(excuse any typos, half-blind from playing with this..)
So, picked up a 1w Luxeon for cheap money for giggles. White, throws an insane amount of light. Now I've been doing reading on the best setup with these (ideally a constant-current source with internal feedback), and then immediately doing the exact opposite, as is my usual experimentation method (I love inhaling vaporized doping materials and plastic).. 2 AAA batteries and a 2 ohm resistor later, I'm happily blinding myself. I know, I'm underdriving it (3.17v from batteries, with an LED Vf of 3.42v, hehe.. tolerances? specs? Humbug!)
What occurred to me is that the whole current-limiting thing is to prevent overheating and thermal runaway.. the limiting factor is the heat being generated... in other words, it could probably take a LOT more than 350ma spec, if cooling were taken into account..
So, anyway, if my desired application for the Luxeon is basically a strobe light whose duration will be measured in milliseconds... there's little time for heat issues.
That being the case, how high a drive current do you think a 1w luxeon could take, assuming the Vf is run at rating, 3.42v? Of course it will be out of spec. Then, carry that forward, and what do you think the current/lumen capacity might be... especially if the strobe pulse is quite short or managed via PWM?
it could probably take a LOT more than 350ma spec, if cooling were taken into account
No that is with cooling and an infinite heat sink the limit is from the thermal resistance between the chip and the case.
how high a drive current do you think a 1w luxeon could take,
The data sheet should give you a pulsed current rating at a specific on / off ratio. Typically is X10 for a 10:1 ratio. You can do more that a data sheet says but then you run into reliability issues, it might fail after say 5 or 6 months of being operated like this. Again the data sheet should have some absolute maximum values.
I say should but a lot of LEDs are from China and they just don't do a proper job so you are on your own.
Well, Phillips carries the lumen data out to 800ma, only slightly over twice the rated current; and that's based upon full-time illumination (with active cooling via fan and heatsink). The lumen output stays fairly linear, though I'm sure it jumps the cliff at some point..
This is mainly a conjecture thing over a cup of java at the moment, might do a little more research. It's not particularly practical of course, as we have both noted- the smoke test will the best determining factor, as the specs have been left on the side of the road.
Just occurs to me that if the desired illumination from the LED is intermittent (like my strobe thought) then the heat factor may be less of an issue and the LED might be able to handle very short, very high current pulses. Haven't seen any others having done this, but an interesting thought experiment that may need to see a breadboard..
What I have in mind is an LED "lightning" strobe (maybe even with a sound ouput.. but I digress) for the front of the house for halloween. I've kinda decided my yard will be animatronic this year if I can pull it off, and using Arduinos to do it if possible, even if it doesn't make a lot of sense to do so. Sort of an Everest "because it's there" thing, I think.. we'll see if I actually bring any of it to fruition.. (another pet project, I've taken to "possessing" a discarded Furby toy as a concept at some point.. though a Teddy Ruxpin would be better... I'm committed to the idea of reverse engineering one of these toys and making it as demonic as possible)
The problem you get with high current as well as the heat is the mechanical stress on the very thin wires going into the chip, caused by the magnetic field created. This is often more damaging than the thermal stress. The other factor is that high current depletes the semiconductor of majority carriers causing an increase in resistance.
Wowsa, wouldn't have considered that as a point of failure, mechanical torsion from induced field.. now that's my kind of failure. The math is out there, I'm sure.. that strong a field without a coil, not something you'd normally think of right off. To be clear, I won't be crushed if the Luxeon doesn't survive more than a few hours of operation.. 3-4 hours is all I really need for the kiddies.
So as I diddle with this, I've found a TIP120 lying around and will probably use that for control... somehow I don't think the little 2222's are gonna take this kind of abuse..
wouldn't have considered that as a point of failure, mechanical torsion from induced field
So you never did the student experiment of powering up a 1KW motor and seeing the cable laying on the floor get bushed apart when the current was switched on. As well as the cables the alert students jump as well. The thick ones don't notice. ;D
it could probably take a LOT more than 350ma spec, if cooling were taken into account..
The short answer is that cooling is already taken into account in order to permit the 350mA spec; you can't operate at the 1W level without a moderate heatsink. You might be able to get SOME higher than that with a really good heatsink, but it's the semiconductor junction that has to be kept cool, and at some point the thermal conductivity between the LED die at the package IT is in becomes the limiting factor. LEDs designed for power greater than 1W tend to start getting really exotic packaging, and require careful attention to additional heat-sinking as well...
At least the module I got is mounted on what appears to be an aluminum slug- solder tabs for the LED on pads, etc.. I understand this is a common configuration for these power LED's. I'd like to think there's some type of sinking compound there, rather than just drawing the heat off the leads.. I have some sinks around from various things, one I reclaimed from a car audio amp which ought to be able to dissipate a heck of a lot of heat.. if heat really is an issue, I'll just slap the slug to that beast with some compound... I haven't yet gone as far as considering chilling the whole diode. I was going to say that would be overkill, but the whole idea is overkill from the beginning, so to speak, anyway. Note my use of the word "yet". I've used circulated baby oil as a coolant before... fishtank bubbler+airstone+mineral oil= nonconductive but really dang good heat drawoff from just about anything.... Yes, I know it's a terrible kludge, but that's the kind of foolishness I find the most amusing... but I doubt I'll go as far as milling off the plastic to get close thermal contact... though that's an idea too...
Of course I could just go and get a 5w or 10w (or more) module- but that's too easy. I mean, I'm a shutterbug... I have two studio xenon flashes and a couple of old second-hand-store flashes I use for fill, or I could build a xenon strobe trigger, or "repurpose" one from a disposable camera (great source for em, by the way)... but that's almost too easy. I'm digging the Arduino because it allows me to either take a simple idea and make it needlessly complex, or simply come up with my own ways of doing whatever someone else has already done more cheaply, efficiently, and with substantially less grief.. that IS why we're here, at the base of it, IMO.... lol. I'm learning more about electronics NOW, because I want to fiddle with particular things, than I EVER did during actual education on the subject matter, a few decades back. In this respect, both as an educational tool and just as a very flexible "toy".. it's amazing. Combine that with some of you that really DO have the hardware expertise and a friendly forum like this.. well, suffice to say that I spend time tossing around and prototyping creative (or just plain silly) ideas rather than playing "Left 4 Dead 2" as a timekiller... and that can't be a bad thing. Being a stay-at-home dad has a high "Brain Rot" quotient.
A "good" solution can most likely be bought... probably cheaper and better than I'll ever make... but I think (at least in my case) that the win really IS found in the journey, and not the destination. My first project (and even started a blog on it) was a camera controller- been done many times- "camera axe" is one that's pretty impressive, not far from what I envisioned for my own design. My own design, which became a HDR trigger and such, used many of the same ideas. The thing is, it had been a long time since I actually USED ohm's law. I had to dig around and remember and re-learn voltage dividers and amplifiers and optoisolation.. after a VERY long time since I used any of that. The time, cost, parts that went into it- not huge, but the benefit far outweighs the "overpaying" for a camera trigger, in my mind...
