1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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:

  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?


    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Diodes calculate current Date
I Calculating water pressure and required mass of water Today at 3:53 AM
I Transport in PV Cells (and pn diodes) Jul 18, 2017
I Why is there reverse current in diode equation? May 18, 2017