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PC Fan - reading RPM on yellow?

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gregmcc

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So I've read a ton of threads and I thought it was a simple job of connecting up GND and 12V and then you can read the RPM via the yellow wire.

After unsuccessfully trying to get a Arduino project to work, I eventually connected the yellow wire to my scope and don't see any movement - its 0V.

Its a DC Brushless 12V 0.18A Fan with yellow, red and black wires.

What gives? Am i missing something?

I've also tried pulling up to 5V via a 10k - still nothing.
 
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dr pepper

Well-Known Member
Most Helpful Member
It might be a thermistor controlled fan, the yellow being the thermistor.
 

ericgibbs

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Most Helpful Member
Interesting - anyway I can read it?

hi greg,
If it was a thermistor it would have some resistance value from the yellow wire to one of the other two wires. or the fan frame.

Use an ohm meter and measure the yellow wires resistance to the other wires.

Eric
 

dr pepper

Well-Known Member
Most Helpful Member
Try a 10 or 47k pot from the yellow to ground, see if the fan speed varies, if not then maybe your right its dead.
 

gregmcc

Member
No effect. Guess its toast.

How can you tell if the fan is using a hall effect sensor for speed output, or a thermistor?
 

dr pepper

Well-Known Member
Most Helpful Member
Fan wires are colour coded, black/red for power, yellow tacho, blue is a pwm speed control signal and green for a remote thermistor.
This is only a guideline there are plenty of fans that do not follow this code.
 

gregmcc

Member
That's what I gathered from all the reading up I did as well, which is why I was confused as to why I could not see any pulses on the tacho wire.

I'll return than fan to the shop and try another one.
 

gregmcc

Member
I was too curious and opened the fan up - the yellow wire is not even connected. There is no hall effect sensor even on the pcb! :mad:

Damn cheap Chinese fans!
 

dr pepper

Well-Known Member
Most Helpful Member
you've been diddled.

hey I'm just down the road from you, preston.
 

Mr RB

Well-Known Member
...
Damn cheap Chinese fans!

imagesqtbnANd9GcSPgFAs3uVaw8aNdqSm81S7nr-1.jpg


:D Sorry I couldn't resist a "Chinese fan" reference.
 

dr pepper

Well-Known Member
Most Helpful Member
Its as much use as by the sound of it probably more reliable.
 

ben7

Member
LOL at that fan!

Well, once you get a good fan, hook the yellow wire up to 5v via a resistor (say, 4K7). Most fans have open-collector speed sensing. However, to make sure you don't fry your arduino, double check it once you have the resistor hooked up. If it switches back and fourth from GND to 5V, then hook your arduino up to the sense wire and start programmin'! If it goes from 5V to 12V (or whatever voltage you are feeding the fan), the fan likely has an open-emitter output. You will need to setup a voltage divider via two resistors in order to not possibly fry your arduino.

Fans will have more than one pulse per rev, so beware of that!

So, say, if the fan has 4 pulses per rev, then measure the time it takes it to do 4 pulses. Then using some simple math, you can convert it to revs per second, minute, hour, etc...

Another thing to be aware of is if you are PWMing the fan's power leads. This will cause false readings from your sense wire! You will have to use an adjustable power supply controlled by the arduino, or use a 4 wire fan, which has an extra wire for (usually PWM) speed control.
 

Reloadron

Well-Known Member
Most Helpful Member
All of the 3 wire computer fans I have experimented with need an external 10K pull up resistor to 12 volts. The tach out is normally 2 pulses per rev. Some years ago I did a web page on this stuff and what remains of it can be seen here:

https://bearblain.com/fan_speed_control.htm

The page is hard to follow because it is no longer actually used as a working part of the web. However, it may help a little. Oh yes, while the 3 wire fan shows the resistor as R1 and it looks like it is part of the fan in reality it isn't. :)

Ron
 
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