# I have a hi-power LED and don't know what resistor can I put to get the max power out of LED?

Hi-power LED, 3.7volts, 0.7 amps, and battery has 0.9 Ah so I was wondering what resistor value could I put to get LED-s max power output, and that the led wont burn out
I'm thinking between 1-2 ohms would be ok, but how can I calculate it so I would know for my other projects?

7 years ago
The answer depends on the battery voltage.
R= (Vbat-3.7)/0.7 Ohms. It should be sized to drop ~2V at the rated current, which is quite substantial, and may get pretty hot. At that rate, the resistor would have to be 1.5Watt rated.
jensah6 years ago
High powered leds should not be powered by resistors due to the low efficency. Especially when you use it with a battery.
Try a LM317 or similar. This is a common chip that you most likely will find in your country.
Still not highly efficient, for that you will need a switched constant current driver (LED Driver in short). See ebay and dealextreme. Theese solutions are not too much more expensive then a high power resistor.

In example a 3,7v 0,7A = 2,59w Power led
Resistor 1,5w
Would equal a efficiency of 58% (1,5w/2,59) (A good driver has 90+%)
And the efficency would drop the higher the voltage.
Hope it helps someone :)