On 2007-06-23, Friday (aka Bruce)
was almost, but not quite, entirely unlike tea:
> TimC wrote:
>> http://michaelcarden.net/blog/index.php?p=76
>>
>
> Nice.
>
> Are you/he going to power the LEDs directly or run them via a current
> regulator? Also, if you run them directly, why can't you use the LEDs
> back to back and skip the Shottky diodes, is there a problem with
> reverse biasing LEDs?
It sounded like he'd continue the article when he actually finished
them.
Normal LEDs break down at -5V or so -- they act as a zener diode and
pass the full current available to them when reverse biased, but I
have no idea about white CREE LEDs. In the case of an unrectified
alternator system, as long as no current is being drawn, the voltage
becomes very high. So initially, when voltage is reversing, no
current flows. Then when the voltage increases in response to the
lack of current flow and as the generating cycle goes on, it will get
above -5V, and the LED will start conducting (and not emitting useful
light as a result of this) as a reverse biased diode junction.
I can't tell what will happen then with such a sinewave current supply
-- I guess you'll end with a forward biased device with a 3V drop
across it (3V typically for a white LED), and the necessary current
flow to give 3 watts (since that what these generators supply). So
your 3 watt LED will supply 3 watts of light. But for the other half
of the cycle, as the diode is reverse biased you'll be sinking the
same current, but the voltage across the diode will now be the 5V or
so that the diode drops when reverse biased. No useful light out of
this, excess energy usage, and much excess heat generation in both the
source and the sink.
You'll probably end up with something referred to in the industry as a
semidestructor, a Dark Emmitting Diode (DED), a friode or a reversible
diode. And then you'll have to gather up all the magic smoke it lost,
and stuff it back into the DED before it will become a LED again.
-- Ti "magic/more magic" mC.