I never said it won't work. You are dealing with 20 ma LEDs with a large
(390 ohm) current limiter. I am talking about high brightness LEDs for
lighting applications usually operating at hundreds of miliamps. Don't
jump on a conclusion with a wrong experiment. Your 390 ohm resistor takes
the majority of the voltage (8 volt). Of course you are not going to burn
out your LEDs. If one LED shorts out the remaining LED will increase its
operating current from .02A to .024A. Try two 3 watt LEDs with a resistor
of single digit value. Set LED current to 0.7A. Then short out one LED.

David

--- Mike S <mikes@FLATSURFACE.COM> wrote:

> This is for David Kao, who asked for help in running a simple
> electrical experiment with 2 LEDs. I used a 12V regulated source.
>
> (12V - (2 x 2.1V)) / 0.02A = 390 ohms.
>
> http://www.flatsurface.com/pics/2LEDs.jpg
>
> Works great! Neither is significantly brighter than the other, and
> neither blew up, despite dire predictions.
>

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