> >        IĞm trying to know How can I calculate the cable type (AWG or
> somethig
> >like it) if a know how much current will flow throught it (some
> 40mA).
> >
> >        I need to know it because want to build a bobbin.
> 
> If I understand correctly, you want to wind a coil or solenoid. This
> is
> more complicated than just the max current capability of the wire.
> 
        [snip]

> At 40mA, you're not likely to have any problems unless you have a LOT
> of turns.
> 
> 
        all good points. I'd like to add that skin effect also is
        often a limiting factor, especially at high frequencies.
        To get good efficiency out of the coil, you want to balance
        core and copper losses. As stated above, it can definitely
        get more complicated than just the current-carrying capacity
        of the wire.

        that having been said, a good starting point is to use 500
        circular mils per ampere (of RMS current), where a circular
        mil is the the area of a circle 1 mil in diameter.
        (area in square inches would be (1/4)*PI*E-6).

        if the 40mA is the peak current of a square wave, you need to
        convert to rms(Irms = Ipeak*sqrt(Ton/T).

        Then multiply Irms*500 to get the number of circular mils.

        For example, assume Irms = 40mA, you'd need 
        0.04 * 500 = 20 circular mils. so you need a wire
        with a diameter of sqrt(20) = 4.5 circular mils.
        That's between #37AWG and #36AWG. use #36AWG.

        (#36AWG has a diameter of 5 mils. That's an 
        area of 25 circular mils, or 19.63E-6 sq. in)

        hope this helps.