Resistor Help!

[Lady Revan]

So I'm doing wiring for what I want (for now) to be a AAA Stunt Saber. I checked the wiring diagram in the Wiring thread (diagram for stunt type in question pictured below) and there is a reply that says the resistor changes depending on the LED type. The LED I have does not come with a resistor, and so I was wondering if anyone could help me to determine which resister type I need to get.

Wiring Diagram from thread:



LED Module:



Cree Star Pre-Wired LED & MHS Heatsink Module:

• MHS Heatsink
• Cree XP-E2 copper Star LED
• Cree Star lens holder
• 8.7 Deg Optics
• Thermal tape pad
• JST male connector Blue- White+ 24AWG

There's also a note from the LED listing that "All colors should be ran at 1000mA." Color is Amber

[Rapine]

You'll need the forward voltage for each die.  Typically found on the page they're sold on.

Then plug all of the numbers into an online resistor calculator.   That's easiest.

For example:
TCSS lists that LED at 3.28v @1000mA

A fully charged 18650 has about 4.2v, but has a nominal output rating of 3.7v.  I use 4v for "supply voltage".

So, plugging all of those numbers into this: link, shows a 1ohm/1watt resistor.

That's for a 3.7v supply though (one 18650).  It's different for 2x14500 (7.4v).  Fully charged that would be 8.4v, but rated at 7.4v.  Again, I use 8v.

That gives an answer of a 5.6ohm/4watt resistor.  

Or you could just use a buckpuck at that point.  (7.4v)

4xAAA = 6v

Hope this helps.

[Lady Revan]

Got it! Thank you!

[Rapine]

Right on.

[Chad mikolyski]

Rapine hooked you up pretty well. His calculations are all absolutely correct and the way he is calculating the voltage is conservative to make sure you don't damage your led.

For a second opinion though less conservative: I run all my cree XP2 LEDS closer to 1500mah (Overdrive) for maximum brightness. it does make a difference if you are picky. There have been tests done which claimed that anything between 1000-1500mah is OK for that led type but not for others. (Rebel, seoul i believe)

If your battery set up stays the same at 4 AAA batteries for a total of 6v you could use anything from a high end of 1.8ohm 5watt resistor to a 3ohm resistor. I personally lean toward maximizing brightness and would shoot for something around 2ohm for a combination of safety and utilizing the full potential of the LED you chose.

Choice is yours but here is a super easy LED resistance calculator: http://www.ohmslawcalculator.com/led-resistor-calculator

Best of luck with your build!