Thursday, December 14, 2006

Beware of 4-pin fans (Intel or BTX fan)

I got a rather shocking education in new Intel technology. It seems to be something created for BTX motherboards. Let me step back a bit.

Most of us are familar with the 3-pin fans - 12vdc, RPM feedback, and ground. Motherboards adjust the fan speed by using pulse-width-modulation (PWM). In effect, they send pulses of 12vdc into the fan and the overall speed of the fan is related to the percentage of the time the fan sees 12vdc compared to no voltage. The problem with this idea is many fans "buzz" or vibrate slightly due to the jolts of power speeding up the coasting fan.

So apparently Intel came up with the 4-pin fan or "variable" fan. Instead of "punching" the fan with full 12vdc pulses, the 4th pin is used to send a low-voltage PWM and the fan internally uses this to set a variable speed from its minimum RPM at no pulses to its maximum RPM at a near 100% PWM. Sounds good - but when you plug this into a 3-pin plug it means the 4th pin has no pusles and if you're unluck the fan just stays at its minimum RPM. I learned this the hard way. It seems even the normal PWM of the 12vdc supply doesn't seem to speed up the fan. The fan manufacture offer this work-around: just short the unused 4th pin to the 2nd pin and the fan goes full blast jet-turbine level. Hmm, so it seems WORST case such a 4-pin Intel/BTX-style fan is either quiet but too slow, or super-fast and noisy.

So make sure you look at the photos or product carefully. Unless you have a mobo that supports such 4-pin fans, you don't want them. Online vendors make this a bit more confusing by often refering to fans connectors as 3/4 - which usually means it is a 3-pin connector with a 3-to-4 pin Molex-style power converter.

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Blogger Unknown said...

PMR fans greatly reduce "chatter" due to the pulse method of speed control. You see the fan pops out against its axle stop or ball bearings everytime it recieves a pulse. This is not an issue in a fan running at full speed all the time.
So, if yo have a MOBO that has 4pin plugs for the fans then use them and you will have a much quieter system. The speed is usually adjustable in BIOS and the mobo will manage them. If you have 3 pins then use a 3 pin fan.
But beware of the chatter.

March 20, 2008 at 11:48 AM  
Blogger Lynn August Linse said...

Sure, I accept that - my RANT is that the sellers of fans and MoBo don't make it so clear which is which.

May 1, 2008 at 8:14 PM  

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