How Do You Analyze Circuits Containing Diodes?

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Discussion Overview

The discussion focuses on the analysis of circuits containing diodes, particularly in the context of basic circuit analysis involving voltage sources and resistors. Participants explore the implications of using ideal versus real diodes and the behavior of diodes in circuits with varying configurations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asks how to calculate the current in a simple circuit with a voltage source, resistor, and silicon diode.
  • Another participant suggests assuming a constant voltage drop of 0.6-0.7 V across a silicon diode for basic analysis.
  • A participant references a previous thread that discusses ideal diodes with a forward voltage of 0V and later addresses real diodes with a forward voltage of 0.6 to 0.7V.
  • There is a question about the behavior of a diode connected directly to a voltage source without a resistor, specifically whether the current would be zero or infinite.
  • Some participants mention the diode equation but express that it may be excessive for basic needs, while others caution about the implications of connecting a diode without a resistor, noting potential damage to the diode.
  • One participant asserts that the current is zero when the voltage is below the diode-drop voltage and infinite when above it, but this is contested by concerns about the practical limits of real diodes.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of diodes in circuits without resistors, particularly regarding whether the current can be considered infinite and the implications for real diodes. There is no consensus on the correct interpretation of these scenarios.

Contextual Notes

Participants highlight the limitations of using ideal diode assumptions versus real diode behavior, particularly in terms of power dissipation and the potential for damage when no resistive load is present.

dancavallaro
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How do I do basic circuit analysis of circuits that have diodes in them? For example, if I have a basic circuit consisting of just a voltage source, a resistor, and a silicon diode, how would I calculate the current in the circuit?
 
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For basic analysis (and assuming a silicon diode), assume a constant 0.6-0.7 V voltage drop at the diode.
 
Here's a recent Homework Help thread discussing analyzing a circuit with diodes in it. The thread starts off assuming "ideal" diodes with Vf = 0V, but does address real diodes later with Vf = 0.6 to 0.7V, etc.

https://www.physicsforums.com/showthread.php?t=289232

.
 
Ah, thanks for that link! What happens if there's no resistor in the circuit and it's just a diode hooked up to a voltage source?
 
Google "diode equation"
 
Redbelly98 said:
Google "diode equation"

I found the diode equation, but I think it's overkill for my needs. Mapes, and the link berekman posted, helped a lot for my purposes. The only thing I don't get now though, is what happens when the diode is hooked up with no resistor. Is the current 0, or is it infinite? I'm inclined to say that it would be "infinite", but is that right?
 
If the diode equation is overkill, then the simple answer is:

The current is 0 when V is below the diode-drop voltage (about 0.7V).
The current is infinite when V is above the diode-drop voltage.
 
dancavallaro said:
I found the diode equation, but I think it's overkill for my needs. Mapes, and the link berekman posted, helped a lot for my purposes. The only thing I don't get now though, is what happens when the diode is hooked up with no resistor. Is the current 0, or is it infinite? I'm inclined to say that it would be "infinite", but is that right?

For a real diode, you will probably burn it up. Look at the plot of the diode equation -- the current increases quickly for voltages above the forward threshold voltage. And since power P = V * I, the power dissipated increases quickly as well. Real diodes have limits on how much power they can handle...
 

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