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'[EE] Luxeon LD driving'
2005\04\12@232021 by Russell McMahon

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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

2005\04\13@002839 by Josh Koffman

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Just a couple of things as I'm going to bed and currently pretty tired.

First, you will have to heatsink them, and heatsink them well. Don't
scrimp on this if you can. There are a bunch of app notes on Lumileds
website about this. Or you can pay for the course they teach.
Heatsinking is harder than you think, especially when sharing a
heatsink over multiple LEDs as the slug is not connected directly to
either terminal, it's somewhere in between, so you need to isolate
each LED electrically from the heatsink while still not impeding the
thermal transfer. We use a Loctite product for this (317 I believe).

Trying to match LEDs is tough...we spent a lot of time on this. I
can't tell you exactly what we do, but it's mostly in the software. We
measure a representative from each batch of LEDs we get in and make
calculations based on that. If your volume is the right size, try to
get specific binned LEDs, it will make your job easier.

Ok, that's it for now, off to sleep. If you have any more questions,
feel free to ask.

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

On 4/12/05, Russell McMahon <spam_OUTapptechTakeThisOuTspamparadise.net.nz> wrote:
{Quote hidden}

2005\04\13@041624 by Dominic Stratten

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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 piece
of aluminium about 70mm x 20mm is more than adequate (handy as this is the
mounting bracket I use for my application anyway) with temperature rises at
room temperature of only around 15-20 degrees. I tested these initially on
the bench with a small vaned TO220 clip on heatsink and at 300ma drive it
just about coped.

Don’t 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 removed}

2005\04\13@062214 by Brent Brown

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> 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?

OK, for heatsinking 1W Luxeon's I've done the following. Used 0.2mm thick
FR4 PCB. Surface mounted the LED's, but drilled a 6.5mm hole for the slug
to fit into/through. Put heatsink paste on the back of the slug, press the PCB
against anodized aluminium heatsink. The anodizing gives the required
electrical insulation.

In my case the aluminium was a round tube about 70mm diameter, and the
PCB wrapped around it no worries. 18 x 1W Luxeon's, heatsinking worked
great! The PCB house I used (Capital Circuits) had no trouble producing
single sided PCB's of 0.1 or 0.2mm thickness as these are the sizes they use
to laminate multilayer boards, but can't do through hole plating on them
though.

--
Brent Brown, Electronic Design Solutions
16 English Street, Hamilton 2001, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell/txt: 025 334 069
eMail:  .....brent.brownKILLspamspam@spam@clear.net.nz


2005\04\13@120515 by Harold Hallikainen

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In my application, we're running a 5W "Dental Blue" LED with good heat
sinking at 10W (1.5A, 7V). The ME on the project did a lot of analysis on
temperature rise, etc., and said we could run it at 1.5A. The LED is glued
to a copper heat sink with silver epoxy. The fan in the device runs at low
speed unless the heat sink temperature gets above 60C, when it goes to
full speed. Part of the production test is to make watch the heat sink
temperature rise. If it does not go up as fast as expected, they redo the
adhesive holding the LED to the heat sink.

In this application, we only need to turn the LED on and off. We're using
an LT boost converter with a Maxim current sense chip providing feedback
to regulate the output to 1.5A. There's also a zener between the output
and the LT feedback input to pull the feedback input up if the voltage
gets too high (like an open LED). The whole thing runs off a couple
Lithium ion cells in parallel.

Harold

--
FCC Rules Updated Daily at http://www.hallikainen.com

2005\04\13@134443 by Andre Abelian

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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
  http://www.sipex.com  

Andre Abelian




{Original Message removed}

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