How does a 3pin PWM to 4 pin PWM fan adapter work for monitoring fan speed?

Caveman

Platinum Member
Nov 18, 1999
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I just built a new rig and was surprised by a few things (last homebuilt rig was 8 years ago).

Installed one of the now aging, but still great air cooling solutions: Noctua NH-D14 onto a 4770k mounted to an ASUS Z-87 Pro Edition V MoBo. The MoBo includes a 4 pin connector to monitor the fan speed but the Noctua fans both had a 3 pin connector. Of course I could still plug in the qty 2, 3-pin “PWM” connectors into the 4 pin (PWM) connections on the MoBo but it made me wonder…

How could such a “high end” CPU cooler not already include a 4 pin connector? Certainly 99% of the people that invest in a NH-d14 would also want it to be “smartly monitored” by the MoBo by default. Now I have to go out and buy some 3 pin to 4 pin adapters which was a first surprise.

The second surprise is that this will even work… Maybe I misread something, but I’m baffled about how the “smart pin” that’s missing on the 3 pin fan connector can somehow mate to a 4 pin that plugs into the MoBo and then magically give me the ability to sense/control it???

That seems counterintuitive.

At any rate, I’d like to control all my fans as a function of temperature.

Sneaking in another question: I’ve installed the MoBo’s SATA drivers. Do I need to install drivers for my Blue Ray or DVD drives? I’m assuming “no, dumb question”.

One last comment… In general, I’ll say that after sitting out of the System building game for 8 years, I’m in complete awe of how the technology has changed, especially with respect to the MoBo and the Bios capabilities to reflash “easily” and the general flexibility and ease of use that the new graphical bios interfaces provides… A huge difference from what it was 16 years ago when I built my first PC… I fell like a kid in a candy store.
 

MrTeal

Diamond Member
Dec 7, 2003
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The 3 pin connector monitors fan speed.
Pin1 - Gnd
Pin2 - Pwr
Pin3 - Tach

The fourth pin is the PWM signal, which the NH-D14 fans don't have. You either need to use their resistor based low noise adapters, or use a MB fan header that has voltage control. You don't need 3 pin to 4 pin adapters, since they don't really do anything.

On the drivers, you shouldn't need a driver for an optical drive.
 

Caveman

Platinum Member
Nov 18, 1999
2,537
34
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Thanks for feedback... Ok... I remember seeing the low noise adapters... I was thinking EMI noise or something but I guess the low noise part meant "use these if you want to monitor the PWM signal"... Not looking right now, but didn't these just have a 4th PWM pin? Again, how do I get something (4th pin data) from nothing (only 3 pins physically wired from the fan)... And why didn't a high end cooler just assume I'd want to monitor/control it?

AFAIK, the 4-pin MoBo headers already support PWM monitoring/control but they "can't do it" right now because of the 3 pin connector I have plugged into them... I get power and tach speed in the bios but no way for them to be independently controlled based on MoBo speed without that 4th pin...

Apologies in advance if I'm missing something obvious here... Still doing a little head scratching and interested in learning...
 

VirtualLarry

No Lifer
Aug 25, 2001
56,570
10,202
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Thanks for feedback... Ok... I remember seeing the low noise adapters... I was thinking EMI noise or something but I guess the low noise part meant "use these if you want to monitor the PWM signal"... Not looking right now, but didn't these just have a 4th PWM pin? Again, how do I get something (4th pin data) from nothing (only 3 pins physically wired from the fan)... And why didn't a high end cooler just assume I'd want to monitor/control it?

AFAIK, the 4-pin MoBo headers already support PWM monitoring/control but they "can't do it" right now because of the 3 pin connector I have plugged into them... I get power and tach speed in the bios but no way for them to be independently controlled based on MoBo speed without that 4th pin...

Apologies in advance if I'm missing something obvious here... Still doing a little head scratching and interested in learning...

The "monitoring" line is the Tach line, and that's included in both 3-pin and 4-pin connectors. The fourth pin is for the PWM control signal, you don't monitor it, AFAIK, it's an output-only signal, used to control the fan's RPMs. The input signal is the Tach line.
 

MrTeal

Diamond Member
Dec 7, 2003
3,901
2,631
136
Thanks for feedback... Ok... I remember seeing the low noise adapters... I was thinking EMI noise or something but I guess the low noise part meant "use these if you want to monitor the PWM signal"... Not looking right now, but didn't these just have a 4th PWM pin? Again, how do I get something (4th pin data) from nothing (only 3 pins physically wired from the fan)... And why didn't a high end cooler just assume I'd want to monitor/control it?

AFAIK, the 4-pin MoBo headers already support PWM monitoring/control but they "can't do it" right now because of the 3 pin connector I have plugged into them... I get power and tach speed in the bios but no way for them to be independently controlled based on MoBo speed without that 4th pin...

Apologies in advance if I'm missing something obvious here... Still doing a little head scratching and interested in learning...

Long story short, there's 3 ways to control the speed of a fan.
1. Voltage control - Instead of sending sending the fan 12V you might send it 7V, like if you connect it between 12V and 5V on a four pin peripheral connector. Some motherboards can control fan speed by varying the voltage in software.
2. Power PWM control - essentially this connects and disconnects the 12V to the fan really quickly, which effectively lowers the voltage and speed. It works, but it can screw up the tach signal. You won't see this much on a modern MB as far as I know.
3. Signal PWM control - this uses the fourth wire to send a signal to the fan telling it what speed to run at.

If you use a 3 pin to 4 pin adapter, you won't get control of the fan speed using PWM. You can still read the RPM, but the control won't do anything. Depending on your motherboard, you may be able to do voltage control on the CPU fan header, which would solve your problem. Even if you can't use voltage on the CPU fan header, you might be able to do so on one of the chassis (or system) fan headers, which would give the same result.
 

LTC8K6

Lifer
Mar 10, 2004
28,520
1,575
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Most modern mobos will let you select either DC(voltage), or PWM fan speed control for each fan header.

You likely would never know the difference in operation unless you checked. Both work just fine.

Some cooling solutions may need one or the other, though.