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diode concept question

Discussion in 'General Electronics Discussion' started by max_torch, Aug 1, 2015.

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  1. max_torch

    max_torch

    111
    1
    Feb 9, 2014
    I made a simulation in proteus VSM:
    diodes.png
    The AA119 is supposed to be a germanium diode while the 1N4001 is a silicon one.
    What's the concept behind these readings?
    Why is there a voltage across the two diodes on the left even if no current flows, and what is behind the ratio at which they are following everytime the voltage across them changes?
    Why is it that changing the series resistor changes the voltage levels, I thought Silicon has a 0.7 drop and Ge has a 0.3 drop?
    and in the middle circuit why does the Ge diode have 0 V across?
    This is not a homework assignment, but rather I am just trying to iron out my theoretical misconceptions and I could use some helpful insight because I have had conflicting answers from different people about this.
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,087
    Dec 18, 2013
    Hello
    There will be less than 1 mA of current through your diode. Your Amp meter can only display 1 mA min. The voltage across the diode changes with current. Only at a certain current will you get 0.7 Volts. Have a look at the graphs on the datasheet
    Adam
     
  3. max_torch

    max_torch

    111
    1
    Feb 9, 2014
    This is the current due to the minority carriers?
     
  4. Arouse1973

    Arouse1973 Adam

    5,164
    1,087
    Dec 18, 2013
    This is the current due to an applied electric field across the diode. Drift current is caused by an external electric field (voltage) applied across the junction. Add this to the diffusion current which doesn't need an electric field and you have the total conductor current.
    Adam
     
  5. Ratch

    Ratch

    1,094
    334
    Mar 10, 2013
    There is no "concept", only diode physics.
    Constant current is present, but the AC meter you are using is not registering it. Try using a constant current meter instead.
    You are starving the diodes with the 1 meg resistor in the middle circuit. In the bottom circuit, the increase to 1.0 volt is probably due to the bulk resistance of the N and P slabs at the high current level.
     
    Arouse1973 likes this.
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