G
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.