Strange fan behavior [longish]

[George Macdonald]

Hmmm, I notice that the author doesn't address the issue of
commutation which AFAICS would create a much bigger problem. The
following article appears to support my reservations in this regard.

I thought he was hinting at it in the last diagram... without actually
going there.

"Application of PWM Fan Control":

http://www.coolingzone.com/Guest/News/NL_May_2003/JMC/JMC_May_2003.html

This drawing illustrates the principle of electronic commutation:
http://www.ece.msstate.edu/~hagler/May2001/02/FIG2.JPG

I think you'll agree that the Hall effect sensors will be all over the
place if the supply is chopped. The abovementioned fan control article
suggests that PWM control is only feasible if the power to the fan's
electronics is uninterrupted. According to the article, this
necessitates a 4-wire design.

I understand that Intel is now using 4-wire fans on (some of ?) its boxed
CPUs.

For a PWM-to-DC conversion solution, see AMD's "Open Platform
Management Architecture Specification":
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/32200.pdf

The document states that "the motherboard must provide the necessary
circuit that converts PWM pulses from the mCard output to an analog DC
level that is then amplified to actually control the fans." See Fig
15, section 8.4, page 69 for a suggested circuit.

Hard to say how this, from a 2003 server/workstation doc, translates to
current desktop mbrd applications though.

Here is a photo of your motherboard:
http://www.thetrailingedge.com/images/products/asus-a8n-e-big.jpg

I notice that it appears to use ITE's IT8712F multi-IO chip. Here is
its datasheet:
http://www.iteusa.com/product_info/file/pc/IT8712F_V0.9.1.pdf

Section 9.6.2.2.17 (page 107) lists a selection of 8 PWM frequencies
ranging from 5.86kHz to 375kHz. The example used by the author of
Speedfan assumed that the PWM frequency was of the same order of
magnitude as the rotational frequency, namely some 10s of Hz. I would
think that if the tach signal were chopped at 5.86kHz, say, then each
tach pulse would appear as several hundred pulses. Clearly this is not
happening in your case, so I'm inclined to believe that Asus has opted
for DC averaging. Having said that, I still can't see anything other
than a MOSFET and a capacitor near the fan connectors, and then only
at the CPU fan and front chassis fan. This appears to be confirmed by
the manual which states that "the Asus Q-Fan function is supported
using the CPU Fan and Chassis Fan 1 connectors only". In the absence
of an oscilloscope I'd use the AC voltage range on a DMM to confirm
whether there is an AC component. This voltage would be measurable at
rotational frequencies but probably not at 5kHz and above.

BTW, the Asus motherboard manual shows a BIOS screen with a CPU fan
speed of 4962 RPM and a chipset fan speed of 5443 RPM. The CPU
temperature is 48 degC and Q-Fan is disabled.

Here is an interesting post:
http://www.silentpcreview.com/forums/viewtopic.php?t=18921&sid=4054b97fb8d073b37a98e63a712774c9

The author alludes to an Asus P5GD2 motherboard manual which states
...

"Q-Fan 2 ... allows you to select the type of cable connected to the
CPU fan connector. Set to [PWM] when using a 4-pin CPU fan cable. Set
to [DC] when using a 3-pin CPU fan cable." Options [PWM][DC]

I notice also that your manual states that a fan can draw up to 2A.
This would suggest that, if the MOSFETs were operating in linear mode,
then they could be required to dissipate up to 6W (1A at 6V). In PWM
mode the dissipation would be close to zero. So maybe you're right and
I'm wrong. Is it time to break out a scope?

Unfortunately I don't have a lot of time to put into investigating this.
Maybe I'll try just switching fans to see if the anomaly of increased speed
with reduced drive follows the fan or mbrd... though I'm pretty sure it's
going to follow the fan. At least I have a "solution" with SpeedFan where
I can limit the min drive to 45% and still get 1500RPM at idle, which is a
nice quiet fan speed without flirting with the 800RPM min alarm limit on
the fans which go too slow at 30% drive.

Thanks for the links and clarifications.

