Max Power Graph

Discussion in 'Homework Help' started by camino, Sep 17, 2009.

  1. camino

    Thread Starter New Member

    Sep 17, 2009

    My professor has instructed us to create a graph showing 3 lines: Voltage, Current, and Power.

    The x-axis is to be resistance in ohms.
    The y-axis is to be Voltage.

    The main goal in creating this graph is to figure out at what Resistance Maximum Power is achieved (the peak of the "power" line before it begins its decline).

    We have found the current of the load to be 8.97 mA.
    The circuit has a 90V power source, a 2200Ω resistor in series with 2 resistors, 3300Ω and 4700Ω in parallel. (The 4700Ω is the load).

    I am not sure how to go about making this graph. I am using Microsoft Excel to create it. Any help would be much appreciated.
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Presumably what you are being asked to do is plot the load power variation as the load resistance varies above or below 4700 ohms.

    What you are looking for is the so-called "Maximum Power Transfer" condition which you may be already aware occurs when the load resistance equals the effective source resistance. It will probably be no surprise to you that for your question the load resistance will be different to 4700 ohms when this condition occurs - hence the purpose of the exercise.

    One approach would be as follows

    1. Temporarily remove the load resistor from the circuit
    2. Reduce the remaining circuit to its Thevenin equivalent
    3. Reconnect the load to the Thevenin equivalent
    4. Perform a series of calculations for the load power (plus voltage & current) as Rload varies from zero to (say) 10,000 ohms with some practical incremental change - perhaps 100 ohms to start with.
    5. With load resistance as the x-axis (0-10,000 ohms) then plot load power on the main y-axis, load voltage on a secondary y-axis and load current on a tertiary y-axis.
    6. You should observe the maximum power condition occurs near one of your calculated values
    7. Home in on the actual maximum power resistance value by making smaller incremental changes near that value identified in step 6 above.

    Alternatively you could derive an equation for the power in relation to Rload and the other circuit parameters. Using calculus you would then determine the maximum power condition.