Easy, but tedious to replace all resistors on LED strip.

Chetan Bhargava
http://microz.blogspot.com


On Wed, Oct 30, 2013 at 5:30 PM, c h <cognitive.harmony@gmail.com> wrote:

> > > I'm driving 12V LED load
> >
> >
> > =3D LED strip - 3 LEDs in series + internal resistors.
> >
>
> The internal resistor can be replaced by a npn BJT transistor in
> common-collector
> setup:
> a)  the BJT collector to +12V rail,
> b)  the BJT emitter to "+" of the 3 LEDs chain;
> c) "-" of the 3 LEDs chain to ground;
> d) the BJT base to the Arduino's DAC (the DAC should output up to +12V
> rail, or some rail-to-rail amplifier is needed);
>
> The internal resistors normally takes 2 volts or so at full load, that
> gives 0.27W heat dissipation at 0.135A. At lower currents the voltage doe=
s
> not rise dramatically due to the nature of LEDs. So it looks safe to assu=
me
> that MAX power on the BJT would be about  0.3W even when the BJT is drive=
n
> by DAC, not PWM. With PWM it's not easy to satisfy RF regulations if the
> LED
> strips and wires are long.
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