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Diodes, calculate the current

  1. Apr 17, 2013 #1
    http://img545.imageshack.us/img545/7113/diodeq.jpg [Broken]
    In the circuit shown in the figure (c) above if VA=5V and VB=3V, calculate the current through RL.

    Please explain.
    Thank you
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Apr 17, 2013 #2
    I just finished a unit on circuits in class so I wanted to try this out, but the picture you have provided isn't working.
     
  4. Apr 17, 2013 #3
    hope the image works now :)
     
  5. Apr 17, 2013 #4
    Here you go:

    yhowKZh.png
     
  6. Apr 17, 2013 #5
    Thank you but pardon me. I cannot grasp what is written on the image. It's not clear. And what if the diodes have a Barrier voltage of 0.7V each.
    Thank you.
     
  7. Apr 17, 2013 #6
    I'm sorry, but I thought you preferred really really poor picture quality. Anyway, assuming a diode voltage drop of 0.7V is a good start because by doing that you'll soon enough run into a contradiction (of sorts) and see that one of the diodes is reversed biased. Think about it some more and you'll see it, I'm sure.

    EDIT: SchemeIt is a good alternative for drawing circuits. It's free and very easy to use. Check it out!
     
    Last edited: Apr 17, 2013
  8. Apr 18, 2013 #7
    I got the idea thank you. If VA=VB=5V and if the diodes have a barrier voltage of 0.7V each then voltage across the resistor will be 4.3V. Am I right?
     
  9. Apr 18, 2013 #8
    Assuming you're using standard silicon diodes (voltage drop about 0.7 V), the current that flows through RL is 4.3 mA, only diode A conducts and diode B is reverse biased.
     
  10. Apr 18, 2013 #9
    Yes. By the way, your circuit is also functionally equal to a _____ gate. Do you know which one?

    Logic_gate.gif

    P.S Let a logic '1' be any voltage > 4.3V and a logic '0' be any voltage < 4.3V. The gate output 'F' from the figure above would be the node between the diodes and load resistor.
     
    Last edited: Apr 18, 2013
  11. Apr 23, 2013 #10
    Thank you.
     
  12. Apr 23, 2013 #11
    I hope it is the OR gate. Thank you for helping, I have a better idea now.
     
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