Question 1, what are the currents i1, i2, i3? Question 2, what is the voltage at node X? Solve using superposition. My problem is that I tried solving it using superposition and I get $I_1=6$, $I_2=2$, $I_3=8$, $V_X=80$. But the textbook (and the circuitlab simulation) both say the answer is different, $I_1=5$, $I_2=2$, $I_3=7$, $V_X=70$. What am I doing wrong? |
by dnadya
November 17, 2016 |

OK here's how you solve it using superposition. First only look at the voltage source. So get rid of current source I1 and solve everything, call this A: Just by looking at it you get $I_{1a}=I_{3a}=6$, $I_{2a}=0$, $V_{Xa}=60$. Now only look at the current source, so get rid of the voltage source V1 by making it short circuit (0 volts): Just by looking at this you get $I_{1b}=-1$, $I_{2b}=2$, $I_{3b}=1$, $V_{Xb}=10$. Now you add up the totals from the two sources: $$I_1 = I_{1a} + I_{1b} = 6 - 1 = 5$$ $$I_2 = I_{2a} + I_{2b} = 0 + 2 = 2$$ $$I_3 = I_{3a} + I_{3b} = 6 + 1 = 7$$ $$V_X = V_{Xa} + V_{Xb} = 60 + 10 = 70$$ That matches your textbook. Not sure what you did wrong but that's how to do it right. |
+2 votes by gavers7 November 17, 2016 |

dnadya, I figured out what you were probably doing wrong by looking at @gavers7's answer. When you apply superposition, you have to "set the other sources to zero" -- but that has a different meaning for a current source and a voltage source. For a current source, it means $I=0$, zero current, i.e. an open circuit. So you can just remove the current source entirely. But for a voltage source it means $V=0$, which means a I think the mistake you made was to remove the voltage source V1 and just leave it open: When you do it this way you must have found $I_{1b}=0$, $I_{2b}=2$, $I_{3b}=2$, $V_{Xb}=20$. So then when you added things back together you found: $$I_1 = I_{1a} + I_{1b} = 6 + 0 = 6$$ $$I_2 = I_{2a} + I_{2b} = 0 + 2 = 2$$ $$I_3 = I_{3a} + I_{3b} = 6 + 2 = 8$$ $$V_X = V_{Xa} + V_{Xb} = 60 + 20 = 80$$ That matches what you (incorrectly!) found. Hope that helps! Remember: superposition = set sources to zero, which means different (almost opposite!) things for $V=0$ and $I=0$. |
+1 vote by mrobbins November 17, 2016 |

firstly voltage source will be short circuit,,,and current source will be open circuit,,,,then follow the direction of current......,,,then add current,,,,,for i1,,,,,will 5,,cause,,,for S.C i"1=-1,, for O.C i'1=6,,,follow the step.i think you will be able to solve it |
+1 vote by MR_HIMEL December 06, 2016 |

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