I have a few cheap LED flashlights that I would like to power with a 7.6 volt R/C car battery pack. I used to power a regular bike light with it, but the bulb was for 7.5 volts. I'd like to go LED for better battery life. All the LED lights use 3 1.5 aaa batteries. Do I need to add anything (resitors, capacitors, etc.), or are they already in there? Any help would be great.
That is a complex problem. First you need to know what specific bulb you are dealing with . Most white LED's typically run between 3.7 to 4.2 Vdc , and require 20-30 mA each to run at their full output. You say that the 3 AAA batteries are what they ran on originally, so that means that they could have only been setup as a parallel circuit, if they were in series that would require at least 36 volts. But you do not know the specifics, so here is how you start: Assume that the max voltage of the bulbs is 4.0 Vdc x 9 bulbs @ 20 mA each , powered by 7.6 vdc One 22 ohm 1 watt resistor @ 30mA each you would need one 15 ohm 1 watt resistor , but be careful you could shorten the life , or not, trial and error is the only way to know. Radio shack does sell a 22 ohm 1/2 watt resistor, and it would get hot, so you need to go to a specialty electronics(radio shack is not anymore) shop and find exactly what you need for around a buck. Oh and you cant take two 22 ohm 1/2 watt resistors and run them in series, that would result in 11ohms at 1 watt, and a bulb life of around 40ms.
Your 3 batteries add up to 4.5 volts so you may risk damage to the leds, One method would to add a 5 volt regulator which has 3 tabs--input (where you put the positive lead of the battery, ground, and output which should supply 5 vdc to your leds
