Re: Digital Current Control for LED array

On Fri, 23 Dec 2005 20:46:07 GMT, "John_H" <johnhandwork@xxxxxxxx>
wrote:

>"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
>news:j8loq15pka1b4c7e3ro8au0njn01q2bhsp@xxxxxxxxxx
><snip>
>> Yes, that's why toward the end of the article I wrote:
>>
>> "So, under those conditions it looks like what's needed is about a
>> 15 ohm 15 watt adjustable power resistor."
>>
>> What I was thinking was along the lines of an adjustable vitreous
>> enamelled wirewound reesistor like a an Ohmite Type 210 or a
>> Huntington AVT, with a Huntington 25 ohm 50 watt unit available from
>> Digi-Key for $4.50 qty 1.
>>
>>>If the passive
>>>approach were used, a combination of fixed resistors for a set point along
>>>with more reasonable potentiometers might be warranted.
>>
>> I agree, but I think that using a single adjustable fairly
>> high-power resistor instead of a rheostat and an assortment of fixed
>> resistors per LED would be even cheaper. Hard to beat $4.50 per
>> LED, no?
>>
>> --
>> John Fields
>> Professional Circuit Designer
>
>It's easy to get under $4.50 an LED and have efficiencies significantly
>higher than the 30% you propose (10V supply, 3V load). The MOSFETs are
>about $0.50 each, the 4-channel opamp is under $2 ($0.50 per channel), the
>discrete resistors are negligible, and the cheap pots are down to $0.25
>each. It isn't just a nest of wires but the supply can be made to
>accommodate a very small dropout voltage for very high efficiencies. The
>solution is elegant rather than brute force. I'd hate to throw away 50W of
>power for 21W of light.
>
>Most folks designing with the high-power Luxeon LumiLEDs are interested in
>efficiency above convenience; otherwise incandescent lamps would be fine.

---
My intent when posting the article wasn't to describe an efficient
system, it was to give the Jackal something to chew on. I,
personally, have no particular interest in high-powered LEDs, but if
you do, go for it! And good luck!


--
John Fields
Professional Circuit Designer
.

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