What happens to the node after a source transformation?

  • #1
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I have one circuit and another with a source transformation figure a and b respectively. My question after transforming the 30 mA source shouldn't the current going into node 5 equal the current going into node 4?

Screen Shot 2017-10-12 at 12.27.36 AM.png

Figura a. Before the transformation of the source

Screen Shot 2017-10-12 at 12.27.14 AM.png

Figure b. After the source transformation.

To clarify my question shouldn't the current going into the node before the transformation (34.50 mA) equal the the current going into the node after the transformation so again 34.50 mA)? If the two current's aren't supposed to be equal explain what concept I am missing about source transformation and how I can check if the source transformation is correct?
 

Answers and Replies

  • #2
cnh1995
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To clarify my question shouldn't the current going into the node before the transformation (34.50 mA) equal the the current going into the node after the transformation so again 34.50 mA)?
No.
Before source transformation, there are four paths for current division at node 5. During source transformation, one of them (resistance in parallel with the current source) gets shifted in series with the voltage source. So, current supplied by the voltage source= current supplied by current source-current through the parallel resistance (which is now shifted).
Hence,
Current supplied by the voltage source=
30mA-19.5mA=10.5mA.
 
  • #3
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No.
Before source transformation, there are four paths for current division at node 5. During source transformation, one of them (resistance in parallel with the current source) gets shifted in series with the voltage source. So, current supplied by the voltage source= current supplied by current source-current through the parallel resistance (which is now shifted).
Hence,
Current supplied by the voltage source=
30mA-19.5mA=10.5mA.
Can I add the two resistors the 4 ohm and 2 ohm?
 
  • #4
cnh1995
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Can I add the two resistors the 4 ohm and 2 ohm?
No.
 
  • #6
cnh1995
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That’s because of the node right?
Yes, they are not in series.
 
  • #7
donpacino
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Just to point something out... garr6120 you are partially right. The current going into the main circuit should and will be the same before and after the source transformation. But you forgot that the 2k resistor is part of the source the way you presented the problem. The current going out from your old and new source should equal each other, but you need to include the resistance.

30-19.5=10.5
 
  • #8
donpacino
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You can check your work by comparing the current entering the unchanged circuit elements. if you have difficulty, draw a dotted line around your source, including the source impedance. the IO should be the same
 

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