Help with review before test. Finding Ix, thevanin equivalent, op amp rl

Discussion in 'Homework Help' started by Chaos241, Jul 2, 2012.

  1. Chaos241

    Thread Starter New Member

    Jul 2, 2012
    I am taking circuits over the summer and the final is thursday. I was given a review for the test and it is really scaring me. I am half way done. I have 5 more questions but I haven't gotten to them.

    First problem: I started and I know I am supposed to use kvl and ohm's law but I can't get past what I've got. I am just really confused.

    Question 1

    Second problem: This is the only one I am pretty sure I did right. I used super position and I think it is all good.

    Question 2

    Third problem: Using finding the thevenin equivalent with phasors. I have no idea on this one. I did all of my homework and was never given a single problem like this. So I don't really know where to start on this one. I tried google and youtube but nothing helpful.

    Question 3

    Fourth problem: I know most of this is correct. It is a simple op amp I am just not sure about the very end of my work.

    Question 4

    Fifth question: RL circuit. I know MOST of my work is right. I messed up on steps either 2 or 3 but I don't know where or how.

    Question 5 page 1
    Question 5 page 2
  2. WBahn


    Mar 31, 2012
    First, thanks for posting your work. That is extremely helpful and makes people here much more willing to assist you.

    Having said that, I am having difficulty following your work, in large part because the scans are pretty light and so it is hard for me (with my eyes) to make everything out. Second, there is no clear distinction between what information was given as part of the problem and what notations you have made.

    I'm assuming you are free to choose which solution method you use on each problem? Sometimes exams specify the method to use and you don't get much credit for using other methods because that wasn't the point of the problem. But, for now, I'm assuming you are free to choose. I actually recommend you make the effort to solve each problem several different ways for several reasons: (1) the practice won't hurt, (2) it allows you to check your prior work, and (3) it let's you develop the feel for what things to look for that hint at particular techniques being particularly useful.

    Other suggestions:

    Use and track the units at every step of the way. Most mistakes you make will mess up the units and, if you track them, they will tell you when you have gone off the straight and narrow.

    Be neat and clean in your writing. Label the diagrams clearly and consistently. If node labels aren't given, supply them.

    Solve the solution symbolically (using variables) as far as you can. Plug in values only at the end, if possible. If a constant is given in the problem, replace it with a symbol and then substitute the constant at the end. This will help you come up with a more general solution and will also significantly increase the effectiveness of checking the units and being able to track down, correct, and propagate those corrections to the end without having to redo the whole thing.

    For that first problem, it seems like the easiest way to solve it is just to walk the solution from left to right.

    First label the nodes as follows:

    G: Upper left node. This will be the ground (0V) node.
    A: Top left node shared between the voltage source and the current source.
    B: Bottom left node.
    C: Bottom right node.

    Now, I'm assuming from your diagram that part of the given information is that the 4Ω resistor in the left-most branch has 2A of current flowing from Node G to Node A, correct?

    Assuming that to be the case:

    Q1) What is V_A (the voltage on Node A relative to ground)?
    Q2) What is V_B?
    Q3) What is I_AB (the current from Node A to Node B) through the 5Ω resistor?
    Q4) What is I_BC through the 4Ω resistor?
    Q5) What is V_C?
    Q6) What is I_AC through the 2Ω resistor?
    Q7) What is i_x?

    Once you have an answer, use KCL at Node A to find the current in the 3V supply and compare this to what KCL at Node G tells you about this same current.
  3. WBahn


    Mar 31, 2012
  4. WBahn


    Mar 31, 2012
    You haven't done any circuit analysis using phasors?
  5. WBahn


    Mar 31, 2012
    You don't indicate which amplifier input is which (inverting vs. noninverting).

    The 'I' that you solved for: Is that the current you are looking for?
  6. WBahn


    Mar 31, 2012
    I can't make out enough of the given circuit to even start. You're going to have to post a clearer image that has more contrast.
  7. Chaos241

    Thread Starter New Member

    Jul 2, 2012
    Thanks for all the input I'm going to change the contrast in just a minute and it makes it a lot more readable :)
  8. Chaos241

    Thread Starter New Member

    Jul 2, 2012
    First I'm going to put the original problem here.
    Problem 1

    Problem 2

    Problem 3

    Problem 4

    Problem 5

    All right moving on. The first problem makes total sense now. I'm going to solve it tomorrow and make sure eveyrhting comes out good.

    Second Problem: Problem 2 work

    I was pretty sure my polarity was off on V when I was solving it..

    Third Problem: Problem 3 work

    I have done phasors before but never for finding a thevenin equivalent. I know Vth is 10 angle 60 but I have no idea what Zth is or how to find it. And I can't seem to find anything about it in my book.

