> The other challenge brings back something I asked on this 
> list a while back.
..


Why does it need to be inductorless (RFI?).
I'd say that this was a reasonably simple task to do if 
inductors were allowed.
Also feasible inductorless BUT with rather more effort.

Outline:

Buck regulator that gives programmed current to LED 
regardless of voltage + current control circuit based on 
input voltage. As Vin rises the desired current is raised 
and the buck current increases.

If Iled is dropped across a resistor and then compared to a 
reference, if the reference is derived by dividing the input 
voltage you get almost exactly what you want.

I'd design you one of these for the money offered BUT given 
the above description, you or someone else could do it 
easily enough.

___

Inductorless you can use a capacitive divider that charges 
caps in series and discharges them in parallel.
As 12V works OK, even using a single stage 2 cap system 
should do. The 'bottom' cap is always grounded. The "top" 
cap has its bottom driven by a totem pole betwixt cap1 + and 
ground. output to LED cct is provided only when the two caps 
are low (2 diodes and 1 FET or bjt).

You could follow this by an identical stage which would give 
Vin/4 - a bit out which should be a useful level to run the 
LED from.


>  I will even give a $200 reward for an idea I
> can use.

Hmm. Was that for the idea or an actual circuit? :-).
if the above so inspires you that you want to start waving $ 
around you could donate it to james for PICList support.
If not then, would you like an actual circuit ? :-)

Note that an inductor based cct would be easier.

        Russell






----- Original Message ----- 
From: "Brian Kraut" <brian.kraut@engalt.com>
To: "Microcontroller discussion list - Public." 
<piclist@mit.edu>
Sent: Tuesday, December 04, 2007 4:57 PM
Subject: [EE] Challenge II:: LED driver


> The other challenge brings back something I asked on this 
> list a while back.
> I did not get any responses I could use that time, but I 
> like the idea of
> presenting it as a challenge.  I will even give a $200 
> reward for an idea I
> can use.
>
> I manufacture a fiber optic instrument light for aircraft. 
> The illuminator
> can be seen at http://www.engalt.com/newpage2.htm.  It is 
> actually much
> smaller than that now.  The LED is a 100 mA LED.  I also 
> have a 150 mA LED I
> can use, but I am currently only driving the one I use at 
> 75 mA due to heat
> dissipation (in the dropping resistors, not the LED).  The 
> inside of the box
> is about 3/4" cubed.  I actually have less space to work 
> with than that
> because the gland that the fibers go into and the 
> connector takes up space
> in the enclosure.  The enclosure is potted with potting 
> compound.
>
> Here is my problem.  The LED is about a 2.5V LED.  I have 
> these units set up
> for either 12V or 24V input.  Actually, they run about 
> 13.5V or 26V in use.
> With the 12V unit I am dropping about 11V at 75mA which is 
> .825 W of heat I
> need to get rid of.  I use two dropping resistors now, one 
> on each leg of
> the LED and orient them near the top of the potting 
> compound.  That does a
> good job and does not get very warm.  24V is a completely 
> different story.
> That gives me 1.76 W of heat to get rid of and the unit 
> just overheats.  I
> really want to run the unit at the full 150mA brightness, 
> but that gives me
> over 3.5W and that is really too much.
>
> Heat sink is not an option.  External big resistor is how 
> I am having to do
> the 24V units now and that is a very bad solution.
>
> A switching regulator is the answer, but one catch is that 
> it has to be
> dimmable.  That is easy enough to do with off the shelf 
> parts now, but
> presents a problem in this unit.  I only have a positive 
> and negative input
> and can not add a third lead for dimming.  What I need is 
> something that can
> go in the place of a light bulb and work with the existing 
> dimming rheostat
> already in the plane.  I want full brightness about 13V 
> and dimmed to about
> nothing at around 3V.  All of the off the shelf driver ICs 
> are made to
> output a constant current with a varying voltage and have 
> a third input for
> dimming.  I need something that has the output current 
> varying linearly with
> the input voltage.  It needs to be inductorless, very 
> small and low parts
> count, and cost no more than a buck or three.  I can build 
> the boards with
> different value parts for the 12 or 24V units.  Ideal 
> would be something on
> a half inch square board or less with no more than one low 
> pin count IC and
> a few supporting parts.
>
> If you want to send me your ideas direct my email address 
> is
> brian@engalt.com.  If you think that you can offer a 
> complete solution but
> don't want to give it away for a few hundred bucks and 
> think no one else
> will either send me a proposal and we will talk.
>
> Brian Kraut
> Engineering Alternatives, Inc.
> www.engalt.com
>
>
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