Re: Internal resistance

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Ludwik Kowalski wrote:

> The value of r was expected to be the same for
> all currents, in the range for which the power
> supply was designed.

Why should we expect that?  Ohm's law is not a
God-given law of nature.

> But it turned out to be

> different for different currents.

I expect it.

Maybe it's not regulated and is simply a transformer driven by a
"variac" followed by a full wave rectifier and cap. filter.  (not even a
choke input LC filter)

A 50% change is a bit extreme for a regulated supply.

I expect < 1%   Agilent claims mine 0.025%

Their new low cost series is 0.01% * +/- 2 mV.

L's cheapie is cheap, as in shoddy, but then may be it didn't cost $1080 (Mfg. list for E3632A - 15 V. 7 A.; 30 V. 4 A.)

I couldn't find a site for Lab-Volt P/s's

* Minimum for the repeller in a reflex klystron.

bc, who thinks L's P/S is perfect for a lab. exercise.  He could perhaps wire up a regulator (see Horowitz and Hill) to follow his P/S with it for more exercise.



John Denker wrote:

> Terminology point:  On the side of the tracks
> where I grew up, "resistance" means the small-signal
> resistance,
>
>      R := dV/dI
>
> as discussed at
>   http://www.av8n.com/physics/resistance.htm
>
> I am beginning to suspect that Ludwik might have
> been using some other definition of R.
>
> Ludwik Kowalski wrote:
>
>
> > The value of r was expected to be the same for
> > all currents, in the range for which the power
> > supply was designed.
>
> Why should we expect that?  Ohm's law is not a
> God-given law of nature.
>
> > But it turned out to be
>
>
> > different for different currents.
>
> I'm not surprised.
>
> > Nowhere in our
>
>
> > textbooks was this possibility mentioned.
>
> The textbook assumes that everything in the world
> is ohmic?  You need a better textbook, stat.
>
> > Many numerical problems would make no
>
>
> > sense if r were current-dependent.
>
> The real world would not make sense if everything
> were ohmic.
>
> > What is
>
>
> > wrong with testing this experimentally? What
> > is wrong with asking how r depends on I?
>
> Nothing wrong.  Excellent idea, actually.
>
>
>
> > How to explain the observed dependance?
>
> Draw the (I,V) curve.