CPU coolers... Thanks for the idea!
It isn't new but I could not rapidly locate the thread where I first saw them used in a bike light. It was a German cyclist.
"You have the LED-MCPCB (you'll have to have, if you don't direct mount, and you don't need or want to go that route), MCPCB-shim, and shim-heatsink interfaces. That adds only one more interface to the usual and gets you focussed properly, a fair trade off especailly considering the overkill heatsink size."
You kind of lost me on this part. What is an LED-MCPCB? and a MCPCB-shim?!
My bad. Acronyms are great once your'e familiar with the terms.
Let me back it up. I actually left out the greatest resistance in the thermal chain. The heat is created at the LED junction. So LED temperature limits talk of 'junction temperature'. The thermal resistance from the junction to the LED thermal pad is a feature of the LED and out of our control as buyers/users other than the recent LEDs have lower resistance and higher power capabilities as a result. I left it out because it is what it is for whatever LED you decide to use.
The next resistance is from the LED pad to the Metal Core Printed Circuit Board or MCPCB. These are often 'stars' of 20 mm or so size and are six sided with cutouts at the apexes for wire access, screws and optics alignment posts. They can be larger, smaller, square, or round, etc. On one side are the printed circuit and thermal pad in copper, with a mask often white covering all but the solder points (PCB). There is a layer below the copper to insulate it from the aluminum core (Metal Core). They can have one or more LEDs. The LED is soldered to the thermal pad and the + and - terminal pads. You have to electrically isolate the thermal pad in many LEDs from the negative, positive, or both. So it makes direct mounting of an LED to a heat sink a huge challenge for beginners and only worth it if you have a mess of heat to shed in a hurry like a 20 watt LED in a small flashlight. So most of us buy a 'star' or 'light engine' which is the LED premounted on its circuit board. The LED-MCPCB resistance can be affected by a poor solder job or poor MCPCB design, but a quality supplier like Cutter stands behind the products they sell. You can get 'cheaper' in both meanings of that term.
The Metal core comes in because that thermal pad needs to pass the heat and it needs to be spread out to aid transfer to the next part in the thermal path at that junction. Ideally, that would be the heatsink. But you are placing an LED in a reflector and will have to find the focal point or at least the place where the LED works best. I suggested a shim to project the LED into the light and have the big heatsink right behind. So you would have the interface between the LED and the MCPCB and another between the MCPCB and the shim (hence: the interfaces I labelled LED-MCPCB and a MCPCB-shim, these are not official technical terms but made up of the two parts either side of the junction, I picked that labeling method up somewhere).
Visit Cutter
site and look up CREE XP-G and see how many MCPCB's you can get with one, 3 , 4, 5, 6, or even 7!
I'm still not sure how I could get a couple of LEDs epoxied to a cpu cooler to look like a respectable bicycle light. I'm afraid it will end up looking hideous. haha. I guess I'll just dig up my cpu cooler and see what it looks like first, then start brainstorming how I could modify the looks.
If you want to adhere to a completely stock look, you are likely going to have a hard time. If there is a lot of depth behind the reflector and before the back of the light, you might sneak the cpu cooler with a bit of trimming in there and only need to get air by it. One modder used a forward air scoop on top and a rearward one on the bottom to do just that. If care is taken that any water entering that way can't get to the electricals, that would work. If there isn't room instide for the hidden sink method, then if you are OK with it looking stock from the front but it can look a bit techy from the side and rear you have some flexibility with a heat sink on the back.
If you are VERY lucky the back of the headlight shell is aluminum and not chromed steel. That would allow a circular LED mounting disc often called a 'firewall' the diameter of the shell where you want the firewall to sit. Again, you would shim the LEDs into the reflector likely needing a bigger access hole than the former bulb needed. If the light has a high and low beam bulbs, you can use both and a second firewall for the second LED. There is another classic bike light thread here showing one that uses small mounting firewalls one for each LED using the reflector as a sink but not extending to the shell. That would be fine if you have an aluminum reflector and shell. An aluminum reflector and shell will conduct the heat all over themselves and finally to the air. Steel can't do that fast enough. So an aluminum shell means you don't need fins outside the headlight. (note: if you have a high/lo beam 2 LED light, care must be taken switching them on and off as pulse currents can blow the LED. That part of a design will need someone else's help.)
You likely have steel. If so, only you can decide how retro-techno you can stand. I decided against this route for my bike a 30 year bike in the same idiom as Steve's Powers bike, refitted as a 3 x 10 speed:
DIY lights that don't look DIY unless you stare into the front, up close. You can see why a generator hub would suit this bike. Note the small black bullet shaped light is from
here. It's the second last item on the page. Another classic shape. They are $10 plus shipping. Aluminum with plastic end and mount piece. You can use a brake crown mount for the Schmidt lights or the handle bar quick release mount on the same page with the bullet light. Don't forget plastic mounting bolt, glass lens in case you can't lift the one off the bulb without breaking it, and anything else you think you'll need so you can save a double mailing.
I also used two for the tailights so you can see them from another couple of angles:
Hope this helps.
. Here is a picture of the light housing.
