How much the voltage can vary at different current for zener diode.

Discussion in 'General Electronics Chat' started by hp1729, Jul 15, 2016.

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Mod Note:
    This thread was split from -- Zener diode problem.


    Zener diode exercise results. Not for a 1N746 but it gives you an idea of how much the voltage cab nary at different current. Your same basic circuit of a resistor and a Zener, varying the resistor values and noting diode voltage, resistor voltage, current and effective diode resistance. Using Excel we can graph the results. This was a 4.3 Volt 250 mW Zener rated at 8 mA and at about 8 mA it has the 4.3 Volts across it.
     
  2. ScottWang

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    Aug 23, 2012
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    @hp1729
    You made an example for the Zener is 4.3V, could you explain more details about these in your attached Excel, also explain the meaning of vlaues for all the vertical of graph.
    Ohms V app Vd V Res Id Rd
    Thanks.
     
  3. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Ohm is the series resistor value used at that step.
    V app is the applied voltage if it varied any at that step
    Vd is the voltage measured across the diode at that step
    V res is the calculated voltage across the resistor
    Id is the calculated current through the diode
    Rd is the calculated effective resistance of the diode.

    At each step you use a certain resistor, measure V app, measure the diode voltage, the calculations are made by Excel. Go on to the next lower resistor. Continue until you exceed the diode specifications.
    You can do this with most any diode, LED, or whatever. Yes, it can take time but you come away with a good grasp of what the components behave like. When you get done you can measure the voltage across a diode and know how much current is flowing through it.
    Similar exercises can be done with transistors and see how gain varies at different collector currents and such.
    These exercises were part of the course at the school where I used to teach. The objective was component level troubleshooting.
     
    Last edited: Jul 16, 2016
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  4. ScottWang

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    @hp1729.
    Thanks.
    You didn't explain what are the meaning of values for the vertical and horizontal of the three graphs.
     
  5. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    ???
    How much is needed? The graph of the diode voltage is the voltage across the diode at each reading. Current through the diode is current through the diode. Diode resistance is diode resistance. I guess familiarity with Microsoft excel helps.
    Such stuff is covered in a class on Excel prior to this electronics part of the course. The electronics portion relies heavily on using Excel for design and component analysis.
     
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  6. ScottWang

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  7. Techno Tronix

    Member

    Jan 10, 2015
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    So that means zener diode is "magically" able to change its resistance to allow more current to pass through it.
     
  8. hp1729

    Thread Starter Well-Known Member

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    The change of resistance is the result of different current. The voltage is not precise and consistent.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No magic expressed or implied. The dynamic or effective resistance of *all* diodes changes as a function of the current. Whether in forward conduction or zenering, that is what diodes do. A theoretically perfect standard signal diode sits there with a constant 0.6 or 0.7 V across it as the current through it varies from 0.1 mA to 100 mA. That is a 1000:1 change in current but no change in voltage drop. As a series element, it's effective resistance must also have changed by 1000:1.

    Using basic Ohm's Law, you can calculate the effective resistance change of a 1N4004 rectifier:
    R = E / I
    R1 = 0.6 V / 0.1 A = ?
    R2 = 0.8 V / 1 A = ?

    Note: Despite tons of sexy multiplexer chips developed over the last 50 years, RF circuits still use "diode switches" because they work.

    ak
     
  10. hp1729

    Thread Starter Well-Known Member

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    On a graph of the voltage the vertical is voltage. Horizontal is the reading step in the Excel chart it relates to.
    I'm sorry, I didn't think graphs were a mystery.
     
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  11. crutschow

    Expert

    Mar 14, 2008
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    No mystery at all to us. Most of us are completely familiar with Excel and graphing. Do you know how to add axis labels in Excel?
    But your condescending attitude has no place here.
    Proper (and common) procedure when doing a graph is to label the two axes with the units of what the numbers represent.
    If you look at the graphs in a data sheet for example, you will see that they are all labeled that way.
    It would seem you are the one not familiar with normal graph labeling procedure. :rolleyes:
     
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  12. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    I/m sorry. I thought it was obvious enough. Yes the graphs alone might be vague. Accompanied by the data should make them clear enough.

    (edited to add ...)

    I can add a legend to the vertical and horizontal entries but I don't see a way to put resistance values along the horizontal axis. Any suggestions would be helpful.
     
    Last edited: Jul 16, 2016
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