Maximum power transfer

Discussion in 'Homework Help' started by chadchoud, Feb 2, 2009.

  1. chadchoud

    Thread Starter Member

    Dec 18, 2008

    I am unable to solve this problem. They're asking for R so that maximum power is transfered to RL.

    I understood this as: RL is Rthevenin for maximum power to be delivered, so in this case, if R = 10 ohms, Rthevenin becomes equal to 15 ohms and the problem is solved.

    But their answer is R infinite. First I thought it was their mistake, but to confirm I took several values of R (10, 100) and I remarked that the power is increasing as we increase R. I understand why: it is because less current is passing as we increase R, so more current will pass in the other branch, thus more power.

    I need to know if there is a rule on this. Being on an exam, my first guess would be 10ohms. I'd like to know why I was wrong.

    Thank you :)
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  2. S_lannan

    Active Member

    Jun 20, 2007
    this is a pretty stupid question, in the way they have set it up. Maximum power transfer theorem states maximum power will be transferred when the impedance of the supply is the impedance of the load.

    basically the resistance they are referring to is bypassing the load and drawing more power as it decreases, so basically it should be infinite for maximum power to be transferred to the load.

    a worthwhile question would have the resistance in question being the load or supply impedance being the resistance in question...
  3. chadchoud

    Thread Starter Member

    Dec 18, 2008
    Well the normal questions are to find the resistance/impedance of the load for max power transfer, but in this case the resistance is given. I saw many exercises of this style, though, so I don't believe they're stupid :)

    Edit: I just would like to know why my reasoning was wrong when I assumed that R thevenin is 15 and went backward.
  4. Ratch

    New Member

    Mar 20, 2007

    The power transfer theorem is not applicable here. They are not asking you to change the load resistance. Instead you are changing the shunt resistance. Any value of shunt resistance less than open is going to divert power from the load. No need to get involved with Thévenin either. It just does not apply.

  5. chadchoud

    Thread Starter Member

    Dec 18, 2008
    Okay. Thank you Ratch :>
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    It IS stupid, the way it's asked, but the solution shouldn't even require any calculations. With "R" missing, you're going to have the greatest voltage possible at the node where R was.
  7. Cabwood


    Feb 8, 2009
    I don't think the question is stupid - I think it is designed to see if you can extrapolate what you do know to solve problems that are not classical question paper and classroom exercises.