Thevenins Theorem Help!

Discussion in 'Homework Help' started by THEDOC92, Nov 27, 2013.

  1. THEDOC92

    Thread Starter New Member

    Nov 27, 2013
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    Hi, first post so a little about myself. I'm a HNC Student in Fabrication and Welding but have a unit this year based on DC Network Analysis and more that we are yet to cover yet.

    Im struggling with Thevenins Theorem. I've used the DC section on this website for help (http://www.allaboutcircuits.com/vol_1/chpt_10/8.html) and I understand the basics of what to for that particular circuit however I'm stumped when it comes to applying the laws to a circuit laid out differently.

    My question is laid out as..

    [​IMG]

    With a little assistance on how to utilise the batteries, im confident i can work out the Thev Equivalent. Im just unsure on where to start.

    Thanks
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Welcome to AAC.

    Remove the load.
    Analyze the resulting circuit to find the voltage across the terminal where the load was and call this Voc (the open-circuit voltage).

    Put a short circuit (a wire) where the load was.
    Analyze the resulting circuit to find the current through that wire and call this Iss (the short-circuit current).

    Voc is the Thevenin equivalent voltage. The equivalent resistance, Req, is Voc/Iss.

    This is not the only way, but it is pretty much guaranteed to work on any linear circuit.

    Next you need to post YOUR best effort to solve the problem. That will give us a basis for further discussion.
     
    anhnha likes this.
  3. THEDOC92

    Thread Starter New Member

    Nov 27, 2013
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    Thanks for your help.

    [​IMG]

    Basically, ive assessed the circuit as if it where two seperate circuits with single batteries. However, im pretty sure im falling at the first hurdle with this. In the example shown on this website (link in first post) the overall voltage is shown at 21V. Im assuming this figure is based on 28V - 7V. However, with my particular circuit being laid out differently, im unsure on how to begin my calculations once I've removed the load.
     
  4. THEDOC92

    Thread Starter New Member

    Nov 27, 2013
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    My biggest issue is that in class I've been taught and practiced on numerous layouts of circuit all of which have just one battery. The work ive been given for home utilises multiple batteries and I'm unsure on how this affects the working out.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    You haven't analyzed ANY circuits that had multiple batteries? At all?

    If I gave you the above circuit, telling you that Rload was 6Ω, and just asked you to find the current in the Rload, would that be something that you haven't done anything like it at all before?

    Have you been taught analysis techniques such as node voltage analysis and mesh current analysis? If so, have none of those ever involved multiple batteries?

    You haven't been taught superposition at all?

    I need to get a feel for where your at before I can help you move in the right direction.
     
  6. THEDOC92

    Thread Starter New Member

    Nov 27, 2013
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    The majority of the things I've learned have came from this website, I really can't say much for our teacher. He's a Doctorate in his field but he just cannot come down to our level to teach us.

    Basically, this is where I'm at and what I'm comfortable with:

    Ohms Law
    Analysis of Series Circuits
    Analysis of Parallel Circuits
    Analysis of Series/Parallel Circuits
    (But only using one Battery)
    Kirchoffs Law

    We've touched on Thevenins, Nortons and Ive looked into mesh analysis myself but my knowledge is very limited at present.

    Im sure you an appreciate my frustration! If I had a better understanding of how to analyse circuits with Multiple batteries i'm sure i could figure it out.

    Thanks again.
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,715
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    If that's where you're at, I can definitely sympathize with your frustration. Node Voltage Analysis, Mesh Current Analysis, and Superposition are bedrock techniques for analyzing circuits and, in my opinion (apparently shared by most textbook authors) is that a fair level of proficiency with them should be attained before even beginning with Thevenin and Norton equivalent circuits.

    I'd recommend hitting your text on these subject or delving into Chapter 10 of the E-book here.

    http://www.allaboutcircuits.com/vol_1/chpt_10/1.html

    Again, keep in mind that the E-book tries to use electron flow and not charge flow.
     
  8. THEDOC92

    Thread Starter New Member

    Nov 27, 2013
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    0
    Hello again!

    After some research and help of others, these are the calculations ive managed to come up with.

    Voc = (V1/R1+R2) + (-V2/R1+R2)
    = (12/15+10) + (-9/15+10)
    = 0.48 - 0.36
    = 0.12

    V2 + VR2 = 9 + (0.12X10) = 10.2v

    Therefore, Open Circuit Voltage = 10.2V??

    From this, i gathered that Iss = Voc/R

    =10.2/(R3+(R1^-1+R2^-1)^-1)

    = 0.56A

    Any assistance on whether this is correct or where I've gone wrong would be greatly helpful!
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Four things you need to do before I will be willing to look it over.

    1) Define the variables you are using. If throw around R1 and V2 and such yet those appear nowhere in the circuit diagram. Don't require people to play detective and figure out what you mean.

    2) Clearly define the polarity of the voltages and especially the currents you are using.

    3) Clean up the expressions. The expression (V1/R1+R2) means ((V1/R1)+R2) where as you probably mean (V1/(R1+R2)). Don't make people violate the rules of arithmetic in order to correctly interpret what you meant to say.

    4) You need to start using units properly and consistently. A voltage is a quantity having both a magnitude and units. While 12 and 0.012 are NOT the same, 12V, 0.012kV, and 12000mV are. Saying V=0.12 is meaningless since 0.12 is a number, not a voltage. Learning to track and check your units throughout your work will significantly decrease the number of mistakes you make and let you catch the ones you do. Don't just tack some unit on the end and call it good.
     
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