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Diode question

Discussion in 'Homework Help' started by hupsenfg, Oct 6, 2017.

  1. hupsenfg

    hupsenfg New Member

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    upload_2017-10-7_10-39-4.png
    Hi i think if I would like to find Vd and Id, i can replace the diode with a 0.7V voltage source. How then should I proceed to do this question?
    Thank you
     
  2. hupsenfg

    hupsenfg New Member

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    For Id, it should (5v-0.7v)/ 10k right?? Or we can ignore 0.7v

    And Vd = 5v? After voltage divider
     
    Last edited: Oct 6, 2017
  3. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    Y0u only need one thread for one subject...
     
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi hup,
    The question is misleading.
    It asks for Vd, but it states 'assuming a practical diode', which suggests a diode with a 0.7v forward voltage drop.

    The circuit diagram shows Vd across the diode, so Vd must be 0.7V

    E
     
  6. ci139

    ci139 Active Member

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    FIG.1
    is't like 10kΩ || (10kµ+diodeΩ) -- where the first 10k & 5k likely can be considered as "primary voltage divider" and
    the second 10k and the (BW or FW)diode as the secondary voltage divider for voltage drop on the first 10k ... PS! the foreward diode is usually some 2k55 ohms (i donno for this circuit . . . the LT Spice shows so at 5V total supply for 1N4148)
     
    Last edited: Oct 7, 2017
  7. Ratchit

    Ratchit Well-Known Member

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    It should be OK to give you the correct answer since a month has passed since you asked for help.

    The diode will definitly turn on, because if it were off, there would be 20/3 volts across it. Use node analysis to find the voltage Vo. The equation is:
    hupsenfg1.JPG
    The solution is Vo = 4.825 volts

    Next find the current present in the diode.
    hupsenfg2.JPG
    The solution is Id = 0.0005015 amp or about half a milliamp.

    Note: If you gave me the saturation current of the diode, I would not have to make an estimate of 0.7 volts across the diode. I could calculate the diode voltage by the current present within it.

    Ratch
     
    Last edited: Nov 7, 2017

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