Thanks for all the responses, I will try and answer them in turn, since they are all valuable information. I had a MUCH longer reply before, and my computer keeps getting the BSOD after plugging in a USB card reader... so it'll be a bit more brief this time.
First some pics of my project... I know it's a bit "rough" but given the challenges, it's totally functional at least... now just to get the control part down.
The SunLED Apollo chips are PLCC-4 and require the cathode/anode to be heatsinked... basically they rely on a metal core board, but if you don't have that, you have to find a slug of coppy to mount them on, and then electrically isolate that from a bigger heatsink. Hence the pink sil pads... it was a challenge to solder them on. I had to seperate the copper slugs (made from copper tubing flattened and sanded) from the aluminum heatsink, otherwise I would have never been able to solder anything onto the copper slugs.
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I'm not too worried about running 2 in parallel since Lumileds specifically reccomends that as there really isn't any alternative with a limited Voltage Supply... There are LED drivers which fix this, but I'm not quite to that point... wanted to do something on my own anyways.
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So running strings in parallel with ladders across every few LEDs will help equalize the differences in forward voltage drop. I think my creation is something like figure C. Lumileds PDF is here:
https://www.electro-tech-online.com/custompdfs/2006/12/AB20-3.pdf
The SunLED Apollo chip is a 2.5v Vf at 350mA... So 4 in series is 10v Vf-total. And 3v for the LM317. I measured 14.5v at the battery with the motorcycle engine running, so I should be ok, no? I measured the voltage drop across the LEDs, and they are almost always 2.3-2.4v on spot...
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I think my problem is that the LM317 is only rated for 1.5amps... Am I overpowering this chip? I figured I was using 700mA for the LEDs, and maybe some overhead for the internal resistance of the chip, but still under 1.5mA?
Now my test Power supply is only 11.5v @ 5amps so that may be sagging a bit. I have a 16v @ 7.5v PSU I will try out to see if maybe that was the problem...
Part of my problem is just the limited space, and electrically isolating, while also heatsinking them... Tapping the aluminum block was a pain too... So I don't really want to redo everything if I can find a solution.
Now it says I can run the LEDs at 1/10 duty cycle for .1mS... I will have to test this out with my PWM kit I got... Could try that...
I will have to test the logarithmic scale of vision as well, probably do like 50mA and 350mA or 100 and 700mA something in that range. Brake lights are supposed to be in the 7:1 brightness range, so I'm sure the math doesn't work out precisely, but 100:700 should work. But when driving them with such low current they tend to become orangish not red... so PWM is necessary I think to get that low of brightness...
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I know I missed some comments, but this is the 3rd time writing up a long post, and am brain dead, and need to tinker a while to resolve some of the questions you all have brought up. Thanks again, cheers