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Get Current and Voltage with a Circuit with Diodes

  1. Oct 20, 2016 #1
    1. The problem statement, all variables and given/known data
    Find the V and I in the circuit shown below:

    upload_2016-10-20_12-21-44.png
    2. Relevant equations
    • D1 or 2 is off=short circuit
    • D1 or 2 is on=open circuit

    3. The attempt at a solution
    So, my confusion lies with deciding the on and offs of D1 and D2 . My assumption is that both of them are on and that is due to them in forward bias. However, when I looked online, D2 was off and D1 was on. Based on how the current is going into D2, how is D2 off?

    Any help is much appreciated!
     
  2. jcsd
  3. Oct 20, 2016 #2

    berkeman

    User Avatar

    Staff: Mentor

    Your Relevant Equations are backwards, BTW. On=short and Off=open for diodes.

    What is the typical forward voltage for a silicon diode? What is the anode voltage for D2 if it is off? And if D2 were on, what would that do to the Anode voltage?
     
  4. Oct 20, 2016 #3
    Thank you for the response!

    Aww, I'm sorry. Yeah it does look like they are bakwards (was in a rush). Thank you for that.

    Im sorry, but what do you mean by anode voltage? My assumption was that d2 was forward positive because:
    upload_2016-10-20_18-46-39.png
     
  5. Oct 21, 2016 #4
    Basically, I think I want to know why D1 and D2 both cant be on.
     
  6. Oct 21, 2016 #5

    gneill

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    Staff: Mentor

    Look for circuit behavior contradiction(s) if you assume that both are on. For a quick evaluation, assume that the diodes are ideal (no forward voltage or resistance). What would be the potential V on your diagram? Determine all the currents through the components (should be simple if you know the potential drops on the resistors and KCL for the junctions). What do you get?
     
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