
A Christmas Tree with LEDs - gus_massa
https://gus-massa.blogspot.com/2015/01/a-late-christmas-tree-with-leds.html
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sjburt
Couldn't comment on the blog, but yes, you should have one resistor per LED.
The I-V curve for LEDs (which is looks like a cliff and has a flat region we
simplify as the "forward voltage drop") varies from component-to-component and
also with the temperature. Attaching them directly in parallel means that an
uneven amount of current will flow through each LED. Not only that, but the
LED with the most current will heat up, and its forward voltage will decrease,
causing even more current to flow through it. Depending on what equilibrium is
reached, this could result in that LED burning out (and the effect continuing
on other LEDs), although the more typical result is just one LED that is much
brighter than the others (since ultimately some steady-state will be
achieved).

With multiple resistors, the ID curve for each resistor-LED pair doesn't have
a flat region, but instead a sloped region, and the resulting currents will be
more or less equal.

