First timer, question on the resistor

[Shaitan]

Trying to build my first (easy) saber. I'm planning to use a 4AAA battery pack (6V) with a RED Cree XP-E2 Star LED (2,65V at 1000mA), no sound. Is a 3.9 ohm 5W resistor the right choice? Thanks in advance for the help!

[Generic Jedi]

A 3.7 volt lithium ion battery would be the best choice, but if you really want to use alkaline batteries, use 2 AA batteries instead of 4 AAA batteries. You'll get longer run time.

[Shaitan]

Thanks a lot for the suggestion. For now I think I'll use 2AA batteries. In this case a 2.2ohm 5W resistor should be fine right?

[jbkuma]

What math are you doing to come up with this number?

[Shaitan]

The resistance should be (4.5 - 2.65) / 1 = 1.85 ohm while the power is (4.5 - 2.65) * 1 that is again 1.85 doubling it up it should be near 3. The nearest higher resistor is a 2.2 ohm with 5 W. Am I missing something?

[Silver Serpent]

The 4.5 in your calculation is the battery pack voltage. If you are using 2 alkaline batteries, that would be 3v, if you are using 4 alkalines it would be 6v. 4.5 is the value for 3 alkaline batteries.

Using 2 AA batteries is an acceptable solution if you can't use a li-ion cell (3.7v)

[Shaitan]

so you are telling me that 1.5 + 1.5 is 3... :mad: thank you so much for wasting your time with a fool!! I'll go for a 1ohm 2 watts

[Silver Serpent]

No worries. Sometimes a second set of eyes is necessary to see what has been missed. Good luck with your build!