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

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?

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## 2 Replies

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.

jensah (author)2011-04-24

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 :)