Man-wai Chang to The Door (+MS=32B) said:
From the back-emf??? Or.... what? Is it accurate after all?
You can see an example of adding monitoring to a fan here.
http://tipperlinne.com/fan-tach
A brushless DC fan motor needs a way of switching the
coils on and off, in such a way as to encourage rotation.
The transistor added in that project, is picking up a
signal from when the coil is energized. So as long
as there is enough potential applied to the coil,
the transistor is forward biased. The purpose of
using the transistor, is to make sure that only positive
potentials are available on the monitoring wire. The
transistor supports open collector logic, which means
a pullup resistor should be present somewhere, to make
the "logic 1" when the transistor is not conducting.
The hardware monitor chip could have 3.3V or 5V logic, while
the signal from the fan could operate at 12V. Some hardware
monitor chip datasheets, include a zener diode in the
circuit, for clipping the signal amplitude to a value
suitable for not damaging the hardware monitor. In the
example on that web page, the designer has opted to
clip the circuit to 5.1V, on the assumption that the
motherboard has no protection. I don't know if
there is a "standard" for fans, that defines what
the level should be - whether it should be clipped
to no more than 5V, or whether fans should be allowed
to put out a 12V amplitude signal.
If you want to see a typical example of a recommended
interface, at the hardware monitor chip end, try PDF
page 36 (Figure 2.4) here. They offer four different ways
of building an interface. Presumably one of those
circuits, has the best compromise between low parts
cost and best compatibility.
http://web.archive.org/web/20061118133939/http://www.winbond.com/PDF/sheet/w83627hf.pdf
Paul
