Simple question about Diode circuits

Discussion in 'Homework Help' started by Bz1984, Nov 3, 2006.

  1. Bz1984

    Thread Starter New Member

    Nov 3, 2006
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    If a diode is in Parallel with a resistor, does any current pass through the resistor? and in another case if a diode was in series with a resistor and those were in parallel with another resistor, how can one acquire the total resistance if it was possible? (assuming in all cases the diode is forward biased and is on)

    many thanks~
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    In the first case, diode in parallel with a resistor, the answer is yes. The resistor has 0.7 Volts across it. How much current flows depends on the resistor, and the current rating of the diode. If the diode tries to pass too much current it will fail. If the resistor tries to pass too much current it too will fail.

    In the second case both branches have the supply voltage across them. Apply Ohm's law for the currents, remembering to figure the forward drop of the diode at 0.7V for a slicon diode, 0.25 V for a Schottky diode, and 0.2v for a Germanium diode.
     
  3. vikram_kumar

    New Member

    Nov 2, 2006
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    dioe havingtwo cases
    (1)anode voltage(Va)>cathode voltage(Vc) diode act as short circuit
    (2)Va<Vc diode act as open circuit

    SHOWN In fig
    If a diode is in Parallel with a resistor,diode in forword bias ,no current pass through the resistor
    another case if a diode was in series with a resistor and those were in parallel with another resistor, the total resistance is parrel combination two resistances ()
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    Your answers are complete nonsense. A diode will never act as a short unless it has failed, and it will definitely be different then two resistors in parallel.
     
  5. Bz1984

    Thread Starter New Member

    Nov 3, 2006
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    thanks for the replies :)

    i was actually confused cus i read somewhere that the diode does act as a short circuit therefore taking all the current from the resistor, thanks for the info ;)
     
  6. WeirdmanJ

    New Member

    Dec 15, 2009
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    I may be wrong here as it's been a long time since I've done any actual serious look at circuitry, so please correct me if I am in fact dense. Automated test equipment made me lazy.

    If you have a resistor and a diode in parallel on a DC circuit and the diode is forward biased its a short (or very close to it) and the current/DC signal passes through it instead of the resistor.

    If the diode is reverse biased then it becomes an open (or very close to it) on that same DC circuit and the current/DC signal goes through the resistor instead, making the diode pretty much useless.

    I don't remember what situation if any this would be practical for.

    The only exceptions that I can think of to this would be a zener diode (reversed biased at breakdown voltage) or photodiode (forward biased with light applied). I'm not sure if the zener diode would work, but i think the photodiode could actually be used with a sensor on the same line as the resistor as a crude NOT function for light.
     
  7. WeirdmanJ

    New Member

    Dec 15, 2009
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    Now if these components are on an AC circuit in parallel it seems that the current would pass virtually unrestricted through the diode one way, and be forced to go through the resistor the other way. I imagine that this would result in a lopsided sin wave. Is this correct?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    WierdmanJ - the thread you replied to is 3 years old. It's a good idea to check dates on posts.
     
  9. kirti_sonawane999

    New Member

    Jul 9, 2011
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    If Diode is forward biased, Voltage drop will appear 0.7V . and other voltage drops will appear as u apply KVL to circuit.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    kirti_sonawane999 - the thread you replied to is 2 years old. It's a good idea to check dates on posts.
    And even 3 years before that.

    Bertus
     
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