Diode Circuit ON/OFF Model Problem

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SUMMARY

The discussion focuses on solving a diode circuit model problem involving two diodes, D1 and D2. The key equations established are that when the diode is on, Vd equals 0.7V and Id equals 0, while when the diode is off, Vd is less than 0.7V and Id is greater than 0. The user, Shaunzio, is advised to analyze the circuit by considering different cases for the diodes' states, particularly focusing on the voltage Vx and its implications on the current through resistors R1 and R2. The discussion emphasizes the importance of systematically evaluating each scenario to determine the correct circuit parameters.

PREREQUISITES
  • Understanding of diode characteristics, specifically forward and reverse bias conditions.
  • Knowledge of circuit analysis techniques, including Kirchhoff's laws.
  • Familiarity with voltage and current relationships in resistive circuits.
  • Basic proficiency in using circuit simulation tools for verification.
NEXT STEPS
  • Study the effects of forward and reverse bias on diode behavior in circuits.
  • Learn about Kirchhoff's Voltage Law (KVL) and its application in complex circuits.
  • Explore circuit simulation software like LTspice to model diode circuits.
  • Investigate the impact of varying resistor values on diode operation and circuit performance.
USEFUL FOR

Electrical engineering students, circuit designers, and hobbyists looking to deepen their understanding of diode circuits and improve their problem-solving skills in electronics.

Shaunzio
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Homework Statement


T2r13.png


Homework Equations


When Diode is on Vd=0.7 V and Id=0
When Diode is off Vd<0.7 and Id>0

The Attempt at a Solution



I've been working on this problem for hours now but I just can't logic it out properly. I know the right model is supposed to be D1 ON and D2 OFF, but I can't figure out why or what the value of Vx is. Any help would be greatly appreciated.
 
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Hi Shaunzio. You need to consider all possibilities, taking one at a time.

Case (1). D1 is forward biased, D2 is forward biased.
With D2 forward biased, what value does that make Vx?
With that value for Vx, how much current is going through R2, and in what direction?
And so on ... until you are satisfied you have uniquely determined all circuit parameters.

Case (2). ...
 
I think there is a slightly shorter method in this case...

I'd start by looking at the case with D2 ON. For D2 to be ON the voltage Vx must be < 3V-0.7 - 2.3V

Try assuming Vx=2.3 V and calculate the current that would flow through R1. That current must also be flowing through R2 (it can't flow the wrong way through D2) so it sets a minimium voltage for Vx. If that's greater than 2.3V then the assumption that D2 is ON is incorrect.

If V2 is off the problem simplifies greatly.
 

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