Understanding the output waveform and using LTSpice

In summary, the conversation discusses finding the output waveform for a circuit with an AC voltage source, resistor, diode, and battery. The voltage drop across the resistor and the voltage across the diode and battery are constant at a given instant. The AC source can be neglected in some cases depending on its instantaneous voltage compared to the fixed voltage value. The conversation also suggests using LTSpice for analysis and increasing the input magnitude to gain more insight. Finally, it is recommended to consider the diode as either conducting or not at any moment and analyze the circuit in each state.
  • #1
JC2000
186
16
Homework Statement
Find the output waveform for the given circuit.
Relevant Equations
Not sure about which equations to use here (if any)
Screenshot 2020-07-21 at 12.33.48 PM.png


For the following circuit I need to find the output waveform.

A. Conceptually I am at sea here but I will have a stab at it. My though process is as follows :
  1. At a given instant, the AC voltage is at a fixed value and this value drops (##V_drop##) across the resistor ##R##.
  2. The voltage (##V_drop##) across the diode and battery as well as the voltage across ##R_L## is constant for a given instant.
  3. Current flow would be affected across the diode when the voltage across it is less than the cut-in voltage of the diode (0.7 V), however this information seems irrelevant in finding the output voltage.
  4. At a given instant the AC source is the same as a DC source which is parallel to the voltage source ##V##. Two cases emerge here, one when the instantaneous voltage of the AC source is greater than ##V## and the other when the instantaneous voltage of the AC source is lesser than ##V##. In the former, V acts as a short (?) while in the latter the AC source can be neglected (?!).
  5. Can't quite put a finger on how V affects the output waveform.
How do I arrive at the output waveform?!

B. I tried to use LTSpice to analyse the circuit, could you please tell me if I have replicated the circuit correctly?

Screenshot 2020-07-21 at 12.33.17 PM.png


Thank you for your time!
 

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  • #2
Try to re-run your simulation with a larger magnitude sine wave input, let's say 50V. This will give you more insight. If Vout is less than about 10.7V then the diode will always be off.

You can assume that at any moment in time the diode will either be conducting, or not. So, you can start by analyzing the circuit in each of these two modes (i.e. one with a short circuit, one with an open circuit in place of the diode). Then think about when the circuit will switch between these two states.
 
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Related to Understanding the output waveform and using LTSpice

1. What is an output waveform in LTSpice?

An output waveform in LTSpice is a graphical representation of the voltage or current output of a circuit simulation. It shows how the circuit responds to different inputs and can help in analyzing the behavior of the circuit.

2. How do I interpret the output waveform in LTSpice?

The output waveform in LTSpice can be interpreted by looking at the amplitude, frequency, and shape of the waveform. The amplitude represents the voltage or current level, the frequency represents the rate of change, and the shape represents the waveform type (sine, square, etc.). Additionally, the time scale on the x-axis can also provide information about the timing of the circuit's response.

3. What are some common issues that can affect the output waveform in LTSpice?

Some common issues that can affect the output waveform in LTSpice include incorrect component values, incorrect connections, and simulation settings. It is important to double-check all component values and connections to ensure they are accurate. Additionally, adjusting the simulation settings, such as the time step or simulation length, can also impact the output waveform.

4. How can I use the output waveform to troubleshoot my circuit in LTSpice?

The output waveform can be used to troubleshoot a circuit in LTSpice by comparing it to the expected behavior. If the waveform does not match the expected behavior, it can indicate a problem with the circuit design or simulation settings. By analyzing the waveform and making adjustments to the circuit, you can identify and fix any issues.

5. Can I export the output waveform from LTSpice for further analysis?

Yes, you can export the output waveform from LTSpice for further analysis. You can save the waveform as a text file or an image file, which can then be imported into other software for further analysis or documentation purposes. Additionally, you can also use the cursor tool in LTSpice to measure specific points on the waveform and record those values for further analysis.

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