Hi all, this is my first time posting so please be gentle. I’m designing an item that has LED lights incorporated into it. These need to be as bright as possible while being as small as possible. I am thinking of using 3-4 Cree XM-L2’s. However my question is in the design. I want the dome of the led to fit in a hole (Diameter of the LED) cut in a plastic sheet approx 3mm thick which will then be flush (or as close to as possible). This will then have a backing plate (foam or metal) to seal in the pcm and wiring.
As I’m new to all this can this even happen?
Do these LED’s need space for heat to transfer/circulate?
Will these LED’s be too hot to use next to plastic?
Do they need to be surrounded by metal to transfer the heat?
If this is not possible are there bright LED’s out there that can be close to plastic?
Any help would be great thanks
Hi Alex,
To get anywhere near the output the XM-L2 series is capable of producing (or any high output LED, for that matter), they will need heatsinking. Foam would be very bad. Putting plastic on the underside would not be advisable, as they will be very hot there. Putting it around the sides may be acceptable depending on how well heat is managed on the underside, where most of it will be generated.
The test conditions for the XM-L2, according to the datasheet, is 2.77V at 0.700A, which calculates to an input of 1.94W. No more than 50% of that will be converted to light and the rest will create heat which must be properly dissipated so as to not damage the LED. Heat is the enemy of LEDs. A metal surface will be essential, and the harder you drive them, the larger that area must be. Finned heatsinks are typically used, especially at the higher drive levels.
The die temperature must always be maintained below 150°C for the XM-L2, and if you want it to last any length of time, it should be kept well below that. So you need a metal heatsink and available airflow to effectively drive these to more than a small fraction of their potential output.
I haven’t discussed soldering issues with these. It requires a special board and cannot be soldered with a soldering iron. I would recommend you take a look at, and experiment with some of our similar LEDs which are already mounted on metal-clad “starboards”, such as these:
Starboard LEDs
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Hi David,
Your advice is most grateful and very detailed. Many thanks. I’ve watched a few videos of soldering and that doesn’t put me off but your suggested choice would be better. However these LED’s look problematic for my design.
So it’s back to the drawing board for me then. Do you know if any lights would be suitable for this design format? I could definitely use an aluminium back plate for some conduction. I’m wanting more light for performance, however this would have to be reduced to maintain the design.
Many thanks
Alex
Do you have any targets for lumens per LED? What is a minimum acceptable value, 10 lumens, 50, 150? If I knew that, it would help in looking for possible options.
Do you plan on making your own circuit boards onto which these will be soldered? If so, they will need to be very thermally conductive in order to pass heat back to the aluminum back plate. Ideally, you would use a metal clad board, though I have seen a few designs where a thin FR4 board was used with LOTS of thermal vias to the back side of the board to aid in thermal transfer.
David, the truth is I don’t really know about the lumens. If you haven’t guessed it by now my background knowledge and designs are in the early stages! 
Basically I’m trying to design something that incorporates a powerful headlight into a plastic item. The headlight I currently use has 3 LED bulbs (apparently they are Cree T6 or T8 but it’s a cheap Chinese product so I assume it’s not Cree). So I’m going to say I would like 500 lumen. I have not done any practical prototypes yet to see what LED’s produce the best light for the design. So really I’m after an LED which does not generate loads of heat.
LED strip lights might work? I assume these do not generate as much heat. The lumens is very low but I suppose I need to test it.
With the circuit boards I would be buying one as I do not have a clue where to start on that.
My research has gone to:
- Plastic product to modify
- LED BULB
- PCB
- Driver
- Circuit board
- Power supply
The actual details of these points 2-6 I do not know (loose ideas). I was finding it hard to get this info hence going to forums.
Apologies I’m about to go into detail now!!!
I want the light source to be powerful enough to illuminate a glow in the dark ball but to make the ball really bright. I know a simple low lumen bulb will illuminate any glow in the dark item but I wanted a high level of brightness. Also the reason for this is because the brighter the lumen the bigger the effect on the whole ball. For example if I held my phone LED light up to the ball and held it there for 30ish seconds the ball is only illuminated where the light was leaving the rest of the ball dark. If the light produces enough lumens you don’t have to keep moving the ball; it’s powerful enough to light the whole ball in 20seconds. I thought I could double this up as a torch when not illuminating the ball hence the higher powered LED’s, so as long as it’s bright enough for a good bright ball I’ll be happy. My idea is also to spread the light via 2-4 LED lights placed around a spherical design distributing the light effectively.
Any help would be appreciated
Thanks
Alex
