Thevenins theorem with 3 current sources and no voltage sources

Discussion in 'Homework Help' started by progolfer, Mar 8, 2011.

  1. progolfer

    Thread Starter New Member

    Mar 8, 2011
    Hi all,

    I am currently studying a circuit which I have analysed using mesh and nodal analysis but am stuck on thevenins theorem, the reason being is that I have never used this theorem with 3 current sources and no voltage sources. I know that a voltage source is short-circuited and a current source is open-circuited but I still cannot find the thevenins equivalent circuit in order to calculate the voltage across the 80ohm resistor. Please can someone point me in the right direction. I have attached the circuit. Kind regards
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  2. Vahe


    Mar 3, 2011
    I have looked at the circuit you posted; however, you have to indicate the two terminals where the Thevenin equivalent circuit should be computed. So in your problem lets call the terminal to the left of the 80 ohm resistor "a" and the terminal to the right as "b". The original problem statement should have indicated the terminals. There are two steps in finding the Thevenin equivalent here: (1) equivalent resistance with all sources off and (2) the open circuit voltage across the 80 ohm resistor.

    The equivalent resistance at a-b is Req=80||(100+20)=80||120. The 40 ohm does not come into the picture.
    The open circuit voltage at a-b (in other words Vab) can be computed by superposition. Take one current source at a time (open circuiting the others) and compute Vab. You do this three times and add the results and that is your open circuit voltage. So the end result is Vth in series with Rth that appear at a-b.

    Last edited: Mar 8, 2011
  3. progolfer

    Thread Starter New Member

    Mar 8, 2011
    Hi there,

    I have worked out the equivalent resistance, which is 80||120 = 48Ω. When you say calculate the voltage one at a time do you mean for example the the 3A source in series with the 48Ω resistor which equals 144V, and the same for the 10A and the 5A.

    Kind regards
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Simply find voltages for this three circuit.
    And Vth = Vth1+Vth2+Vth3
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