Russell,

At the moment I also work on luxeon led project
a few things are very important to know about power leds
when they get hot they loose their brightness.

1. isolate the led with thermal epoxy
2. control current (not voltage)
3. leds must be hand soldered 
4. make hole on pcb then from bottom add thermal epoxy then add 
   aluminum plate or your heat sink
5. there are ready made drivers available for $1 TO220 package
   www.sipex.com   

Andre Abelian




-----Original Message-----
From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf
Of Russell McMahon
Sent: Tuesday, April 12, 2005 8:18 PM
To: PIC List
Subject: [EE] Luxeon LD driving

I'm about to implement a small essentially trivial project that uses 
Luxeon 1 watt and 3 watt LED modules. Basically a lamp driver for a 
multi colour bubble tube with extra features. Colours will be able to 
fade to variable depths, flash etc.

I have no difficulty in reading data sheets and app notes and don't 
anticipate any great problems with these BUT

Has anyone got any useful practical input to add. Any thoughts on 
heatsinking, PWM behaviour, linearity of output with PWM %, things 
they have found that others may want to know?

I thought about setting variable maximum PWM LED duty cycles for each 
colour so that I could adjust relative brightness of the colours in 
software (as the various emitters have differing optical efficiencies 
but I'm also thinking that having hardware variable constant current 
control per colour would allow ease of relative adjustment. The cost 
and complexity of a few LM317's or LM350's is of no great importance 
in this very limited volume product.

This is a disability aid for use in a "multisensory room". As well as 
the normal smooth transitions between colours I'm going to add some 
"special effects". Whether each or any of these prove to be a good 
idea will be determined by the professionals involved.



            Russell McMahon 

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