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Hi I want to make a test with a Transistor Darlington PNP TIP137.
If connect the C pin on 12v, the E pin to red cable on the fan and the B pin to ground... what happens ?
Naturaly the black cable, connect to the ground.

Thank you

I Know.. is stupid question  smiley-roll-blue
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what happens ?

A pop, followed by some smoke.
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That puts 12V the wrong way round between collector and base, short-circuiting the power supply.

_emitter_ to +12V, collector to load, base via a 10k resistor to 0V might fare a lot better smiley-wink
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Ok perfect... now i need to do this..
control the pc fan with the positive cable. Need to turn on and off.
I have tried with TIP137 and transistor or with TIP132 and 6n135 but all my test make the negative conclusion.
Not working both solution...
Please help me. What can I do for controll the fan with positive side ? Only with this system I can read the tachio of the fan... otherwise in the yellow cable there is 10V and damage arduino.

Thank you 
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I said:

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_emitter_ to +12V, collector to load, base via a 10k resistor to 0V might fare a lot better

To control the base from an Arduino pin you also need an NPN transistor to level-shift to drive the base resistor of the TIP137.  Remember Arduino pins must not ever go above 5V.
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What can I do for controll the fan with positive side ?

You need a highside switch. Given that the fans are at 12v, you will need a level shifter (a npn for example). A logic '1' turns on the npn, which in turn turns on the pnp thus your fan.
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http://incredulist.blogspot.com/2012/10/high-side-switching.html
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The tachio on a pc-fan is an open collector output. You can connect it to an input pin on the arduino using a pull-up resistor (to 5 volt) or use the internal pull-up resistor, even if the fan is running off 12 V. There is no need to use high side switching. You can attach the fan to 12 V then via the darlington to ground. If you use PWM to control the speed of the fan, the tachio output can be a bit noisy, you may need some filtering / debouncing.
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Thank's to all...
PBROUWER: Can you explain this better ? "If you use PWM to control the speed of the fan, the tachio output can be a bit noisy, you may need some filtering / debouncing."
I Have resolved theproblem with a 4n35 optoisolator and a TIP137 PNP.
If you break the negative, on the tachio cable ther is a 10V and is dangerous for the aurdiono's pin.

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Thank's to all...
PBROUWER: Can you explain this better ? "If you use PWM to control the speed of the fan, the tachio output can be a bit noisy, you may need some filtering / debouncing."
I Have resolved theproblem with a 4n35 optoisolator and a TIP137 PNP.
If you break the negative, on the tachio cable ther is a 10V and is dangerous for the aurdiono's pin.



Nevermind, I got confused with a four pin fan (has a seperate PWM pin), and you are using a three pin fan. If you use high side switching you should be fine, it keeps the tacho sensor's ground pin tied to ground.
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