Help with a thevenin exercise

Thread Starter

drk

Joined Mar 8, 2008
41
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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