100C is not so bad for the surface temp on a 10W resistor. Look up the data sheet and check to see if it requires any power derating at higher temperatures. This is one place engineering comes into play. Nitpick the details.
I know it's been a while since I posted this, but I recently thought about this again and realized that I was wrong, it IS just 3V across the resistors, not all 12. Thus the resistors dissipate a total of about 9W on average. While technically a single 1 ohm 10 watt resistor would therefore work, even using four together, forming a 40 watt resistor, generated well over 100C on each resistor, so I would recommend either making a buck regulator as mentioned by a few others, or splitting it among multiple resistors.
Interesting posting, but I don't see enough details about mounting the LEDs. It6 would certainly work in a car,with the engine on the battery is at least 12.3 volts. And I have an idea to use an ultracapacitor and a smaller battery pack and just get a short flash. A solid copper computer CPU heatsink would be a good choice, I think that I have one of those. Just a second of light for each flash would be good.
Great post. I am truly gratified to find a student, not only interested in engineering, but one who is resourceful and obviously up to a challenge. Great work. The project is also very useful. I don't know if you wish to market this design and device but I certainly feel it would be a good candidate and the sales effort might be very interesting.
sorry I took so long to respond, I haven't checked this in a while. I could have used four LEDs, but the minimum voltage to keep up output is 3V, so if I run four, it would only work as long as its above 12V. A deep cycle battery will drop the voltage a bit when a load like that is applied, and though ohms law will drop the current going to them based on the resistors, the effect would be way more significant if each gets <3V.
As for a driver circuit, I could have, but I was pressed for time initally, as I had intended to bring it to a gadget competition at a camp I was attending. Unfortunatealy, the LEDs didn't ship in time, and I didn't see much of a need to redesign. Another light I made just a few days ago using two LEDs does use a driver circuit. As for calculating the resistors, each LED has a frop of 3V, and thus for each LED, just subtract 3V from 12V, then use ohms law for 3A. I ran three, thus a drop of 9V, leaving 3V. I needed 3A, thus 1ohm. two would leave 6V, thus needing 2ohms, and one would leave 9V, needing 3ohms four would leave 0V, thus 0ohms, but leaving the problems mentioned before.
As for the safety issue mentioned, most of that was precautionary. From more than a few inches it won't burn. Becasue it has a ~120 degree spread, the power will very rapidly dissipate, so it's NOT like a laser, but is powerful enough to distract or irritate someone from afar. Basically, I just meant don't shine it at cars or in peoples eyes. It, and almost all things should just be treated as though they are more dangerous than they really are.
And finially, does anyone have any advice on easy to build 3D printer extruders, specifically, something light that doen't require fabrication access (no milling, latheing (lathing?) 3D printed parts, etc.), just hand tools and such? I'm building a cheap 3D printer/wax CNC device, and the only problem I haven't fixed so far is the extruder, mainly becasue it will likely be fairly expensive (I'm looking for <$50, though), and thus I don't want to experiment more than I have to, as that will get expensive fast.
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