Want to double-check wattage needed for 1 of my LED's resistor(s).

Thanks for taking the time to look at yet another of the myriad resistor question threads. I am hoping a veteran smith/etc can help verify what wattage of resistor I need on one of my lines.

My build has a 3.7W 18650 Li-Ion, Nano biscoutte v2 w/pex powering a R/rB/W Tri-Cree (R and rB wired in parallel; W for FoC).

I've been able to figure out the proper resistors for the W and rB lines that I plan on running at full (safe) power.

For the color blend I want I will be putting 34ohms of resistance on the R line. Now I can do this 1 of 3 ways with the resistors at my disposal:

-a. wire (in series) 4x 8.2ohm, 1W resistors + 1x 1.2ohm, 3W resistor into the circuit.

-b. Wire (in series) 1x 33ohm, 50W resistor + 1x 1.2ohm, 3W resistor into the circuit (the 1.2ohm is already attached to the LED from where I dialed in the color to protect the LED so if it is safe to leave -as far as I understand it is- I figure I may as well leave it in the circuit).

Or -c. Wire a single 33ohm, 50w resistor into the circuit (should be adequate enough resistance for the color I want).

The options using the 50W resistor will take a lot more room in the hilt but should be able to fit. Of course if I can safely get away with the four 1W+one 3W resistors that would give me considerable extra space to work with.

I'm hoping someone with more experience can give their insight to me since I'm not sure what kind of current will be going through the circuit after taking the 34 ohms of resistance into account. (If I'm right, there's a voltage drop from the resistors too which is throwing me off trying to do the math -been far too long since my uni physics classes it seems).

Thanks in advance for your help!

Why such a large resistor. My purple blade used a 3.3ohm 2w ant it dropped the current to about 100mA 34 ohm is higher than the 20mA accent LEDs that I use a 27ohm .25w on. Have you tested the 34ohm resistor? My calculations say that would drop it to less than 10mA, I am not sure a Cree will light at that low of current.


Edit: checked the cree xp-e2 data sheets and their published minimum current for the red is @48mA with about a 20 lumen output.

I'm actually going for an indigo blade. I've achieved it with a pot. To be honest it kind of surprised me that I needed that much resistance too.

Did you measure the resistance from the Pot. Personally it sounds extremely low, especially since the die shouldn't even be lighting bellow 48mA and a 34 ohm resistor should be dropping it to 10.068 mA. You may need to drop the blue as well because it will white out when adding colors. I am running a ohm 1w on my rB to darken the purple as well.

Quote:

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Originally Posted by Jamonk Arturan

I'm actually going for an indigo blade. I've achieved it with a pot. To be honest it kind of surprised me that I needed that much resistance too.

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If you are wanting 34 ohms I don't.think you need 50w based on the math it is significantly lower.

So with a forward voltage of 2.65v and an input of 3.7. A 34ohm resistor yields 30mA of current.

So P=34*0.030^2. P=0.0306W

Edit: I was off on my initial. I thought the red cree had a FV of 2.85.