Three-wire PWM

[WildW]

I was playing with an old motherboard (my old Asus A8R32 MVP Deluxe, a high-end socket 939 board) to see how quiet I could make it, and found some PWM fan control options in the bios. I was a little confused and looked at the fan sockets on the motherboard. They are all 3-wires rather than the 4-wire type I'm used to seeing for PWM operation.

As far as I'm aware, old style fans used 3 wires: +ve, ground, sense
Newer PWM-capable fans use 4 wires: +ve, ground, sense, PWM pulses.

Now, the PWM on this old motherboard works because I was playing with it - so I figure that instead of outputting pulses to a 4th PWM wire, it's just pulsing the +ve voltage line?

I've never seen this before - is it unusual or do modern boards do this at all? It means you can use PWM control on any fan and we could have standardized on 3-pins instead of the mixture of plugs and sockets we have now.

 

[wsaenotsock]

Not sure if this helps you, but I have been running software controlled cooling for years now with 3 pin headers. You can control the speeds with just two pins (gnd and voltage), and the third pin (the one in the middle if I remember right) gives you the RPM feedback. I don't follow the reason why some fans use 4 pins [when 3 pins can do everything?], perhaps someone can shed some light on that.

 

[Zap]

It can pulse the voltage or just adjust the voltage, depending on how it works (and if it really is PWM versus just calling it that).

I've had a drive bay PWM fan controller that used 3-pin connectors. It pulsed the voltage, basically turning my LED fans into strobe lights. Irritating.

 

[alaricljs]

I don't follow the reason why some fans use 4 pins [when 3 pins can do everything?], perhaps someone can shed some light on that.

The reason for 4 pins: +12v/ground/tach/PWM

The +12v is always there, so there is no issue with startup voltage to get the fan going. The PWM smarts inside the fan will always get the fan moving and never drop the fan beneath it's lowest sustainable RPM.

 

[WildW]

The +12v is always there, so there is no issue with startup voltage to get the fan going.

The motherboard in question has a setting for lowest PWM required to get the fan started, so I guess the 4th wire and constant 12V means the fan will always start, rather than relying on the user to set everything up right.

Now that I think about it, the most likely reason for having both a constant 12V and a pulsed PWM line is indeed the fashion for LED-equipped fans.