Simple Diode Confusion

Discussion in 'Homework Help' started by jegues, Feb 23, 2011.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
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    Hello all. I'm having some trouble with analyzing this circuit containing diodes.

    Our professor told us that we have to trial and error analysis of every possible combination between the two diodes. (i.e. whether one is conducting and the other is cut off etc...)

    First question, is there a better, more intuitive way of determining what each diode is actually doing? (i.e. either conducting or cut off)

    Secondly, even after my trial and error analysis in this problem, I'm still not sure how to determine which combination is correct? (i.e. what the diodes are actually doing, either conducting or cut off)

    Thanks again!
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Could you please post a schematic of the original problem, showing the two diodes?
     
  3. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    Here you are.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Here you must consider all the possible permutations of diodes conducting or not conducting, as you have done.

    An acceptable solution requires a forward current in any diode or diodes assumed to be conducting, and cannot have a forward voltage across any diode or diodes assumed to be non-conducting. You will see that only one of your four possible solutions meets these conditions.

    Sometimes it may appear more or less obvious what should be conducting, but using a method helps to get around our assumptions, which may not always be correct.

    Note however that the results from such a simple method assuming zero forward voltages may not always be accurate. Your instructors will probably avoid setting problems where this would be the case
     
  5. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
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    What do you mean by forward current and forward voltage?
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Forward voltage means voltage applied in the forward bias direction, where the anode is positive with respect to the cathode.

    Forward current means "conventional" current flowing from anode to cathode. NB electron flow is in the opposite direction: cathode to anode.

    http://en.wikipedia.org/wiki/Diode

    Figure 5 in the Wikipedia article shows a forward-biased diode. The arrow shows the conventional current direction.
     
  7. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
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    So if we assume that voltage across a diode to always be forward then one can state that the voltage across a diode that is none conducting can never be positive, correct?
     
  8. Georacer

    Moderator

    Nov 25, 2009
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    Correct. Let me rephrase Adjuster's words.
    Assume that some of your circuit's diodes conduct and that some are cut off. Analyse your circuit. If in the end you fount positive forward voltage greater than 0 (or 0.7V) on the diodes you had assumed to be cut off, your initial assumption was wrong, and you have to start with a different combination of conducting and cut off diodes.
    The same goes if you find voltage less than 0 (or 0.7V) on diodes that are supposed to be conducting.
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You have to make a series of different sets of different assumptions (conducting and non-conducting) to cover all the possibilities.

    You must reject any set of assumptionsassumption which produces a contradictory result, such as conduction in the reverse direction, or a forward voltage across a diode assumed to be blocking.

    Remember however that what is being accepted or rejected is a given set of assumptions. The fact that you may reject one choice because a diode is assumed blocking but actually has forward bias does not necessarily mean tht it will be conducting in the correct solution.

    Perhaps you will make more progress by working through some examples than by thinking about it in the abstract.
     
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