KCL and Groud point

Discussion in 'Homework Help' started by drao, Mar 12, 2013.

  1. drao

    Thread Starter New Member

    Mar 11, 2012
    16
    0
    Untitled.png
    For this circuit, in the ground point i can apply KCL??? I3, I2 and I1???
    Why do i ask you this :D.
    I solved the second circuit, and if i consider Ie of transistor = Ie of resistor and i apply KVL for Ie*R+Veb=0 (i forgot to mark the loop) i dont obtain the same current as if i apply KVL for the marked loop . WHy???
    Untitled.png
    BTW, im really sorry for not posting in the right section :|.
     
    Last edited: Mar 12, 2013
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    You have not applied KCL correctly. The currents on the left hand side are not Ie. There is an entirely different portion of the circuit to consider and that is the portion containing the supply VCC. Draw in the supply with its ground connection and try again with KCL.

    Define KCL. This will help us understand your understanding of the concept.

    Make sure you identify the nodes you are calculation KCL on.
     
  3. drao

    Thread Starter New Member

    Mar 11, 2012
    16
    0
    Why???
    Dont we consider Ib=0 and Ig=0???
    And arent the picture circuits equivalent???
    I dont know how to draw the supply. I tried, but i dont know if its what i had to do.
    Untitled2.png
    Supply.png
     
    Last edited: Mar 13, 2013
  4. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    Now compare your Supply.png drawing to your untitled.png (and try to give your images some kind of meaningful title, it doesn't have to be much). In your untitled one, you concluded that any current that went down to ground in the right hand branch had to come out of ground and go up through the left hand branch. But now look at the Supply image. Do you see that this is not the case? Some of the current that current that goes into ground from the right branch can come up out of the ground but then take a left turn and go to the supply and not go up through the middle (used to be left hand) branch. In fact, ALL of the current that comes down the right hand branch can take the left turn and there can be a completely independent current that comes down the middle branch and takes the turn toward the supply before entering the ground symbol.
     
  5. drao

    Thread Starter New Member

    Mar 11, 2012
    16
    0
    Dam, i cant belive how i didnt think at this :|.
    Thx very much for clarification. I gave myself a simple exemple and im feeling bad about my circuit analysis skills, i need improvements :D. But i have 1 more question, if i can edit the circuit like in the untiled2 pic, what happens with the 2A current difference its going thourgh the supply branch???
    pic.png
     
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  6. WBahn

    Moderator

    Mar 31, 2012
    17,757
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    In your pic.png you have 6A going into the ground in the left hand branch and 4A going into the ground in the right hand branch. That gives a total of 10A going into the ground. That 10A is then coming out of the ground and into the negative terminal of the 12V supply. Which is a good thing because, looking at the top of the pic, you've got a total of 10A coming out of the 12V supply.

    And, yes, your circuit analysis skills do seem to be pretty weak. But that's okay -- we were all there at some point. It takes practice and being willing to make up and work your own problems shows a level of effort that relatively few are willing to put forth. So I think you will make the improvements you need and do just fine.
     
  7. drao

    Thread Starter New Member

    Mar 11, 2012
    16
    0
    Again i made a big mistake applying KCL because of not drawing the supply. Anyway, thanks a lot of explanation. All the best!!!
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    Most people need to start off drawing all the connections. Then, at some point, you will just naturally become comfortable leaving certain components and connections, such as supplies, implied.
     
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