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'[PIC]: 3 wire fans, sense line'
2002\07\26@182613 by Brandon Stewart

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Has anyone ever played with a 3 wire computer case fan (where the 3rd line
is a sense line)?  I want to monitor this 3rd line to output the fan's RPMs
on an LCD screen.  What does the waveform look like?  Is the waveform 0-12
volts?


Thanks,
Brandon

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2002\07\26@193336 by Welch, Ken

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some fans just have a binary /fan_fail signal...

those that have a tachometer output (ususally two pulses per revolution 50%
duty cycle) can have a signal amplitude equal to the supply voltage, more
common to have just 0-5 volts -- sometimes open collector

if unable to identify via the fan p/n the best way to determine signal is on
a o'scope...



{Original Message removed}

2002\07\26@214943 by Bob Blick

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Hi Brandon,

It's an open collector line, so it must be connected to a pullup
resistor. That way it'll interface to any voltage you want (within
reason, OK, it's probably meant for 5 or 12 volts.)

Typically it's two pulses per revolution, approximately 50% duty
cycle.

Cheers,

Bob

> Has anyone ever played with a 3 wire computer case fan (where the 3rd line
> is a sense line)?  I want to monitor this 3rd line to output the fan's RPMs
> on an LCD screen.  What does the waveform look like?  Is the waveform 0-12
> volts?

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2002\07\26@215109 by Matt Pobursky

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I've incorporated several 3-wire fans into smart power supply designs
in the past few years (temperature based fan speed control) and have
found two basic kinds of fans are the most commonn: RPM sensing and
Locked Rotor output.

The RPM sensing fans almost all have Open Collector outputs on them.
They usually give 2 or 4 pulses per revolution. Most are generated by a
hall effect switch (or similar sensor feedback) from the brushless DC
motor control in the fan. Very easy to interface with, quite nice edges
and usually very little noise on the signal. Verify with a 'scope, but
this signal works quite nicely when fed into a PIC CCP capture module
with a pullup resistor, for instance.

The Locked Rotor types usually work off a preset RPM in the fan motor
control circuit. It's also usually an open collector signal that's
normally high or low and changes state whenever the RPM of the fan
drops below some preset point.

Grab a 3-wire fan and play with it. They're easy to interface to a PIC,
you shouldn't have any trouble doing what you describe.

Matt Pobursky

Maximum Performance Systems


On Fri, 26 Jul 2002 18:19:24 -0400, Brandon Stewart wrote: Has anyone
ever played with a 3 wire computer case fan (where the 3rd line is a
sense line)?  I want to monitor this 3rd line to output the fan's RPMs
on an LCD screen.  What does the waveform look like?  Is the waveform 0
-12 volts?

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2002\07\27@061743 by Peter L. Peres
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On Fri, 26 Jul 2002, Brandon Stewart wrote:

>Has anyone ever played with a 3 wire computer case fan (where the 3rd line
>is a sense line)?  I want to monitor this 3rd line to output the fan's RPMs
>on an LCD screen.  What does the waveform look like?  Is the waveform 0-12
>volts?

Usually it's an open drain output that outputs pulses. I've seen some that
go low when the fan eneters the designed rpm range. You need a pullup for
this.

Peter

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