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Electric circuit with negative reference voltages and diodes

  1. Oct 15, 2011 #1
    Hi I am having trouble figuring out what to do with negative reference voltages in my circuit. I attached my problem(Problem 3) and I know that some current will be drawn out of the ground and I think all of the diodes will be on but I am unsure how to approach this problem.
     

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  2. jcsd
  3. Oct 15, 2011 #2

    phinds

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    You pic is upside down
     
  4. Oct 15, 2011 #3
    Sorry Here it is right side up
     

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  5. Oct 15, 2011 #4

    phinds

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    OK, you are right about the diodes all being on. You need to show some work here. What have you tried so far? Have you read the forum rules about how to post homework problems?
     
  6. Oct 15, 2011 #5
    Yes I have read the rules regarding homework. So far I have redrawn the circuit with batteries in place of the diodes each = to 0.7 volts. The 0.7 V batteries all oppose their assigned current ( ie the negative end of the battery is the same side as the cathode on the diode.
    So first I tried to figure out each node using KVL starting with Va. But quickly I have figured out this cannot work. So right now I am sort of suck because I am not sure if what I am trying to do is to get each negative potential voltage to zero or if I should somehow find a way to sum up the voltage sources.
     
  7. Oct 15, 2011 #6
    Oh and the current from the +5 Voltage source across R1 is 1mA
     
  8. Oct 15, 2011 #7

    gneill

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    You might want to check that assumption about the diodes all being forward biased.

    Assume for a moment that D2 is not conducting. What would be VA? Compare it to VB.
     
  9. Oct 15, 2011 #8

    phinds

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    Hm ... I worked the whole thing under that assumption and got good and consistent answers. Are you sure I need to redo it?
     
  10. Oct 15, 2011 #9
    ok so I have Va=7.15V and the current on the left side = 1.43 mA
    Vb= 4.3V and current right side = 0.86mA. But how do I know that I can make the assumption to turn D2 off?
     
  11. Oct 15, 2011 #10
    Like with D2 off I get Va>Vb which would mean that my assumption would not turn D2 off.
     
  12. Oct 15, 2011 #11
    Oh wait. Va should be >Vb so the assumption is correct. Thanks for your help. One last question: when choosing diodes to be on or off do you simply take your best guess or is there a more systematic approach?
     
  13. Oct 15, 2011 #12

    phinds

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    Is Va > Vb always a sufficient condition to have D2 on?
     
  14. Oct 15, 2011 #13
    I believe so. The polarity on the open switch that is used to replace D2 would be such that the diode would be turned off. Assuming my Va and Vb are correct
     
  15. Oct 15, 2011 #14

    phinds

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    Well, you need to think about this some more. What does "on voltage" on a diode mean to you?
     
  16. Oct 15, 2011 #15
    Well its the voltage required to turn on the diode to allow current through it. Usually 0.7V for silicon.
     
  17. Oct 15, 2011 #16

    phinds

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    So is it always turned on if that voltage is greater than zero?
     
  18. Oct 15, 2011 #17
    no. If the voltage is less than the "on" voltage then the diode will still be off.
     
  19. Oct 15, 2011 #18

    phinds

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    OOPS --- that IS a bad assumption isn't it.

    Good catch.
     
  20. Oct 15, 2011 #19

    phinds

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    Good.

    Also, note the post I just made about D2 being on.
     
  21. Oct 15, 2011 #20
    OK. So if d2 is on does this change my value for va and vb? I don't think it would.
     
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