Hi guys,

Really simple query for someone, why does my Asus P5b Deluxe have a 4 pin CPU FAN header but all the other chassis headers are 3 pin ?

The Halk · 27 May 2007 at 16:14

3 pin fan headers give power and a sensor.

No idea what a 4 pin fan header would be for, unless it's a standard (much bigger) than a 3 pin fan header, and intended for bigger more powerful fans.

Mortster · 27 May 2007 at 16:17

this is why i'm a bit confused as well, by chance, in fact, the AC7 cpu cooler fan I am using actually has a 4 pin/wire connector which fits the CPU header a treat. All my other 80mm case fans are using the 3 pin fan headers fine (using PWM for controlling RPM based on temps)

chaparral · 27 May 2007 at 16:18

Mortster said:
Hi guys,

Really simple query for someone, why does my Asus P5b Deluxe have a 4 pin CPU FAN header but all the other chassis headers are 3 pin ?

The 4 pin header is used for PWM speed control

The Halk · 27 May 2007 at 16:19

Aha. That explains that then.

Mortster · 27 May 2007 at 16:22

so how does the P5b control my chassis fans with only 3 pin connectors then ? I presume by using voltage control rather than PWM ?

The Halk · 27 May 2007 at 16:24

Mortster said:
so how does the P5b control my chassis fans with only 3 pin connectors then ? I presume by using voltage control rather than PWM ?

Exactly right. Some motherboards will adjust the voltage - I think almost all do now. Used to be rare for motherboards to do anything other than throw out the standard 12v.

chaparral · 27 May 2007 at 16:24

The Halk said:
Aha. That explains that then.

This may explain in a little more detail...

What is PWM? In simple terms, PWM involves rapidly switching the supply to the fan on and off, in this case 30 times a second, (30Hz). By altering the relative on to off times the average voltage "seen" by the fan is also altered. E.G. Assuming a 12V supply, when the on/off ratio = 50%/50% the fan will "see" 6V, likewise when the on/off ratio is 75%/25% the fan "sees" 9V. Obviously 100% output is achieved by having the output on continuously and at 0% the output is off. It is this on/off switching that makes the fan speed sensor inoperative.