Which diodes are On?

Discussion in 'General Electronics Chat' started by ENG99, Sep 2, 2015.

  1. ENG99

    Thread Starter Member

    Feb 13, 2014
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    Hi,
    In this pic I'm only sure about D1 which is ON and for D2 and D3 I'm not really sure about them! So, please help me out how can I determine whether if they are on or off and how?
    thanks in advance.

    ON or OFF.jpg
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    D1 is on
    D2 + D3 is off

    Oops .....Recalculating

    Nope D1 on, D2 + D3 off.
     
    Last edited: Sep 2, 2015
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
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    I get D2=off, D1 & D3 =on

    But the more I look at it the less certain I am:(.
     
    Last edited: Sep 2, 2015
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    Iterestingly enough my sim tells me that not all of them are ON. Sadly I don´t have the mental capability right now to figure out how to reliably come up with the result.

    Edit: it seems there could be a bit of uncertainity coming from the assumed Vf of the diodes, which might turn D3 on as well. My sim seems to assume Vf of 0.56V at this sort of current with 1N4148 and states that D3 is off.
     
  5. GLPro

    New Member

    Aug 21, 2014
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    D1 & D3 on............D2 off?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What assumption are you making about the forward voltage drop of a conducting diode?
    0V or 0.6V or something else?
     
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  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    The symbol is not for a schottky so I assumed a silicon diode with .7 Volt drop.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Ignore D2 and D3. Assume all diodes are perfect. Assume all diode's Vf are 0.6 V.

    Loop current is 14.4 V / 30.2 K = 476.8 uA.

    The node voltages are:

    8.2K/12K = +5.49V. This is greater than 4.4 V, so D2 is off.

    12K/10K = -0.232 V. This greater than -0.6 V, so D3 is off.

    ak
     
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  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    AK, I used the same logic. Agreed (I am sure Mike is plugging this into spice as I type.)
     
  10. ENG99

    Thread Starter Member

    Feb 13, 2014
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    Well you consider D1, D2 on and D3 off! Sorry it's wrong answer!
     
  11. ENG99

    Thread Starter Member

    Feb 13, 2014
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    you got them right! But how? I spent an hour trying to find how but i couldn't. plz show me how you found that, thanks
     
    Last edited: Sep 3, 2015
  12. ENG99

    Thread Starter Member

    Feb 13, 2014
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    What about D2?
     
  13. ENG99

    Thread Starter Member

    Feb 13, 2014
    37
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    Well, you got it, but how?!
     
  14. ENG99

    Thread Starter Member

    Feb 13, 2014
    37
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    0V
     
  15. ENG99

    Thread Starter Member

    Feb 13, 2014
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    Sorry but the answer isn't correct!
     
  16. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Try making the circuit to better understand..
     
  17. Lestraveled

    Well-Known Member

    May 19, 2014
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    Well isn't that special. You present an electrical puzzle and you leave out critical data. Then after several opinions are entered, you release the critical data at the same time you proclaim the winners and losers. Now where is that "Ignore" button I have been hearing about.
     
  18. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I just a simulation on my phone and it say D1 on , D2 &D3 off..
     
  19. ENG99

    Thread Starter Member

    Feb 13, 2014
    37
    0
    This is NOT electrical puzzle this is example I found on google and I have been trying to solve it! And I shared it here so everyone can give me an idea how s/he got it. It's not my problem what do you think and how do you think! I asked question and I didn't write who is winner or loser YOU wrote it. And have fun with ignore button!
     
  20. ENG99

    Thread Starter Member

    Feb 13, 2014
    37
    0
    So, then Mesh analysis? I'll end up with 4 Loops? But how can I determine which one is on and which one is off?!
     
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