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Homework Help: Diode characteristics/dealing with infinity

  1. Dec 10, 2009 #1
    New to the forum, I'll say "Hi," first :)

    1. The problem statement, all variables and given/known data
    Have a physics assignment to hand in. Very simple experiment - increase the voltage in a circuit and measure the current/voltage across the diode in forward and reverse. The problem comes with dealing with infinity in some of the calculations for uncertainty.

    It's really only the data where the current reads as 0A I have issues with (Right before the P-N junction starts to allow a current to flow, and all the results for a reverse-biased diode). I've written them as follows due to division by zero's. The question really is "Is this the correct way I'd write this?"

    2. Relevant equations
    Uncertainty (Voltage) = Minimum readable voltage (0.01V) / Voltage observed
    Uncertainty (Current) = Minimum readable current (0.01A) / Current observed
    Uncertainty (Resistance) = Uncertainty (V) + Uncertainty (I)

    3. The attempt at a solution

    Voltage: 0.38
    Voltage Uncertainty: ±2.63%
    Max Voltage: 0.39
    Min Voltage: 0.37
    Current: 0.00
    Current Uncertainty: ±∞%
    Max Current: 0 < ∞
    Min Current: 0.00
    Resistance: 0 < ∞
    Resistance Uncertainty: ±∞%

    Any suggestions are appreciated.
    Last edited: Dec 10, 2009
  2. jcsd
  3. Dec 10, 2009 #2
    You should be plotting the values of V and I on a graph. You can express the uncertainty in the readings using error bars. Draw the best line through the points.
    What are you trying to calculate? You only need percentage errors if you use a formula with data in it that has been measured to a known level of precision.
    What is the aim of the experiment? If it is to calculate resistance, that can be found from the gradient of the graph.
  4. Dec 10, 2009 #3
    Sorry I do try to be clear but have a tendancy to create the opposite effect :)

    The criteria I have to obtain is simple itself - I just wanted to ensure that the way I am presenting my results is correct. Using the example data I posted


    But I know resistance isn't infinate in reality, so I wanted to make that clear in my results. So would R=0<∞ be the correct way to display that?

    The criteria for this particular piece of work is to calculate and explain the resistance of a component in a complete circuit, and to describe the relationship of current, voltage, resistance and temperature on macroscopic and microscopic levels.

    Also: Whilst on the subjects of charts.... Excel isn't that great - especially when I'm trying to plot a diode in revers and forward on the same chart (given that V/I are positive in both directions...) Anyone know something that will do the job?
  5. Dec 10, 2009 #4
    As I said, if you are varying V and measuring I, you calculate the resistance from the slope of the graph.
    A resistance of infinity is not crazy if it refers to the fact that, despite the applied voltage, no current flows. This happens in a diode!
    It's not particularly useful to worry about the value of R when in reverse connection. No current means, in practice, infinite resistance.
    The forward current and resistance is more important.
  6. Dec 10, 2009 #5
    I tend to worry about the little things! You've been helpful, I'll leave it as infinity.

    The I/V curve is nonlinear, so I'll have to lookup the calculations for the resistance (Havn't been taught it, yet) and try to apply it to my results!
  7. Dec 10, 2009 #6
    This type of question is more about investigating how the current varies with voltage.
    True it isn't linear. The graph is what is important. It should show that there is no current in the reverse direction. The "infinite" resistance is really only a way of stating that mathematically. It's not something you calculate as such.
    The forward resistance clearly varies with current. V/I at any point will give its value at that point. It's usual, if you have such a graph, to measure the slope at the point you are interested in to find the value of R at that point. If you draw the "best" slope and also a "worse" slope (one that fits but is not so good, yet could reasonably fit) - you can quote the answer as "best" and give the "worst" value as a measure of the uncertainty.
    eg if best value gives 50 and worst value gives 55
    Quote value as 50 +/- 5
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