I'm trying to build a model lighthouse (don't ask). :-)
Essentially I'm trying to have an über-bright LED with a motor rotating at 2rpm, both powered from the mains (120V AC). The problem I am having is overheating...
I have constructed the following circuit:
These are the components I am using:
Power source: 12V DC power adapter, 1A
LED: Cree 10W high-power, 2.9-4V forward voltage, 3000mA forward current, 10W power dissipation, 2.5°C/W thermal resistance
Resistors: 1x 3ohm 25W (R1) and 1x 3.3ohm 50W (R2)
The motor part works wonderfully. I can adjust the speed down to 2rpm and everything is good. The LED on the other hand is causing a headache. Initially, I had calculated a 3ohm resistor (R1) based on (12V-3V)/3A. But this caused the LED to flash intermittently - I assumed overloaded - so I bought a slightly larger R2 (3.3ohm), thinking maybe the voltage across the LED was 2.9 and so I should go for something a little larger. Still blinking. On a whim, I put them in series (as in the diagram), and then it stops flashing and everything works as it should.
However, the LED is getting VERY hot, as are the two resistors. I don't run the circuit for longer than 5 minutes because I'm worried about it burning out. The LED comes pre-mounted on a flat heatsink disk, and I could attach a slightly larger heatsink to that, but I'm limited by the lighthouse design and therefore would really like it to get less hot.
The resistor power should, I thought, be 3A*(12V-2.9V)=27.3W.
I feel I must be miscalculating what kind of resistor I should be using. Or maybe there's something I'm missing entirely wrong with the circuit. Any thoughts on this and/or ideas as to how to stop this LED circuit from self-incinerating would be hugely appreciated.
Thank you all!