how do i make a constant power load in LTspice?

Discussion in 'CAD' started by tomrei, Apr 14, 2009.

  1. tomrei

    tomrei Guest

    Hi there,

    this group looks pretty spamed, but i'll try my luck.

    does any one knows how to make a constant power load in the LTspice?

    i've tried a few things but they all involving using dynamic
    variables.

    maybe it's the question of how to use dynamic variable in LTspice.

    for example, the idea is:

    R1 N002 N001 100
    I1 N002 0 {current(p,V(n002))}
    V1 N001 0 10 Rser=1
    ..param p=10
    ..func current(power,voltage) {power/voltage}
    ..tran 0 1m 0 1u startup
    ..backanno
    ..end

    but this won't work since i can't just pass V(n002) as a parameter
    since it's a dynamic variable.

    is there a way i can fix this?

    many thanks

    Ren
     
    tomrei, Apr 14, 2009
    #1
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  2. tomrei

    Charlie E. Guest

    On Tue, 14 Apr 2009 04:36:27 -0700 (PDT), tomrei
    <> wrote:

    >Hi there,
    >
    >this group looks pretty spamed, but i'll try my luck.
    >
    >does any one knows how to make a constant power load in the LTspice?
    >
    >i've tried a few things but they all involving using dynamic
    >variables.
    >
    >maybe it's the question of how to use dynamic variable in LTspice.
    >
    >for example, the idea is:
    >
    >R1 N002 N001 100
    >I1 N002 0 {current(p,V(n002))}
    >V1 N001 0 10 Rser=1
    >.param p=10
    >.func current(power,voltage) {power/voltage}
    >.tran 0 1m 0 1u startup
    >.backanno
    >.end
    >
    >but this won't work since i can't just pass V(n002) as a parameter
    >since it's a dynamic variable.
    >
    >is there a way i can fix this?
    >
    >many thanks
    >
    >Ren
    >
    >


    Ren,
    Not exactly sure what you want to do, but it sounds like you want a
    resistance that always consumes the same amount of power, no matter
    the voltage across it. A little odd, but probably doable.

    In PSpice, you would use a Gvalue part, or a GTable part, where you
    connect the sense leads to the output leads. You then define the
    output (the current through the device) as I(out) = Vsense/Power

    Let me know if you have any questions on the actual execution.

    Charlie
     
    Charlie E., Apr 14, 2009
    #2
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  3. tomrei

    russwallac

    Joined:
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    I've come up with a couple of very easy ways to create constant power loads in LTspice. One is a resistor whose resistance is a function of the voltage across it, and the other is a voltage-dependent current source using a piece-wise linear approximation. Both are bounded to prevent convergence problems, and they work very well for most applications.

    The newsgroup software is blocking my attempts to post a link because I haven't posted here before, so I'll do a followup with the link in a couple of minutes.
     
    russwallac, Feb 28, 2010
    #3
  4. tomrei

    russwallac

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    russwallac, Feb 28, 2010
    #4
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