Graph of waveform across diode shown by CRO

[songoku]

Homework Statement

Please see below

Relevant Equations

None

Option (A) and (B) is wrong because the waveform should be half-wave, not full-wave. But how to know whether it will be (C) or (D) based on the circuit given?

Thanks

 

[DaveE]

Do you know the I-V characteristics of diodes? And the polarity based on the schematic symbol? It wouldn't make sense to ask this question if they hadn't covered diodes in your lectures.

Note that the oscilloscope has very high input impedance (resistance), so you can assume that no current will flow into it. The polarity of the oscilloscope is defined by the ground symbol in the schematic. So the upper input is positive, i.e. positive voltage w.r.t. ground at that input will make the trace go up, a negative voltage there will make the trace go down.

 

[songoku]

Do you know the I-V characteristics of diodes? And the polarity based on the schematic symbol? It wouldn't make sense to ask this question if they hadn't covered diodes in your lectures.

Yes, I know

Note that the oscilloscope has very high input impedance (resistance), so you can assume that no current will flow into it. The polarity of the oscilloscope is defined by the ground symbol in the schematic. So the upper input is positive, i.e. positive voltage w.r.t. ground at that input will make the trace go up, a negative voltage there will make the trace go down.

Based on your hint, I think the answer will be (C) because the current will flow downward through the diode and from higher potential to lower potential. Since the lower potential is zero, the higher potential should be positive so the trace should go up

Is that correct? Thanks

 

[berkeman]

Is that correct? Thanks

No. Does the Cathode Ray Oscilloscope measure current or voltage? Make a plot of each i(t) and v(t) to see how they are related... Can you show us that plot?

 

[DaveE]

You may find the sinewave adds some complexity. First, try finding the current and voltage for a fixed DC value, let's say +10Vdc. Then try -10Vdc. For this exercise, you can think of the sinewave as just a sequence of fixed voltages.

Identify the polarity of the voltages and currents in that diode I-V plot versus your schematic.

 

[songoku]

No. Does the Cathode Ray Oscilloscope measure current or voltage?

Voltage

Make a plot of each i(t) and v(t) to see how they are related... Can you show us that plot?

The input voltage will be:

The input current will be the same shape with different amplitudeThe output voltage will be:

The output current will have the same shape with different amplitude

Is this what you mean?

You may find the sinewave adds some complexity. First, try finding the current and voltage for a fixed DC value, let's say +10Vdc. Then try -10Vdc. For this exercise, you can think of the sinewave as just a sequence of fixed voltages.

For +10Vdc passing through the diode, the output will be:

For -10Vdc, there will no output because the diode will block it.

Identify the polarity of the voltages and currents in that diode I-V plot versus your schematic.

Since there current flows downwards through the diode, the upper polarity will be positive and lower polarity of diode will be negative. Is this what you mean?

Thanks

 

[Delta2]

The way I know it, is that when the diode allows flows of current through it, then the voltage difference on its ends is very small, something like 0.7V. I think for an ideal diode is 0V. If the diode doesn't allow flow of current then you can find the voltage difference on its ends by using KVL and setting I=0 through the resistor (and the diode).

I think that the correct answer is D btw.

I don't know if this will help or confuse you more, but maybe it will be helpful to see the diode as some sort of variable resistance:
If the diode is forward biased then the resistance is zero
If the diode is reversed biased then the resistance is infinite.

 

[songoku]

I don't know if this will help or confuse you more, but maybe it will be helpful to see the diode as some sort of variable resistance:
If the diode is forward biased then the resistance is zero
If the diode is reversed biased then the resistance is infinite.

This helps a lot. From your post, I just realize that I need to measure p.d. across diode, not across resistor.

I also think the answer is (D). When the diode is conducting, the p.d. read by CRO will be zero since the diode acts like short circuit. When the diode is not conducting, the CRO reads the voltage from power supply because the diode acts like open circuit. The polarity of the output will be negative because the current will flow in counterclockwise direction

 

[Delta2]

The polarity of the output will be negative because the current will flow in counterclockwise direction

The current will be zero in this case since we will have infinite resistance in series with 100k resistor. It is more accurate to say that the current will have a tendency to flow in the counterclockwise direction, which of course is prohibited by the way the diode is connected (it allows current only in the clockwise direction).

 

[CWatters]

I don't know if this will help or confuse you more, but maybe it will be helpful to see the diode as some sort of variable resistance:
If the diode is forward biased then the resistance is zero
If the diode is reversed biased then the resistance is infinite.

+1

I would treat the diode as a switch.

Forward Bias = Switch is conducting => the output is connected to 0V
Reverse Bias = Switch is open => the output is just connected to the input via the resistor.

 

[songoku]

Thank you very much for the help and explanation DaveE, berkeman, Delta2, CWatters

 

[Gordianus]

Let me add something tongue-in-cheek.
I haven't seen new Cathode Ray Oscilloscopes in a long time (but I still use my wonderful Tek 465).
The OP's figure comes from an old book indeed.