# Re: NST power rating -- another perspective

```Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>

> Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
>
> To see that the general expression doesn't work, try to compute the
> power when Vsec(s)=V/s, a step function (or DC), and L=0:
> PL=(V/s)^2*RL/(R+RL)^2
> The inverse Laplace transform of this is a ramp, not a constant as it
> should be.
> Note that to obtain the correct value making s=jw you have to consider
> Vsec as a constant, a phasor, not the Laplace transform of Vsec(s)
> when s=jw. A product of Laplace transforms is a convolution in time,
> not a product. Power is a nonlinear function of the voltage, and can't
> be calculated directly from these transforms, although, as you observed,
> some simple tricks give the correct answers.
>
> Antonio Carlos M. de Queiroz

The more I think about what you say, the more I think that my thinking may
only be correct at a constant frequency.  This is how I use these
transforms.

Gerry R.
Ft Collins, CO

```