Help with Diode Circuit with Current/Voltage through Diode

Click For Summary

Discussion Overview

The discussion revolves around a homework problem involving a diode circuit, specifically focusing on determining the value of a resistor (R) that results in a voltage (V) of 80mV across the circuit. Participants explore various approaches to solving the problem, including the application of diode equations and circuit laws.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests using Shockley's current equation to find the current through the diodes, but expresses uncertainty about the approach.
  • Another participant notes the importance of seeing the circuit diagram to provide accurate guidance and suggests using the diode equation or a graphical load line method if parameters like leakage current and temperature are known.
  • A participant proposes that if the diodes are ideal, then no current flows through one diode (D2), leading to a calculation of R as 8 ohms based on a 10mA current through the other diode (D1).
  • Another participant challenges the assumption that no current flows through D2, arguing that the current must be shared between D1 and D2, and emphasizes the complexity of the problem due to the presence of nonlinear elements.
  • This participant outlines several equations that could be used to analyze the circuit, including the diode equations for both diodes, Ohm's law, and Kirchhoff's laws, suggesting that these could lead to a solution for R.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of the diodes in the circuit, particularly regarding current flow through D2. There is no consensus on the correct approach to solving the problem, and multiple competing views remain regarding the assumptions and equations to be used.

Contextual Notes

The discussion highlights the complexity of the circuit due to the presence of nonlinear elements and the need for specific parameters (like leakage current and temperature) that may not be provided. The assumptions about ideal diodes versus real diode behavior are also not resolved.

ischmob
Messages
3
Reaction score
0

Homework Statement



For the circuit shown in Fig. 4.26 both diodes are identical. Find the value of R for which V=80mV

Homework Equations



I'm not quite sure what equation to use here. If you look at the wiki page on diodes http://en.wikipedia.org/wiki/Diode
it might be Shockleys current equation

The Attempt at a Solution



My thought on how to approach this problem is find the current that comes out of the diode and then use that current in a voltage divider using 80mV to find a value of R. Though I have no clue what is coming out of the diode. Though I know 5mA is going to each diode.[URL]http://img218.imageshack.us/i/fig426.png/[/URL]
 
Last edited by a moderator:
Physics news on Phys.org
Hi,
The figure is not visible. Try uploading again.
 
Can you upload Fig. 4.26?

Without seeing the circuit I can't say for certain, but you may be able to use the diode equation if you know the leakage current, temperature etc. Or you may be able to solve graphically with a 'load line' if you have a plot of the diode characteristic.
 
thanks for your guys prompt responses. here's another attempt at the figure upload

http://img218.imageshack.us/i/fig426.png/"

[URL]http://img218.imageshack.us/i/fig426.png/[/URL]
 

Attachments

  • Fig 4.26.png
    Fig 4.26.png
    1.7 KB · Views: 705
Last edited by a moderator:
Just a wild guess. Might be violating the laws of electricity with this answer. :) Is it 8 ohm?

I assumed the diodes to be ideal.
No current flows through the diode D2 as there is no voltage drop across it.
10mA flows through D1.
So R= 80mV/10mA = 8 ohm
 
so your saying that no current will flow through D2. And as far as your answer. I think its a little more complicated than that. I'm sure there's some sort of formula you need to plug in
 
asp_physics said:
Just a wild guess. Might be violating the laws of electricity with this answer. :) Is it 8 ohm?

I assumed the diodes to be ideal.
No current flows through the diode D2 as there is no voltage drop across it.
10mA flows through D1.
So R= 80mV/10mA = 8 ohm

Sorry, but that's just not true. There is current through D2 and D1, since the 10mA current must be shared between D1 and D2, and thus there is a voltage drop across both D1 and D2.

It's not an easy problem, actually, since there are two nonlinear elements. You can write down a few different equations though:

The diode equation (Shockley's) for D1 and D2.
Ohm's law across the resistor.
Kirchoff's current law at the node between the diodes and the current source.
Kirchoff's voltage law around the loop at bottom (that goes from ground, through the diodes and resistor back to ground).

That's 5 equations, though 3 of them are quite trivial, and after some substitution you can solve for R given V = 80mV (I believe).
 

Similar threads

Engineering Full bridge circuit with inductor and resistor
  • · Replies 8 ·
Replies
8
Views
2K
Engineering Analyzing the inductor current in the circuit
  • · Replies 1 ·
Replies
1
Views
2K
Engineering How Does a Capacitor Affect Ripple Voltage in a Diode Circuit?
  • · Replies 15 ·
Replies
15
Views
7K
Engineering Calculating Voltage Drop Across Non-Ideal Diodes
  • · Replies 20 ·
Replies
20
Views
5K
Finding the forward current of diode
  • · Replies 2 ·
Replies
2
Views
3K
Can't handle finding currents on the diodes
  • · Replies 1 ·
Replies
1
Views
1K
Engineering Diodes limiting circuit question?
  • · Replies 3 ·
Replies
3
Views
2K
How do I calculate the voltage of a diode in a simple electronics circuit?
  • · Replies 9 ·
Replies
9
Views
2K
Engineering What is the output voltage of this ideal diode circuit?
  • · Replies 19 ·
Replies
19
Views
5K
Solving for Zener Diode Voltage
  • · Replies 45 ·
2
Replies
45
Views
9K