Diodes & Rectifiers: Homework Solutions

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Homework Help Overview

The discussion revolves around a problem involving diodes and rectifiers, specifically focusing on the transfer characteristic Vo=f(Vs) and the waveform Vo(t) for a circuit with a voltage source Vs=10V and a diode with a forward voltage drop of VD=0.7V. Participants are exploring the implications of these parameters in the context of circuit analysis.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants express uncertainty about how to approach the problem, particularly in sketching the transfer characteristic and drawing the waveform. Questions arise regarding the voltage drop across the diode and the interpretation of the input voltage as AC or DC. There is also confusion about the meaning of transfer characteristics and how to represent them graphically.

Discussion Status

The discussion is ongoing, with several participants seeking clarification on the concepts involved. Some guidance has been offered regarding the analysis of the circuit under different conditions, such as the behavior of the diode and the resistor at various input voltages. However, there is no explicit consensus on the correct approach or understanding of the concepts.

Contextual Notes

Participants are working under the assumption that the problem involves a variable DC source, but there is a suggestion that it may actually be AC. The original poster has indicated a lack of prior instruction on drawing transfer characteristics and waveforms, which adds to the complexity of the discussion.

Cocoleia
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Homework Statement


I am practicing problems like this:
upload_2017-1-21_15-45-27.png

where Vs=10V, R=2kohm, they say it is like a battery + ideal diode with VD=0.7V
I have to sketch the transfer characteristic Vo=f(Vs) and draw the waveform of Vo(t)

Homework Equations

The Attempt at a Solution


I am really unsure how to go about these problems. I have the answer, I just don't know they got to it. Can someone explain please?
upload_2017-1-21_15-49-10.png

I am also unsure of how to draw the waveform, but I think it could be something like:
upload_2017-1-21_15-50-17.png

Is this correct?

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VI = Vs?

Where did you take the voltage drop at the diode into account?

The problem statement asks for V0(t), not VD(t).
 
mfb said:
VI = Vs?

Where did you take the voltage drop at the diode into account?

The problem statement asks for V0(t), not VD(t).
I do not understand, I just found this graph in my textbook. Can you explain even the first part, since it too is just a drawing from the book.
 
Your first plot has an axis label VI. What is that?
Where did you find that graph? The symbols should be introduced somewhere.

The diode won't conduct until there is a 0.7 V potential difference between its sides.
 
mfb said:
Your first plot has an axis label VI. What is that?
Where did you find that graph? The symbols should be introduced somewhere.

The diode won't conduct until there is a 0.7 V potential difference between its sides.
VI is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph
 
Cocoleia said:
VI is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph
Is your voltage source ac or dc? You have shown it as dc but you have drawn the waveforms assuming it to be ac.
 
cnh1995 said:
Is your voltage source ac or dc? You have shown it as dc but you have drawn the waveforms assuming it to be ac.
I don't know, I guess it is DC?
 
Cocoleia said:
I don't know, I guess it is DC?
Well, go with dc since you have shown it that way.
So, your dc source is variable, say from 0V to 10V. The diode drop is assumed to be 0.7V. What can you say about the i-v characteristic of the diode from this? Can you plot the diode voltage first, as a function of the input dc voltage?
 
cnh1995 said:
Well, go with dc since you have shown it that way.
So, your dc source is variable, say from 0V to 10V. The diode drop is assumed to be 0.7V. What can you say about the i-v characteristic of the diode from this? Can you plot the diode voltage first, as a function of the input dc voltage?
Would it be forward biased and therefore a short circuit ? I'm not sure about the plot
 
  • #10
Cocoleia said:
they say it is like a battery + ideal diode with VD=0.7V
images (4).jpg

Now for input voltage Vs≤0.7V, what is the diode voltage? For Vs>0.7V, what is the diode voltage?
 
  • #11
cnh1995 said:
View attachment 111968
Now for input voltage Vs≤0.7V, what is the diode voltage? For Vs>0.7V, what is the diode voltage?
When it is less, it is 0. When it is more it is 0.7?
 
  • #12
Cocoleia said:
When it is less, it is 0. When it is more it is 0.7?
We are talking about "voltage" across the diode. If Vs=0.5V, what is the voltage across the diode? Use the above graph. You are right about Vs>0.7V.
Cocoleia said:
I don't know, I guess it is DC?
Cocoleia said:
draw the waveform of Vo(t)
It seems that the input is supposed to be ac, since the output voltage is a function of time. With dc input, you can draw the transfer characteristic but I don't think they are expecting you to draw Vo(t) for dc input.
 
Last edited:
  • #13
cnh1995 said:
We are talking about "voltage" across the diode. If Vs=0.5V, what is the voltage across the diode? Use the above graph. You are right about Vs>0.7V.It seems that the input is supposed to be ac, since the output voltage is a function of time. With dc input, you can draw the transfer characteristic but I don't think they are expecting you to draw Vo(t) for dc input.
I am unsure on how to draw transfer characteristics and the Vo(t), it was never explained to me in class.
 
  • #14
Cocoleia said:
I am unsure on how to draw transfer characteristics and the Vo(t), it was never explained to me in class.
Ok. We can work that out here. We will take the voltage source as AC, with magnitude 10V.

You are right about Vs>0.7V.
What is the volage across the diode when Vs<=0.7V? Refer the graph in #10. Once you know that, you can draw the transfer characteristic easliy.

Or answer this: If Vs<=0.7V, what is the voltage across the 2k resistor?
 
  • #15
Cocoleia said:
VI is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph
There is still the resistor.

At V0=-10 V, analyze the circuit. What is the voltage at the resistor?
At V0=-5 V, analyze the circuit. What is the voltage at the resistor?
At V0=0 V, analyze the circuit. What is the voltage at the resistor?
At V0=+5 V, analyze the circuit. What is the voltage at the resistor?
At V0=+10 V, analyze the circuit. What is the voltage at the resistor?
With those values (and suitable intermediate values), you can plot the voltage at the resistor as function of the source voltage. That is the curve you need in the first part.

I would expect your voltage source to be AC, otherwise plotting things as function of time is useless.
 
  • #16
cnh1995 said:
Ok. We can work that out here. We will take the voltage source as AC, with magnitude 10V.

You are right about Vs>0.7V.
What is the volage across the diode when Vs<=0.7V? Refer the graph in #10. Once you know that, you can draw the transfer characteristic easliy.

Or answer this: If Vs<=0.7V, what is the voltage across the 2k resistor?
mfb said:
There is still the resistor.

At V0=-10 V, analyze the circuit. What is the voltage at the resistor?
At V0=-5 V, analyze the circuit. What is the voltage at the resistor?
At V0=0 V, analyze the circuit. What is the voltage at the resistor?
At V0=+5 V, analyze the circuit. What is the voltage at the resistor?
At V0=+10 V, analyze the circuit. What is the voltage at the resistor?
With those values (and suitable intermediate values), you can plot the voltage at the resistor as function of the source voltage. That is the curve you need in the first part.

I would expect your voltage source to be AC, otherwise plotting things as function of time is useless.
Ok thank you, I understand now.
 

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