Hi Russell

I havent used PWM for driving the LEDs but use a constant current source
with switchable presets for the brightness output. I use a simple LM317 +
resistor for driving two of these in series with current limited to 300ma
max (min 100ma). The difference in light output to the naked eye between
300ma and 350ma is negligble but the heat produced by the difference is
significant in my application.

Heatsinking is an absolute must !!! I found that bolting each LED to a pi=
ece
of aluminium about 70mm x 20mm is more than adequate (handy as this is th=
e
mounting bracket I use for my application anyway) with temperature rises =
at
room temperature of only around 15-20 degrees. I tested these initially o=
n
the bench with a small vaned TO220 clip on heatsink and at 300ma drive it
just about coped.

Don=92t overdrive these though - I accidentally ran 400ma through one and=
 its
now a slightly different colour to the rest of the batch.

Dom

-----Original Message-----
From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf =
Of
Russell McMahon
Sent: 13 April 2005 04:18
To: PIC List
Subject: [EE] Luxeon LD driving

I'm about to implement a small essentially trivial project that uses Luxe=
on
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 h=
ave
found that others may want to know?

I thought about setting variable maximum PWM LED duty cycles for each col=
our
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 colou=
r
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=20

--
http://www.piclist.com PIC/SX FAQ & list archive View/change your members=
hip
options at http://mailman.mit.edu/mailman/listinfo/piclist

--
No virus found in this incoming message.
Checked by AVG Anti-Virus.
Version: 7.0.308 / Virus Database: 266.9.6 - Release Date: 11/04/2005
=20

--=20
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.308 / Virus Database: 266.9.7 - Release Date: 12/04/2005
=20


--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist