Case Fan Conundrum

Razoo · 29 Mar 2016 at 15:04

I would appreciate some advice on choosing and connecting case fans for a M-ATX computer I'm building (120mm front intake fan & 92mm rear extract fan).

Previously when I've built computers, I've always wired the case fans directly to the power supply via the Molex connectors, so that the case fans are always on and always running at full speed. However, my motherboard for this build (Gigabyte GA-Z97M-D3H) provides three 4-pin system fan headers. I assume these headers will allow me to fit either 3-pin case fans, or 4-pin PWM case fans - but which to choose?

Presumably if I fit 4-pin PWM case fans to the motherboard system fan headers, the fans will be speed-controlled by the motherboard using Pulse Width Modulation control (the same speed control as is used for the CPU cooling fan)?

And if I fit 3-pin case fans to the 4-pin motherboard system fan headers (and assuming I can change the mode of operation for the system fan headers in the BIOS), the fans will be speed controlled using a feedback loop that varies the applied voltage to alter the fan speed?

Another thought (as negative pressure cooling is generally considered preferable to positive pressure), would be to always run the 92mm rear extract fan at full speed, but allow the 120mm front intake fan to be speed controlled. Note that the CPU cooler fan will also be tending to operate as an intake fan, as it is located directly in front of the (unused) case side panel intake grille. I think having the rear extract fan always running at full speed would tend to bias the case cooling system towards negative pressure, unless/until the front intake/CPU fans were cranked up by their motherboard speed controls.

So, considering the two modes of case fan speed control available, which would you recommend I use, 3-pin variable voltage or 4-pin PWM?

And why?

Would always running the rear extract fan at full speed be a good idea?

So many questions!

Thanks.

BTW, I intend buying Be Quiet Silent Wings 2 case fans.

Ayahuasca · 29 Mar 2016 at 15:08

I would go for 4 pin PWM's if you're looking to control them through the BIOS or the motherboard may have software you can download to change them on the fly within Windows.

Running the rear at full speed shouldn't be an issue unless it's too noisy.

Razoo · 1 Apr 2016 at 21:27

Oops, nearly screwed up here!

It transpires that although my Gigabyte GA-Z97M-D3H motherboard provides three SYS_FAN 4-pin headers, these 4-pin headers are NOT PWM enabled. Only the CPU_FAN 4-pin header is PWM enabled.

The three 4-pin SYS_FAN headers only provide 3-pin voltage-controlled fan speed control. The 4th pin, which would normally carry the PWM (Pulse Width Modulation) signal, is in fact annotated as VCC and powered at +5V DC.

The pin-out is:

Pin 1 - GND
Pin 2 - +12V/Speed Control
Pin 3 - Sense (speed pulse return)
Pin 4 - VCC (+5V DC)

I've no idea of the purpose of pin 4, or the effect +5V DC would have on a 4-pin PWM fan.

There's no facility in the BIOS to change the mode of the SYS_FAN headers either.

This means there's absolutely no point in using 4-pin PWM case fans, and basic 3-pin case fans are all that is needed. 4-pin PWM fans would work in these 4-pin SYS_FAN headers and they would be speed-controlled. However, the speed control would be by varying voltage (on pin 2) and not by Pulse Width Modulation (on pin 4).

The lesson here is, don't assume all 4-pin fan headers on motherboards are PWM controlled, like I did

I guess I've just answered my own originating question!

Tealc · 1 Apr 2016 at 22:12

Pwm fans use a 5v pulse signal to switch the fan on and off. 5v constant would just run it at 100%.

Razoo · 2 Apr 2016 at 15:20

Tealc said:
Pwm fans use a 5v pulse signal to switch the fan on and off. 5v constant would just run it at 100%.

Thanks for the information.

It seems that the Gigabyte SYS_FAN 4-pin headers are designed to provide variable-voltage speed control and work normally with a basic 3-pin fan (which leaves pin 4 of the header unconnected). If a 4-pin PWM fan is plugged in, the +5V constant on pin 4 will tell the fan to run at 100%, except the 4-pin fan will still be speed-controlled by the motherboard because the voltage (+12V DC) on pin 2 is being varied.

However, I read recently (tomshardware.co.uk) that a 4-pin PWM fan could be used in a 3-pin fan header:

"...a 4-pin fan plugged into a 3-pin port. The fan will receive the first three signals [GND, +12V/(speed control) & Sense], but no PWM signal to use. So it will not do anything to alter the flow of current from the supply through the motor. But since this is a standard 3-pin port, the +VDC line is varying, not fixed at +12 VDC, so the fan speed CAN be controlled by the motherboard."

However, in this case the signal to the fan on pin 4 will be 0V, rather than +5V, so the PWM circuit would see nothing. Logically, if +5V = 100%, then 0V = 0%, so the fan would not run.

Maybe you can't use a 4-pin PWM fan in a 3-pin fan header unless you provide a separate +5V DC constant supply to the fan's PWM circuit on pin 4?

Relentless81 · 2 Apr 2016 at 15:27

This is why I bought the Phanteks PWM Hub, it needs one PWM connection from the motherboard, you plug your PWM CPU fan into the speed sense port and the rest of the case fans go into the three pin sockets and everything gets controlled by the PWM signal.

Cpu gets hot all the fans ramps up, Cpu cools down all the fans slow down. It couldnt be simpler

For £15 its a fantastic little piece of kit and solves so many problems.

ScoTTyBEEE · 4 Apr 2016 at 18:26

Are you not overcomplicating it. They're 4 pin headers, stick a 4 pin fan on them. Pin 4 runs at either 0v or 5v and quickly flicks between the two to achieve desired speed, hence it's pulsing (I think this is how it works)

Razoo · 5 Apr 2016 at 21:53

ScoTTyBEEE said:
Are you not overcomplicating it?

No, but I think Gigabyte are making things complicated.

They're 4 pin headers, stick a 4 pin fan on them. Pin 4 runs at either 0v or 5v and quickly flicks between the two to achieve desired speed, hence it's pulsing (I think this is how it works)

Although there are 4-pin SYS_FAN headers provided on the motherboard, they are NOT PWM enabled. Consequently there's no point in buying or fitting 4-pin PWM fans.

Basically the 4-pin header operates as a 3-pin header (providing normal variable-voltage control of a 3-pin fan), plus a fourth pin (pin 4) that carries +5V DC constant, so that if a 4-pin PWM fan is plugged in, the fan will still run OK, but only as a 3-pin voltage controlled fan.

For 4-pin PWM fans, the headers are a compromise. Easiest to simply fit 3-pin fans.
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Razoo