On Tue, 10 Apr 2007 09:19:44 -0700 Jim Cathey <[EMAIL PROTECTED]> wrote:
> > How much power am I really wasting at the resistor or LM317? I want to > > > > come in under 5w, how much do I plan for at the resistor? > > Given that you aim to run the LED's at a specified current, you can get > awfully close just by using Ohm's law, in this case P-I^2R. For the > LM317, also you lose 1.5I in it as well. Actually, he wants P=IE, where I is the current drawn by the LED and E is the voltage across the LED (for power dissipated in the LED) or the voltage across the LM317 (for the power dissipated in the LM317). The internal bias current of the LM317 will be insignificant compared with the current of the LED. That said, the LM317 is performing like an electronic resistor. If you have 1 amp flowing though the LED and 3 volts across it, the power dissipated in the LED will be 3 watts. If you're running it from 12 volts, the voltage across the LM317 or resistor will be 9 volts. The 1 amp current will cause it to dissipate 9 watts. The total power draw from the battery will be 12 watts, implying the efficiency to light the LED is 3/12 = 25% (rather low). There are a lot of switchmode regulators used to light up strings of LEDs. I've seen several advertisements at http://www.electronicproducts.com/, but don't recall any specifically at the moment. Coincidentally, their featured product today is, 85-lumen LED-based MR16 lamp replaces incandescents http://www.electronicproducts.com/ShowPage.asp?SECTION=&PRIMID=&FileName=hlap01.jun2007.html It has a 4 watt emitter and takes 12 volts AC/DC at 0.7 amps (8.4 watts, which implies an efficiency of 48%). Craig
