how do i make a constant power load in LTspice?

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

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

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.

Here's the link to my blog post, "Simple Constant Power Loads in LTspice":
http://russellsrandomthoughts.blogsp...n-ltspice.html