Hi!

In a typical 3 wire DC fan, two wires are power and one is sense. i'm curious what the signal on that sense wire actually looks like. Is it the same frequency as the fan rotation? How much current can it drive? Is it open drain typically?

Thanks

 

A typical DC fan is not really 3 wire, unless BLDC! DC brushed is 2 wire.
Is this a room fan, PC fan???
Max.

 

If it's a typical PC fan, then it's BLDC. The third wire usually outputs a square wave signal whose frequency is related to its speed.

 

Yep a BLDC 3 wire PC fan

 

If it's a typical PC fan, then it's BLDC. The third wire usually outputs a square wave signal whose frequency is related to its speed.

Yeah but is it an open drain output? Will i need a pullup resistance? What is the typical driving capability of this square wave signal?

 

Probably not much, just intended as an indicator, what are you trying to do?
Max.

 

Was planning on driving an LED to indicate the fan is still running (it isnt in a visible place)

 

I would use a 2n7000 mosfet driver etc.
Max.

 

Ok cool. But do you think it will be open drain or push pull output from the fan?

 

Yeah but is it an open drain output? Will i need a pullup resistance? What is the typical driving capability of this square wave signal?

Good question ... I'm not sure. But Max is right, it's best to use a small transistor to drive the LED, and the nFet's gate can be easily be driven by the fan's output wire.

 

It seems that your intuition was right, this fan's datasheet shows the following connection to its wires:

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Thanks a bunch!

 

Yep a BLDC 3 wire PC fan

There are a few caveats to some of this which may or may not apply. If it is a PC fan with a straight 12 volts applied then no problem as they typically output 2 pulses per revolution of the fan. With 12 VDC applied most I have played around with typically run at about 2700 to 2800 RPM or about 46 to 47 revolutions per second which gives you about 93 to 94 Hz out of the tachometer out line of the fan. All that I worked with had an open collector output as illustrated by cmartienez in post #11.

Here is the slight caveat. PC fans which have some form of fan speed control generally use a PWM (Pulse Width Modulation) to control fan speed and those systems which do this do not always have a nice tachometer output. This generally won't matter but if you are using the open collector Tachometer Out make sure your pull up (shown as R in the above example) is tied to a constant voltage like 12 or 5 VDC and not to the fan power shown as Vcc above. LED flicker would be normal but you will know if the fan is turning.

Ron