How do I calculate the resistor value I need?

[LightHearted]

Hi all,

This is in regards to the Rayovac LS headlamp mod (Thanks LUX!). The sample headlamp I have (thanks again LUX!) has a 1W LS side emitter, and I am using 3 NIMH AAAs (700mah each). The light output is very bright and pleasing, but it sounds like this may be too much for the LS as some people have ruined LEDs using this configuration. Is there a way to calculate the least ammount of resistance I would need to put in the circuit to get the most light output without causing the LED to die an early death? When I mod my own I want to keep it as bright as possible, but not if I will need to replace the LS after just a few battery cycles. I would appreciate it if you could explain this in the simplest terms, as I am very new to all of this. Also, why would 1.2v NIHMs need a resistor to be added to the circuit when 1.5v Alks do not?

Thanks much,
Ken

 

[chrisse242]

http://www.candlepowerforums.com/ubbthreads/showflat.php?Cat=&Number=346129&page=0&view=collapsed&sb=5&o=93&fpart=1

look at htis link, it's four threads above yours at this moment. Download the prog and start calculating.

Or use: http://www.markhannahsurplus.com/led-resistorcalc.htm

For everything else do a little search around the forum and you will find out nearly everything you want to know.

Chrisse

 

[LightScene]

A simple calculation isn't going to answer your question, because each luxeon is different, and each flashlight is different. NiMh batteries supply more current than alkalines, so they are said to have a lower "internal resistance".

You need to measure the current when your light is turned on briefly. Then you will know if you are overdriving the lux by too much. You would want the current to be at or below 700mA. I doubt that 3 AAA's can overdrive a luxeon, but I'm not sure.

You also need to be know how well the lux is heat-sinked. If it doesn't have much heat-sinking, then you would want the current to be much lower - like maybe 300mA or less.

If you've been using your light for a while with NiMh's, then that's pretty good evidence that it's OK to continue doing it. The guy who modded your light for you can probably answer this question better than anyone else, since he knows the details.