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Output of a fan RPM wire

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Raptor77

Registered
Joined
Dec 27, 2009
Location
UK
What's the output of an RPM wire from a case fan?

I'm guessing that it changes between ground and a set voltage, or between two voltages, but what are they? And with what frequency? I'd assume one pulse per spin of the fan. Tried Googling for this and couldn't find the answer.
 
2 Pulses per revolution, not one.

Edit :

2 pulses type is common, but some also there are some type with 4 pulses/rev, though they're rare.
 
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The tach wire is also the trigger for the magnets internally.
There is a hall effect sensor that senses the position of the magnets (and hence, the fan rotor) and triggers the electromagnets when the fan rotor hits a certain spot.
That trigger is also fed down the tach wire.

Depending on how the hall effect is wired, it'll either be a momentary dip in a voltage, or a momentary voltage spike from zero.
 
The tach wire is also the trigger for the magnets internally.
There is a hall effect sensor that senses the position of the magnets (and hence, the fan rotor) and triggers the electromagnets when the fan rotor hits a certain spot.
That trigger is also fed down the tach wire.

Does that mean that fans with no tach wire could very easily have one added? If their electromagnets are being triggered from a signal that's the same as a tach signal.

Cheers for the help.
 
Does that mean that fans with no tach wire could very easily have one added?

It needs additional circuit to capture that rotational pulse, and filter it if needed, and amplify it enough for the tach wire reader (usually at the mobo) to understand it. Yeah.. I would'nt call it very easy or practical. :D

In the olde day, when tach's fan was quite rare, this kind of method was quite popular amongst electronic DIYers.

Now with the wide availability of tach's fan, I think its not worth the trouble.
 
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I'm just asking for interest. What kind of voltage does it have to be amplified to for a mobo to pick it up?
 
That tach pin puts out a logic level signals at 12 Volt.

Usually that tach line needs to be attached to a resistor to the +12 Volt rail, tech speak called it "pull up", and from there the tach sensor reader observes and counts that output for the pulses it generated.
 
Cheers, so if the pull up is connected to +12V, are the pulses usually at 0V? I've done a bit of electronics, though not really since my A2's.
 
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