> You can't start learning to EE without learning Ohm's low first. The
> discussion is senseless.

> Why didn't you learn Electricity at school? You seem to be more than
> capable to get it. Language problem at that age for the immigration?

His major educational field is in the biological sciences. Electronics
is a hobby unrelated to his school curriculum.

That said, Ohm's law is part of the "MRS GREN" of the electrical
world. It is so fundamental to everything that you do that it MUST be
learned to the stage where you are comfortable with the implications
in real world situations without having to go and look things up.

The original question would have benefited from a simple Ohms law
analysis - most of the very useful answers that people have given are
essentially conveying information that would be intuitively obvious
given a basic understanding of the applications of Ohms law in the
real world, and even an hour or less playing with a few cheap meters
and some LEDs and resistors and a power supply. Such an investment in
'basic research' will not only help you to ask better questions and
not attract such adverse comment, but will also - probably more
importantly - provide you with some Eureka moments that can't be
easily replaced by having people tell you stuff. The non linear V/I
relationship of any LED, the reason that LEDs are best current-driven,
the point at which they emit coloured smoke, or stop emitting light,
the interesting results of reverse bias at about zero current as
voltage is increased, and much more, will embed itself usefully in
your brain far more easily, and with a far greater sense of
satisfaction, than it will by getting answers to very basic questions
from other people.

Try it, you'll like it.

PS: Ohm's law always works, by definition. ie I = V/R. R = V/I. V = IR
ie it's essentially a tautology.
BUT as noted - for some substances the ratio of V/I (=R) changes with
V and or I. R can also changes with temperature, pressure, exposure to
EM field etc. For most purposes I = V/R is valid enough to be useful.

For semiconductor devices the ratio of V/I can change inmmensely with
V. Which is exactly what happens in the LED case being discussed on
this thread. Aty say 20 mA an LED may have a Vf of 3.0V, so R = V/I =
150R. However, at 30 mA Vf may be 3.1V. And at 100 mA Vf may be 3.3V.
Giving R's of 103 R and 33 R. Clearly R is VERY dependant on V. We
don't usually think about this example in these erms, but doing so
demonstrates that Ohms law, as it is usually understood, can be easily
'broken' in everyday life.




           Russell.
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