Help with a thevenin exercise

Discussion in 'Homework Help' started by drk, Dec 8, 2009.

  1. drk

    Thread Starter Active Member

    Mar 8, 2008
    41
    0
    The exercise asks to determine the resistor R such that it will have a current of 1A through it.
    My idea is to find an equivalent of thevenin with that resistor as the Rload, and then calculate the resistor value.

    So i 1st calculated the Rth(see picture).
    I turned the 'triangle' to 'star', then series/parallel and got Rth=2R

    Then i needed the Vth.
    Vth is the voltage from a to b(see picture).
    I used the node method:

    i1-i2+i3-i4=0

    i1=2A
    i2=(v1+10)/4
    i3=(2-v1)/8
    i4=(v1-18)/2

    doing the math i got V1=10V

    then:
    i4=(10-18)/2=-4A
    Vb=V(lower resistor of 1R)=-4*1=-4V

    i3=(2-10)/8=-1A
    V3=-1*6=-6V

    Vab(not sure)= -2V (?)

    Then with the thevenin circuit(see picture) it would be:

    I=V/R

    1=-2/(R+2)

    R=0 (?)

    The solutions tell me that R=7.93R , but can see where is the mistake.
    thanks
     
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