Loading effect - No load voltage?

Discussion in 'Homework Help' started by jegues, Sep 13, 2010.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    See figure attached for problem statement.

    Under question 2. in Figure 3 I am given the Thevenin equivalent attached to an Oscilloscope.

    In part a) they ask what the value of VNL is in terms of Vs and Rs.

    How do I go about finding this using Figure 3? (again, see attachement)

    If the Oscilloscope can be modeled as a resistor then this would be a simple voltage divider circuit and I could find VNL in terms of Vs, Rs, AND the internal resistance of the oscilloscope, but in the question it says SOLELY in terms of Vs and Rs.

    I dont see how I can do this. What am I not seeing?

    Any help/tips/suggestions/comments would be greatly appreciated.

    Thanks again!
     
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  2. Ghar

    Active Member

    Mar 8, 2010
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    No load means open circuit on the terminals, infinite resistance. There is nothing connected there, that's why it's called "no-load"
    What is the voltage on the output if there is no current through Rs?
     
  3. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    Well if there is no current through Rs, then there shouldn't be any voltage across the output, right?
     
  4. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Write a loop equation:

    Vs = IRs + VL

    With no load, VL = VNL

    Vs = IRs + VNL

    What's I...?
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    VNL = RS
    No load gives infinite resistance. results no current, which interms gives the maximum voltage.
    Interms of Rs and Vs ......VNL = Vs
     
  6. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    In you figures, the oscilloscope is considered merely as a way to measure the voltage across the terminals. It is supposed to have infinite resistanse, so you can treat it as an open-circuit, much as an ideal volt meter. In fact, oscilloscopes do have very high resistance, much greater than any cheap volt meter.

    So don't be fooled by the picture. The first figure is to be treated as a source without any load, and the second as a source with only Vl attached to it.
     
  7. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    Just to be sure I can visualize whats going on I drew a quick sketch, is this right? (see figure)


    Okay now back to my loop equations:

    I = (Vs - VNL)/Rs

    Sorry if the answer should be obvious by now, but I still don't see how we get VNL in terms of Vs and Rs.

    Maybe another nudge and I'll get it.

    EDIT: WAIT! I is 0, we have an open circuit! (DOH!)

    So back to our node equation Vs = VNL!

    Thanks everyone for all the helpful replies, this has cleared everything up for me!
     
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