Engineering Help with Diode Circuit with Current/Voltage through Diode

AI Thread Summary
The discussion centers on determining the resistance value (R) in a diode circuit to achieve a voltage of 80mV. Participants suggest using Shockley's diode equation and Ohm's law, noting that both diodes are identical and share the current. Initial assumptions about current flow in the diodes are debated, with clarification that current does flow through both diodes, affecting the voltage drop. The complexity of the problem is acknowledged due to the presence of nonlinear elements, and multiple equations are recommended for solving the circuit. Ultimately, the solution requires careful application of circuit laws to find the correct resistance value.
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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]
 
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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
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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).
 

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