computer fan wiring

From a previous thread which gave me an idea,
I find myself needing to wire a computer fan,
A bit perplexed I found a red, black and yellow wire.
All have a resistance,
I have a 12v power supply with a much larger current rating,
I really don't want to blow anything up,
proberbley a stupid question what goes where,

Baz

What could possibly go wrong, ?

Positive side of the supply to red, negative side to black, and ignore the yellow.

The yellow is used as a speed detector or fan fail alarm, if you are not using it, just insulate it and tie it back out of the way.

JimB

thanks for a straight forward answer,
chears

Baz

What could possibly go wrong ?

You are welcome.

I had the same problem myself two weeks or so ago.

My thought process to find the solution went like this:
The Red and Black are 99% certain to be the supply.
The Yellow could be a control wire of some kind, or from previous experience more likely a signal wire to tell if the fan has stopped.
Connect the power, the fan runs.
Check the yellow with the 'scope, there is something there.
Connect a 1k pull-up resistor between red and yellow, scope shows a nice square wave, 99% certain to be a speed signal.
I don't need a speed signal, so just insulate the yellow and tie it back.
Job done.

JimB

The μp fans can now be the optional 3 or 4 wire, on the 3 wire type the 3rd wire is the speed or operation detection, the 4th wire is the speed control, varies rpm with temp..
Max.

The third wire could be a fan speed control *input*, wither an analog voltage or PWM. Either way, these usually default to full speed when left open.

ak

Yellow is a squarewave tacho signal.
Blue on 4 wires is a pwm input, the higher the duty cycle the slower the fan goes.
Any pc fan I've played with connecting red & black to power and leaving any others dis makes it run minimum speed, connecting the blue to + makes the fan run flat out, the only exception being ones that have a built in temp sensor.

Just in case anyone lands here:

From my yesterday's test on a 4-pins connector fan, having the black wire on your left, you get:

black - GND
yellow - +12V
green - tacho signal - 2 pulses per revolution
light blue - PWM ctrl (see below)

The 3-pin fans may be used here knowing that you do not have that PWM ctrl available.

As per the governing standard (reccomendations?) the PWM control should be a nominal 25KHz signal (0-5V).

Worth to note:

In tacho output I used a 10K pull-up resistor to 12V.

With no signal in pin 4 the fans goes fast.

0V on pin 4 gets the fan running to a supposed "minimum" of 30%.

Steady 5V on pin 4 means NO change in speed.

PWM ctrl (between 20 and 80% that my function generator allows) seems to have a very linear response.