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Basic queries on using LT Spice

Discussion in 'Electronic Design' started by Terry Pinnell, Nov 30, 2004.

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  1. These are very basic questions, some no doubt in the Duh! category.
    FWIW I've read the tutorial, albeit quickly. But a few quick pointers
    from the established LT Spice users could save me hours please! Or
    maybe there is an existing source for learning practical details which
    I've not yet found? I'm so familiar with CM that getting comfortable
    with LT Spice may take me a while, but I'd like to master it as its
    simulation facilities are plainly superior.

    1. What's the difference between Move and Drag? (Both appear to move a
    selected part or group of parts around, until left click fixes them in
    new position.)

    2. With a circuit drawn, I click Run and get the 'Select Visible
    Waveforms' dialogue, with choices like V(n001),V(n002),etc. But how do
    I know what those nodes are? IOW how do I first ensure the nodes are
    displayed on the schematic.

    3. In the absence of such indicators, suppose I just choose the first
    and get a plot headed
    'V(n001)'. Can I *now* get the schematic to show that choice? Is there
    any other way of finding what voltage or current I'm seeing, apart
    from clicking each node with the cursor and observing the result?

    4. If I decide to take the Help's advice "The easiest method is to
    simply probe the schematic. You simply point and click at a wire to
    plot the voltage on that wire," how do I by-pass that initial
    dialogue? Even with none of its choices selected, if I click Cancel,
    it still plots V(n001). I could of course then close that, to get a
    clean slate ... if I knew which node to click!

    5. With several voltages plotted, how do I separate them into
    individual windows?

    6. To edit the schematic, I take it you must first close all plots?

    7. In CM I can select a 'component' called '.IC' and connect it at a
    node. In LTS I see I add this as a Spice Directive. But how do I
    connect it?

    8. Is there any way to re-assign shortcuts? For example, I have ctrl-z
    ingrained for Undo, and Ctrl-c for Copy, not F9 and F6.

    9. Where can I find step by step examples of adding/importing new

    10. For a Transient Analysis, with whatever defaults running, what are
    the steps for changing the key parameters: Start Time, Stop Time, Step
    Time, Max Step Time. Are these all entered as Spice Directives? If so,
    is there a succinct summary of the main directives and their syntax
    please? (In CM, I use only the GUI.)
  2. LTSpice is my first Spice ever, so can't really answer
    many of your questions Terry..... but I do have to say
    that I'm mightily impressed with some of the simulations
    it has done on known circuits.

    Left click in the schematic window to make that window the
    one with the input focus. Then wander around around nodes
    and components to see the scope probe or current display
    being offered. As each one is offered it's description
    in given down in the bottom left corner, in text.
    No, just left-click in the schematic window to make it the
    active window. When done, re-run the simulation and a new
    plot will overwrite the old one.
    Drop down the 'Simulate' pane, 'Edit Simulation Cmd' is
    the bottom option.
  3. Thanks Tony. 3 down, 7 to go. Until my next 10 <g>.
  4. I forgot to say. After the schematic is drawn, and
    before a run, you can wander the cursor around the
    schematic and the identity of each node also comes
    up in the bottom left corner.
  5. Thanks YD. Discovered that one a little later <g>. How about
    connecting the .IC command though?
  6. YD

    YD Guest

    You can also place a label (F4) at the nodes of interest. Press F4 to
    give it a name and definiton (In, Out, Bi, None). Then drag and drop
    it onto a wire in the node. Or place it somewhere and connect it.

    - YD.
  7. John Smith

    John Smith Guest

    ----- Original Message -----
    From: "Terry Pinnell" <>
    Newsgroups: sci.electronics.cad,
    Sent: Tuesday, November 30, 2004 3:42 AM
    Subject: Basic queries on using LT Spice

    A dedicated LT Spice newsgroup:

    If the component is connected in the schematic, using drag will pull the
    wires along with the component when you move it. Move moves the component
    without stretching the wires. Note that you can draw a rectangle with drag
    or move to select an entire area to move or drag.

    See Help/Schematic Editing.

    I believe this question has been answered. If you don't want to pass the
    crosshairs over the wire to read the node ID below, you can label the wire.
    See Help/Trace Selection.

    No. Not that I aware of. Note that you single click a node to plot it.
    Clicking the same node twice in succession discards all plots but that node.
    This is not what you asked, but it is handy to know.

    On a new schematic, or when you have closed the plot window, LT Spice
    doesn't know which node you want plotted, so it asks. You must either tell
    it which node to plot or it will plot node 001. After the initial run, it
    will continue to display the node or nodes previously selected until you
    close the plot window again. If you have a display which you like, you can
    click on the plot window somewhere to make it the focus, then save the plot.
    Save the plot with the same name as the schematic. The next time you do a
    run after closing the plot window (or after closing LT Spice and then
    reloading), LT Spice will look for a plot file and use it to display your
    plots as selected previously.

    See Help/Save Plot Configurations

    I think this has been answered, but, right click the plot pane. Select Add
    Plot Pane. Click the node of interest.

    I've never used this, but look under Help in Shortcuts.

    Look in Help/FAQs/Adding Third Party Models

    With focus on the schematic, click Simulate/Edit Simulation Command. A
    dialog box opens for you to insert the values without resorting to
    directives. Otherwise, see Help/Dot Commands.

    Note that my answers here are based on my experience and there may be other
    and better answers.

    LT Spice comes with example programs and pretty good help, especially for a
    free, powerful program. I strongly recommend you read the Help and run the
    examples. I should do this, too.

    Good luck.

  8. Thanks for that very thorough and helpful reply, John. Much
    appreciated. Have now joined the Yahoo Forum.
  9. john jardine

    john jardine Guest

    It's added as one of those arcane, bloody, directives. Eg. " .IC V(n003)=0 "
  10. Jim Thompson

    Jim Thompson Guest

    In PSpice you just place an IC marker on your schematic... "IC1"
    establishes initial conditions for a point relative to ground, "IC2"
    establishes initial conditions between two points.

    Generally you can also establish initial conditions within the setups
    for capacitors and inductors.

    ...Jim Thompson
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