Need help on what resistor I need to use?

[Randy555]

Hi guys, hope someone can tell me what resistor values I need to use to connect a cree led to a 6 or 12 volt sla batteries, that a friend gave me. I am thinking of making a simple emergency light.

Randy

 

[f22shift]

http://led.linear1.org/1led.wiz

maybe this can help?

 

[Oznog]

You're gonna notice that running an emitter at a low voltage at hundreds of mA is gonna be a LOT of heat for a resistor, the little 1/4W stuff you buy in huge packs won't do it. So look at the wattage of what resistor you need- this is really inefficient too. I suppose for an "emergency light" it could be ok to be inefficient. The world went to switching power supplies for a reason, you can get 12v ones from Kaidomain or Dealextreme sometimes.

 

[Randy555]

Thank you, its very useful. I will also check about installing a diver from DX or Kaidomain but not sure what are the pros and cons of plain resistor and a driver. I am a bit new and hope someone can share more explanation.

Randy

 

[Gunner12]

A driver would be much more efficient and would provide longer runtime.

From what I understand:

Resistors burn off the extra power as heat.

A perfect driver driving a 1 watt LED at 3.7v from a 12v source will take in 1 watt of power. Add in the resistant of internal components and you should be seeing around 1.2-1.5 watt from the source.