 

[George Macdonald]

it could be a faulty thermistor. its a rare occurance with computers,
but it happens.

Two good ones and two bad ones?... on the fans?

 

[George Macdonald]

Well, the following document suggests otherwise, although I can't see
how you could sense the processor's temperature unless the thermistor
were screwed down onto the heatsink.

Precisely. The CPU has an on-board thermal diode which can be read by
firmware/software to control fan speed; all the mbrds I've seen also have
an external CPU temp sensor which is read through the sensor chip. Both
can be read by SpeedFan.

Cool ‘n’ Quiet™ Technology Installation Guide for AMD Athlon™ 64
Processor Based Systems:
http://www.amd.com/us-en/assets/content_type/DownloadableAssets/Cool_N_Quiet_Installation_Guide3.pdf

"All AMD Athlon 64 Processor-In-A-Box packages include thermally
controlled fans. A thermally controlled fan detects the current
temperature of the processor using a thermistor, and when a lower
temperature is detected the fan speed and noise is then reduced. Upon
detection of a higher temperature the fan speed is resumed at full
speed to appropriately cool the processor."

Here is one fan with an external thermistor:
http://www.nidec.com/designoptions/thermalfan.htm

Yep, looks like the one I already talked about with the "blue blob" and two
legs but obviously that's not going to control fan speed based on CPU temp.

 

[Ed]

"All AMD Athlon 64 Processor-In-A-Box packages include thermally
controlled fans. A thermally controlled fan detects the current
temperature of the processor using a thermistor ....

Sounds to me like AMD is trying to make it sound better then it is.

The thermistor isn't anywhere near the CPU or even touching the
heatsink, so how's it going to adjust the fan speed for CPU temp?

If the fan is at it's minimum of 3K RPM and you put a full load on the
CPU and the CPU die temp jumps from say 30C to 50C the fan will still be
spinning at 3k, it'll only spin faster when the temp of the air passing
through the fan warms up. End of story.

Cheers,
Ed

 

[George Macdonald]

Sounds to me like AMD is trying to make it sound better then it is.

The document is >2 years old.

The thermistor isn't anywhere near the CPU or even touching the
heatsink, so how's it going to adjust the fan speed for CPU temp?

If the fan is at it's minimum of 3K RPM and you put a full load on the
CPU and the CPU die temp jumps from say 30C to 50C the fan will still be
spinning at 3k, it'll only spin faster when the temp of the air passing
through the fan warms up. End of story.

Exactly! The fan-mounted thermistor is simply to give a higher airflow
when the ambient temp, i.e. inside the case, is higher.

 

[Franc Zabkar]

Sounds to me like AMD is trying to make it sound better then it is.

The thermistor isn't anywhere near the CPU or even touching the
heatsink, so how's it going to adjust the fan speed for CPU temp?

If the fan is at it's minimum of 3K RPM and you put a full load on the
CPU and the CPU die temp jumps from say 30C to 50C the fan will still be
spinning at 3k, it'll only spin faster when the temp of the air passing
through the fan warms up. End of story.

Cheers,
Ed

What you're saying makes a lot of sense. In fact AMD's own docs appear
to contradict themselves in this regard. One refers to airflow
temperature, another to processor temperature. <shrug>

Anyway I've posted a related question to alt.comp.hardware.homebuilt:

http://groups.google.com/group/alt....cc8a1?lnk=st&q=&rnum=1&hl=en#9e792cb5771cc8a1
http://tinyurl.com/yhuzhd (short)

One person states that "on a prior Asus board (A7N8X) with ITE IT8708
Super I/O, just before the fan header there's a 100uF cap, then an
HJ772 (PNP transistor) that looks driven by an LM358".

I suspect that the OP's motherboard probably uses a similar design, in
which case it would be DC rather than PWM. The question as to why a
fan speeds up when its voltage decreases remained unresolved, though.
I still think my own explanation is plausible, in which case the OP
would be advised not to run his fan at any voltage other than 100%. I
mean, why would you want to control a fan that is designed to control
itself?

- Franc Zabkar