A couple of Luxeon questions...

I am trying to change the LED on my L4 and I have a couple of questions:
- Is it safe to remove the emitter from a Luxeon star (I just found out that the L4 only needs the emitter) or is there the risk of overheating it?
- Am I wrong or the emitter only and the star have different anode/catode terminals?
- Do I need any thermal glue between the emitter and the sink?

Thanks everybody for the help! /ubbthreads/images/graemlins/bowdown.gif

1) Yes it is safe to remove the emitter. In fact it allows better heat conduction because there is one less junction.

2) The emitter of course has two tabs to solder wires onto -- positive and negative. The positive end has a longer tab than the negative end. This allows you to distinguish between the two.

3) Yes Thermal glue will help greatly. I'd recommend Artic Alumina.

Luke,

Thanks a lot!
Can you tell me if the emitter is actually glued to the star and if so should I take any particular precautions in removing it?

Surefire doesn't actually use thermal epoxy to mount the emitter. I think they use Arctic silver. It has better thermal conductivity than thermal epoxies. I tend to not use epoxy wherever I can get away with it because of this. You do need to make sure you minimize the gap between the LED and the mounting surface when using only thermal compound. Any air gap will degrade heat transfer. The big advantage of the epoxy is that you can let it set undisturbed which tends to minimize the LED moving around while you solder it in place.

CM

CM -- I'm not familiar with the L4: how do they hold the emitter on?

As far as removing the emitter goes:
Grab a toothpick and your soldering iron. First you need to desolder the leads on the emitter from the Star board. Press the hot iron tip onto one of the leads until it starts bubbling while you simultaneously pry (not too hard now, you don't want to break it off) the lead from the star board. Repeat with the other lead. Heat up the bottom of the star until it is just warm to the touch. Grab two pairs of pliers and gently twist the star board until the emitter 'pops' off.

CM:
Yes I can see a problem in centering the emitter (and keeping it as close as possible to the sink) just by soldering the leads; epoxy would probably allow a more precise alignement but I see your point on heat dissipation.

Luke:
Thanks for the info on emitter removal, I guess that the glue used by Lumileds must be epoxy, does it leave residues on the emitter?

Thanks again

It rarely leaves much if any. You might clean off the bottom of the emitter with alcohol and a q-tip if you want.
As far as using Thermal epoxy goes:
It will be able to transfer heat just fine if there's not a more natural way. All of the extremely high powered LED mods (SNII, Megaclops) all have emitters that are AA'd to the heatsink.

I guess I will try first with AA and if I do not succede in centering and/or keeping a low gap I will try with the epoxy.

Thanks again for the help

lapping the bottom of the emitter after removing it off the star is a very good idea. remember, the smoother you get it, the better it will transfer heat.
i always lap and polish all my emitters, whether from a star or not /ubbthreads/images/graemlins/grin.gif

I've only popped an emitter off once where it WASN'T still covered in epoxy. I usually have to scrape the adhesive off the slug with a knife (and dig the bits from the little channel between the slug and plastic housing)

Where does one buy Artic Alumina?

the best place is the sandwhich shop /ubbthreads/images/graemlins/grin.gif

Thanks again. CPF to the rescue!!!!!

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LukeK said:
CM -- I'm not familiar with the L4: how do they hold the emitter on?...

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The emitter is held in place by the solder connection. At least that's the case with three KL4's I've disassembled. I was surprised they didn't epoxy but they were probably trying to maximize heat transfer since the KL4 is small and generates a lot of heat.

CM

Rothandir,
May I ask you what you use to polish the slug?
I was thinking of building a little clamp to hold the emitter in place while soldering, do you think it could be helpful?

Thanks

I was just reading the assembly instructions for the Mclux where it is indicated to glue the emitter with a mix of 50% AA and 50% epoxy.
This could be a decent compromise but will it guarantee a sufficient termal transfer for a 5W LED? /ubbthreads/images/graemlins/thinking.gif

[ QUOTE ]
alexgian said:
I was just reading the assembly instructions for the Mclux where it is indicated to glue the emitter with a mix of 50% AA and 50% epoxy.
This could be a decent compromise but will it guarantee a sufficient termal transfer for a 5W LED? /ubbthreads/images/graemlins/thinking.gif


[/ QUOTE ]

Another option that I prefer is to use the AA [or your other favorite thermal compound] under the metal slug of the emitter, and a few very small dabs of epoxy under the black plastic outer ring of the emitter. For both the AA and the epoxy, try to use the minimum amount possible. The thinner the joint, the better the thermal transfer.

That's a great idea Doug, I'll try that as soon as my hotlips arive /ubbthreads/images/graemlins/thumbsup.gif