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On Wed, Nov 14, 2007 at 10:17:06PM -0500, Spehro Pefhany wrote:
> >Any suggestions for the boost regulator? I've never designed a switch
> >mode regulator circuit before, something simple would be good.
> >
> >If you got any better ideas, do tell!
> 
> A really simple current regulator suitable for this purpose would be a
> BJT with base connected to 5V and a fixed emitter resistor. Eg. for
> 15mA, use about 300 ohms. You can switch the base between 5V and 0V
> for the PWM. It will have a slight positive temperature coefficient,
> (dI/dT ~= 0.04%/K) which may or may not compensate for LED brightness
> variations with  temperature, but small percentage changes in current are
> generally not very visible. The power dissipated in the resistor is
> 4.3 * Iled; the power dissipated in the transistor is Iled*
> (Vsupply - Vled - 4.3V).

So essentially the base is set at a fixed voltage, I built it with 3.3V
actually, but anyway if current is zero, there is no voltage drop on the
resistor and the transistor is fully on, with 33ma flowing through the
transistor.

As the voltage current increases the voltage drop accross the resistor
increases, thereby lowering the effective voltage seen on the base
relative to the emitter. Once the voltage drop gets to 2.6V, the
transistor starts turning off, maintaining a level equilibrium as the
voltage continues to rise.

         V+
         |
        LED
         |
	 C
3.3V---B<
         E
	 |
         R
         |
        GND

'Cause I built one, and it seems to do all sorts of weird and wonderfull
things only somewhat related to current limiting... Not sure if I'm
understanding it correctly.

- -- 
http://petertodd.org
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