Why is my solution for real diode analysis wrong?

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    Analysis Diode
AI Thread Summary
The discussion focuses on a homework problem regarding diode analysis, specifically the incorrect application of the equation I=Ise(V/VT). A key error identified is the neglect of the thermal voltage (VT) in the exponent, which is critical for accurate calculations. While VT is typically around 0.025V at room temperature, its actual value can vary based on diode doping, necessitating precise determination for the problem at hand. Participants suggest using simultaneous equations to solve for the saturation current (Is) and VT before calculating resistance (R) to ensure both sides of the circuit equate to 10mA. Accurate diode analysis requires careful consideration of these parameters for correct results.
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Homework Statement


9XUbpS3.png


Homework Equations


I=Ise(V/VT)

The Attempt at a Solution


The solution I got is wrong. Does anyone know why?

Here is my attempt, the image was too large for this forum so here is the link:
http://i.imgur.com/4tuGxdO.jpg
 
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You ignored VT in the exponent.

ehild
 
ehild said:
You ignored VT in the exponent.

ehild

Isn't the VT value the same (approximately 0.025V at room temperature)?
 
theBEAST said:
Isn't the VT value the same (approximately 0.025V at room temperature)?

The approximation is just that, an approximation. It is a good "typical" value for run-of-the-mill diodes. The actual value for a given diode depends upon the details of the diode doping. You should determine its true value for this problem.
 
theBEAST said:
Isn't the VT value the same (approximately 0.025V at room temperature)?

Even in case it is, I1/I2 = e(V1-V2)/VT

ehild
 
You may need to run a few simultaneous equations. Here's my suggestion: Use the generic information provided to solve for Is and VT before proceeding further. Then from there you can solve for R using the fact that both sides of the circuit must equate 10mA.
 
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