Diodes, calculate the current

1. Apr 17, 2013

Proff Young

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.

Thank you

Last edited by a moderator: May 6, 2017
2. Apr 17, 2013

AnTiFreeze3

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.

3. Apr 17, 2013

Proff Young

hope the image works now :)

4. Apr 17, 2013

gnurf

Here you go:

5. Apr 17, 2013

Proff Young

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.

6. Apr 17, 2013

gnurf

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
7. Apr 18, 2013

Proff Young

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?

8. Apr 18, 2013

Gordianus

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.

9. Apr 18, 2013

gnurf

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
10. Apr 23, 2013

Proff Young

Thank you.

11. Apr 23, 2013

Proff Young

I hope it is the OR gate. Thank you for helping, I have a better idea now.