How Do Diodes Behave in Different Circuit Configurations?

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Discussion Overview

The discussion revolves around the behavior of diodes in various circuit configurations, particularly focusing on homework problems involving diode characteristics, voltage inputs, and output behavior. Participants explore theoretical and practical aspects of diode operation in circuits, including zener diodes and their response to different voltage levels.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the behavior of two diodes under positive and negative voltage conditions, suggesting that the output voltage (Vout) is zero for positive inputs and follows a specific relationship for negative inputs.
  • Another participant emphasizes the importance of resistance in determining diode conduction, questioning the choice of voltage drop values (Vd) for the diodes based on current levels.
  • There is a discussion about the output voltage when the input voltage is at specific negative values, with one participant suggesting that Vout would be -2.3V when Vin is -1.9V or -2.1V.
  • Participants explore the conditions under which the zener diode conducts, questioning the differences in behavior for positive versus negative input voltages.
  • One participant expresses confusion about the circuit configuration and the implications of multiple diodes, indicating a need for clarification on the voltage drop across the diodes.
  • Another participant calculates the current through the bottom diode for a specific input voltage and discusses the implications of exceeding a certain current threshold on the choice of Vd.
  • There is a question about the maximum voltage the anode of the top diode can assume, indicating ongoing exploration of diode behavior in the circuit.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the circuit configurations and diode behavior. There is no consensus on the correct approach to some of the problems, particularly regarding the choice of voltage drops and the output voltage under specific conditions.

Contextual Notes

Some participants note the dependency of diode behavior on current levels and the importance of resistance values, while others highlight the complexity introduced by multiple diodes in the circuit. Unresolved assumptions about the circuit configuration and the specific values for voltage drops contribute to the ongoing discussion.

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Homework Statement


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Homework Equations





The Attempt at a Solution


My instructor gave us some sample problems he said might be on the exam, I tried working them out but I was having difficulty with 3 of them:

For the 1st problem, when there is positive value for voltage, I got the 1st diode as off (open circuit) and the 2nd diode as on (short circuit). This would mean Vout is always 0 for positive voltage. When the voltage becomes negative, the 1st diode becomes a short and the 2nd becomes open circuit. So 20||20=10 and Vo=(0.5)Vin using voltage division. Graph for Vout would be 0 when the 5V peak sine wave is positive curve, and a curve of half the amplitude when it is a negative curve (min=-2.5).

For the 2nd problem, I was used to working with 1K ohm resistors so the 500 ohm kinda confused me. But if the value of resistor doesn't matter, I find that diode starts conducting when Vi=2.5 and conducts fully when Vi=2.7 (+2V). This would mean graph is a linear line (Vo=Vi) until Vi=2.7 when it becomes a horizontal line. For the -3V, diode starts conducting when Vi=-2.5 and conducts fully when Vi=-2.3. Graph would look similar to the +2V one.

For the 3rd one, I had a zener diode for another problem so I am assuming there is positive value at the negative end. Also VZ=VZ0 because rz is very small. For +20V, I got current flowing through diodes B, C, D. the voltage is 20-2Vd-Vz=10.4V. For +5V, I got a negative voltage after the voltage drop across zener diode so I found no current would flow through the loop. For -5V, I got a positive voltage after the voltage gain in zener diode, so zero current again. For the last one (-20V), I found current flowing through doides A, C, E if I assume zener diodes can conduct voltage in both ways. Then the voltage is -20+Vz+2Vd=-10.4V.
 
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(a) is A-OK.

For (b), the resistance certainly does matter.
When Vin = 0 what is the current flowing thru the lower diode? You have to try both Vd = 0.5V and Vd = 0.7V and pick the right one.

Then, run Vi from -5V to +5V to get the output. What is Vout when Vin = -1.9V? When Vin = -2.1V?

Does the top diode ever turn on?

You gt (c) right also.

Why would the zener not conduct current the same way for a negative 20V input as for a positive 20V input? The zener current is in the same direction for both +20 and - 20V inputs.
 
(b) I don't really understand this type of circuit, usually with the previous problems i had only 1 diode and voltage battery. When vi=0, the current through lower diode would be (0-Vd+3) diovided by resistance. I'm not really sure what you mean by choosing Vd=0.5 or 0.7 though. I get I=5 or 4.6mA depending on this.

When Vi=-1.9 and -2.1, current has to flow through bottom diode only. I think vout would be -2.3V for both because that's when the diode finishes conducting.

am I doing this right so far? Maybe the vout graph has to include both values then, for both the +2 and -3V sources. So I should graph both for vi=-5 to +5V. But I think my only problem is what vout would be at Vin=0V, -2.3V or just 0V.

also, for (c), would I be right in thinking that if I graphed VR, it would look like the sin wave input except with amplitude of 10.4V? And V0 would be a constant value instead of a curve, +/- 9.6V depending on the sine value.
 
Last edited:
If the input is -1.9V, and the voltage drop across the bottom diode is 0.5V, what would be the current?

And what about the top diode? What input voltage would turn it on?
 
asdf12312 said:
(b) I don't really understand this type of circuit, usually with the previous problems i had only 1 diode and voltage battery. When vi=0, the current through lower diode would be (0-Vd+3) diovided by resistance. I'm not really sure what you mean by choosing Vd=0.5 or 0.7 though. I get I=5 or 4.6mA depending on this.

Remember what the problem stated: if i < 1 mA, Vd = 0.5 and if i > 1 mA then Vd = 0.7v. So which do you think you should choose?
 
I see now. If input is -1.9V, the current through the bottom diode is (-1.9-Vd+3)/500=1.2mA. But this is >1mA..so I should choose the bottom diode Vd=0.7V right? For the top diode using a similar problem I was assigned I found it starts conducting at 2.5V.
 
For the top diode: what's the highest voltage the anode can assume?
 

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