I would disagree, Ruben. Ohm's Law is not a fundamental circuit law
(like Kirchoff's Voltage Law). It is an empirical law which says that
for most conductors, V=3DIR where R is a constant. If R is not a
constant (i.e., if it is a function of V or I), then what you are
doing is linearizing the non-linear V to I relationship about one
operating point. In that case, you really need to add more constants
to make Ohm's law useful: V=3D(I-Io)*R+Vo where Vo is the voltage drop
when I=3DIo. This is what small-signal analysis is all about. In a
general sense, the "resistance" of an LED, for example, has little
meaning. If anything, it is usually taken to mean the actual Ohmic
resistance of the wires and contacts which make up the LED, not the
ratio of the entire voltage drop to the current.

Sean


2009/7/6 Ruben J=F6nsson <ruben@pp.sbbs.se>:
> All currents flowing in a circuit follow ohm's law.
>
> However, as soon as the current is not a DC current and/or the load is no=
t a
> pure resistive value or changes with voltage, time, temperature or whatev=
er it
> becomes a bit more complex. To complicate matters even more, the load can=
 have
> inductive or capacitive components which makes the current, voltage and
> resistance (or rather resitance + reactance) to change when the frequency=
 is
> changed. The current actually also becomes out of phase with the voltage.
>
> /Ruben
>
>
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
> Ruben J=F6nsson
> AB Liros Electronic
> Box 9124, 200 39 Malm=F6, Sweden
> TEL INT +46 40142078
> FAX INT +46 40947388
> ruben@pp.sbbs.se
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
>
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