Transient analysis

How To Run And Probe Circuit Using LTspice (Transient Analysis)

Simulation Commands

To run a simulation, specify the type of analysis to be performed. In today’s tutorial we will see Transient analysis.

There are six different types of analysis:

  • Transient analysis
  • Small signal AC
  • DC sweep
  • Noise
  • DC transfer function
  • DC operating point
Transient analysis

As we have seen the different type of analysis in the above section, we will learn first analysis – Transient analysis in detail.

The transient analysis is probably the most important analysis you can run in LTspice, and it computes various values of your circuit over time. Two very important parameters in the transient analysis are:
  • Stop Time.
  • Time to Start Saving Data
  • Maximum Time step

The ratio of Stop Time: Maximum Timestep determines how many calculations LTspice must make to plot a wave form. LTspice always defaults the start time to zero seconds and going until it reaches the user defined final time. It is incredibly important that you think about what timestep you should use before running the Simulation, if you make the timestep too small the probe screen will be cluttered with unnecessary points making it hard to read, and taking extreme amounts of time for LTspice to calculate. However, at the opposite side of that coin is the problem that if you set the timestep too high you might miss important henomenon that are occurring over very short periods of time in the circuit. Therefore play with step time to see what works best for your circuit.

In the above figure we need to apply the voltage. If you want to apply DC voltage then right click on the voltage source. Then you will get a pop up window where you can specify the DC value. In case, you want to apply AC voltage then click on the advance button in the pop up window and you will get another window with types of AC sources. Eg: puls, sine, Exp etc.

In our example we take Pulse as the input. In order to design the pulse we need to provide the values for the following parameters.

  • V initial (v) = Initial voltage
  • V on(v) = On voltage
  • T delay(s) = Time delay
  • T rise(s) = Rise time
  • T fall(s) = Fall time
  • T on(s) = On time
  • T period(s) = Time period
  • N cycle = Number of cycles

Editing Simulation Commands

In order to edit the simulation commands you need to

  • Left click on Simulation menu.
  • Left click on edit Simulation Command
  • Left click on Transient tab
  • Enter a Stop Time

Refer the below figure.

Once you press OK buttons and subsequently on run button you will receive the below screen.

After clicking the RUN button the screen will get divided into two. One is for circuits and the other is for plotting the wave form. By default you will have single plot pane. In case if you want to plot multiple wave forms then right click on the plot pane and select “Add plot pane” from the pop up menu. In this way you can add n number of plot panes to your simulator.

Probing a circuit and Wave form viewer

  • Left click on any wire to plot the voltage on the wave form viewer.Prober
  • left click on the body of component to plot the current on the waveform viewer.probe

See the below simulation result. It indicates input wave form, output wave form and current through the capacitor.

Average /RMS current for Voltage Calculations

Hold down Ctrl and left click on the I or V trace label in the wave form viewer.

You will get the values for Average / RMS voltage in a pop up window. In the same way you can find the values for Average / RMS current.

Intaneous & Average Power Dissipation

  • Intaneous Power Dissipation
    • Hold down the Alt Key and left click on the symbol of the component.
    • Pointer will change to thermometer.
    • Plotted in units of Watts
  • Average Power Dissipation
    • Hold down the Ctrl key and left click on the trace label power dissipation waveform.

Generating BOM

BOM – Bill of Materials

  • Left click on view menu
  • Left click on Bill of Materials

There is lot more in Transient analysis, but it is not possible to specify each one in a single post. So I conclude this section and will try to include any miss out points in the future post.

Happy reading… do write to us if you like our post..


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