KVL use problem

Discussion in 'Homework Help' started by rip_r, Nov 26, 2011.

  1. rip_r

    Thread Starter New Member

    Nov 13, 2011
    2
    0
    Hello,

    I'm sorry if I've broken any rule by posting this, but I have some issues with English (not being an english speaker and all).
    So here's my problem... I missed a few electronics seminars and I gotta do a set of about 20 problems, so I asked a friend to help me out with an example.

    The image is the problem(first circuit) and how he solved it.
    What we have to do is find out the voltage marked with "u" by using source transformations.

    I understand how he used source transfomations to get to the final circuit and all the calculations, but I don't understand how he used KVL to find out the voltage.I didn't ask him so I wouldn't look as dumb as I actually am.
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    Hi rip_r. Welcome to AAC! :)

    You have posted this in the right section.

    I am looking at your work, and I am trying to figure out how you got 12 volts across the (9.5?) amp supply. When you transform a current source with a parallel resistor to a voltage source with a series resistor, you need to multiply the current by the value of the resistor (ohms law) in order to get V. I do not see where you did this. Perhaps I am just not seeing it correctly. I wonder if there's another way you could post the problem and your work here, that is easier to see? I'm sure it will help a lot.

    Best wishes,
    Der Strom
     
  3. rip_r

    Thread Starter New Member

    Nov 13, 2011
    2
    0
    Thank you for the welcome

    The left current source is 1.5A.Transformed it into a 12V voltage source with the 8ohm resistance.Then added what's now 2x 8ohm serial resistors into a 16ohm one and transformed it back into a 0.75A current source, and then I added the two 16ohm and 4ohm parallel resistances which gave me a single 3.2ohm resistance.Then again transformed it into a 2.4V voltage source... and this is as far as I got.

    l.e. :

    At some point, after using source transformation, I will get to the circuit below.
    Can I add the current sources? And maybe work from there.
     
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    Last edited: Nov 26, 2011
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    1,328
    Ah, I see. I misread your writing :p

    so your 12v source with an 8 ohm resistor in series is correct. Then you need to transform the right-hand current source in the same way. This will give you two voltage sources and two resistors in series. When you have two resistors in series, of course, you can simply add them together. Once you do that, you should have two voltage sources and one resistor. From there, you can find the current (combine voltage sources and use Ohm's Law) through the circuit. Finally, you use Kirchoff's voltage law. Start with the left-most battery, go around the circuit (subtract R*I1--R is the resistor, I1 is the current through the circuit, and multiplied together you get the voltage drop across the resistor), and finally (depending on the orientation of the second battery), either add or subtract the final voltage. KVL states that all these voltages must add up to 0. So basically, you'll have V1-(R*I1)+(or -)V2=0. V2 is what you are looking for, so Ohm's Law and simple algebra should give you the voltage you are looking for.

    Let me know if you have any more questions!
    I hope this helps.
    Regards,
    Der Strom
     
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