Source transformation principle - I am doing something wrong.

Thread Starter

Xhendos

Joined Dec 1, 2018
9
Dear AAC,
I am trying to solve practive problem 4.6 which asks to find Io using source transformation principle as can be seen in the screenshot down below.
2021-01-02-175435_1136x389_scrot.png

What I tried to do was using source transformation principle to greatly simplify the circuit and then apply nodal analysis to get two constraint equations and find the voltage. Using that voltage V2 and subtract voltage 35/4 gets me the net voltage. I find the current flowing through that resistor 35/12 and calculate the amount of current flowing through the 7 ohm resistor since it is parallel with the 1 + 4 ohm resistors. My calculations can be seen below.
WhatsApp Image 2021-01-02 at 17.53.03.jpeg

However, when I solve I find that Io is equal to approximately 3.97A which does not match the book which says it is 1.78A. Could anyone point me on my mistake since I am stuck for quite a while and can't seem to find it.
 

RBR1317

Joined Nov 13, 2010
633
The only technique I ever use is nodal analysis, so this may be a dumb question, nevertheless if you are trying to find the current through the 7 ohm resistor, why do you transform away the 7 ohm resistor? How do you get the 7 ohm resistor back in order to find the current through it?
 

The Electrician

Joined Oct 9, 2007
2,834
You included too many resistors in your source transformations. Leave the 3 ohm and 7 ohm resistors alone. Convert the 5 A source which is in parallel with 6 ohms into a voltage source in series with 6 ohms. Convert the 3 A source which is in parallel with 5 ohms into a voltage source in series with 5 ohms. Now you can use mesh analysis to determine the 3 mesh currents, and from them you can get Io.
 

Thread Starter

Xhendos

Joined Dec 1, 2018
9
WhatsApp Image 2021-01-03 at 11.12.28.jpeg

Thank you for your replies. I shouldn't source away the branch with the 7 ohm resistor indeed. This is my solution and I get ~1.78A which agrees with the solution of the book!
 
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