    Forth Problem: Problem 4 work

    It is an inverting op amp. I think I did it right up until the last step but after that I'm not sure.

    Fifth Problem:

    Problem 5 work page 1

    Problem 5 work page 2

    Thank you so much for everything you've explained so far. Its been a big help. I hope the contrast on the images is better.
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    For Q5

    At t=0+ the inductor current cannot have changed from the value at t=0-. The inductor current at t=0- will also be 0A.

    Hence the inductor current at t=0+ is still 0A. Treat the inductor branch as an open circuit at t=0+.

    The 9A source current must divide only between the 6Ω and the 30Ω at t=0+.
  10. WBahn


    Mar 31, 2012
    Why do you think the Thevenin equivalent voltage is going to be the same as the source voltage when you have a voltage divider between it and the output?

    The exact same concepts apply for AC linear circuits (in steady state) as DC linear circuits. If I gave you a DC circuit with a 10V battery in series with a 6Ω resistor and an 8Ω resistor, how would you find Vth and Rth?

    Use the exact same approach for the AC circuit. The only difference is that the numbers associated with the voltages, currents, and impedances are all complex numbers instead of purely real numbers. If you solve it first using V, I, and Z symbols, the solutions are identical.
  11. WBahn


    Mar 31, 2012
    It is "Fourth Problem", not "Forth Problem" (unless you are from a part of the world that does it different than we do).

    Look at the problem statement. Notice how they specify the polarity of Vin. They also specify the polarity of Vout and the polarity of the dependent voltage source. Voltages and current have both magnitude and direction. Both are important. Ask anyone that has ever tried to jump start a car and didn't think that voltage polarity mattered! When you worked the problem, you completely ignored them and that makes it impossible to follow your work.

    While what is inside that triangle might be an opamp circuit, the triangle itself wouldn't appear to be "an opamp". Opamps are simply high gain differential (neither inverting or non-inverting) amplifiers where the output voltage is

    Vout = Av*(Vp-Vn)

    Where Vp is the voltage at the positive pin and Vn is the voltage at the negative pin and Av is typically a number larger than +100,000.

    So think of this as just an inverting amplifier with a gain of -100 (+100 as drawn because the model circuit incorporates the minus sign by having the dependent voltage source "upside down").

    So, in your solution, the same questions I asked before hold here.

    When you come up with I=0.002A (I'm supplying the units that you were too lazy to -- note I will be merciless on the point of units, so get used to it), what does that represent? It is the current WHERE in the circuit? And which direction is it flowing? It is critical that the person looking at your work be able to determine, unambiguously, both of those these for every thing you do.

    Once you have determine what that 'I' is, then tell me whether that is the current that you are supposed to be finding.
  12. WBahn


    Mar 31, 2012
    You should try to make your lower case 't' look more distinct from a '+' sign. Sooner or later you will mistake one for the other and chaos will ensue. Either curve the bottom to the right, or add a little "up tick" to the bottom.

    You need to be much neater with your work.

    Word to the wise:

    The Proper Care and Feeding of Homework Graders

    Consider things from the grader's perspective. You have something like 50 to 100 or more papers to grade. Most of them are chicken scratches that you have to decipher. How much effort are you really going to put into deciphering them. The answer's wrong but you can't figure out where the mistake was made, whether there was only one mistake, or whether it was a conceptual mistake or just a stupid math error. How much benefit of the doubt are you doing to give them?

    Now you come upon one that is neat, well laid out, well labeled, and easy to follow. What's your first opinion going to be toward this work? If the person makes a minor mistake that is obvious to see that it is minor and that it is a simple math blunder (the person added 2+3 and got 7) and that all of the other work is consistent with the error, how likely are you going to be to assess just a slight penalty for the math mistake?

    I've actually had a grader give me a perfect score on an assignment with the notation, "You made a slight math error on problem #2, but this was so beautiful that I just couldn't bring myself to take off points."

    As side benefit of taking the time and cadet to make your grader's life easy is that it makes it much more likely that you will think about the problem in a sufficiently coherent and organized way so as to have a much better change of getting it correct -- and it makes it much more useful as review material for the exams than if it were chicken-scratch city.

    So, with that dispensed, in your solution you talk about a capacitor. What capacitor?

    For the second part, you state that "inductor = current source" but give no hint of what you mean by that or what the relevance of it is.

    You appear to be combining the 10Ω and 30Ω resistors in parallel. Is that reasonable at t=0+?

    Q1) What is the current in the inductor at t=0-?
    Q2) What is the current in the inductor at t=0+? Why?
    Q3) What is the current in the 10Ω resistor at t=0+?
    Q4) Which resistors share the 9A from the current source at t=0+?

    The remaining parts appear to be okay, or at least consistent with the error in Part 2.