Question : Problem: Adding a 2nd front fan to the cpu case

The problem is that my motherboard have only 1 connection with RPM control in front, and the extra fan have the yellow cable available... so my idea is this: can i weld the yellow cable of the extra fan to the yellow cable of the existing fan?? obviously the oders black and red cables will be attached to the power supply with a generic connector

Answer : Problem: Adding a 2nd front fan to the cpu case

If you have 3 fan wires the third wire is not a 'control' it's a 'monitor'.
It's an RPM -SENSOR-, not a speed control.

http://www.interfacebus.com/ATX_Motherboard_Fan_Connector_Pinout.html

What it senses is 'read' by an IC chip on the motherboard and the +12v power to the fan is adjusted to control the fan speed. (It may be adjusted either on the +12v or ground side of the fan header. That depends on the specific controller design.) That IC chip usually also has an input from a temperature sensor and it uses both inputs to set the fans speed.

You should -NOT- splice RPM sensors together or the IC chip will 'be confused' set the fans to the wrong speed.

You could splice the power and ground wires wires from a motherboard header to a second fan and NOT splice the sensor wire,,, that would cause both fans speed to be controlled based on the RPM of one fan which should be okay.

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4 wire fans are the same as far as those 3 wires.
Do NOT splice sensor wires.
In 4 wire set-ups the 4th wire is a control (not just a sensor) and the 4th wire could be spliced.

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One other caution.
Motherboard fan headers have current limits. It's usually not much.
Two fans to one header may exceed that limit and burn out the fan controller IC chip completely.

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