Help on LED Resistors

[brthegreat117]

Hey all. I need some help on my resistor knowledge.
I am planning a Graflex 2.0 build and only need the resistors.
I am using a 18650 3.7 Lithium Ion battery, a NBv3 and Tri-Cree B/B/W XP-E2. This will be on a Padawan Chassis from Goth.
I was wondering what resistor would I need to run the B/B in parallel and White for FoC.
I did some calculations with Ohm's Law, but I believe they are incorrect.

I also posted this on the question section of the "Star Here". Noob mistake.

[darth_chasm]

You'll run each at 1000mA I presume. If that's the case just subtract your led forward voltage from the supply voltage and that gives you both ohms and wattage requirements. We usually recommend using a resistor on EACH die.

Ex. 3.7-3.3 = .4 ohms/watts (just an example)

You may need to round up to the next most common resistor size. In the example above you could go with a .47ohm. Ohms affect brightness watts do not.

[brthegreat117]

Illumination Color: Blue
Wavelength: 485 nm
Luminous Intensity: -
Luminous Flux/Radiant Flux: 45.7 lm
If - Forward Current: 350 mA
Vf - Forward Voltage: 3.1 V
Mounting Style: SMD/SMT
Power Rating: 3 W

Info For The Whites

info for the WHITES
Illumination Color: Cool White
Color Temperature: 10500 K
Luminous Flux/Radiant Flux: 360 lm
Color Rendering Index - CRI: 65
Luminous Intensity: -
Viewing Angle: 125 deg
If - Forward Current: 1050 mA
Vf - Forward Voltage: 3..5 V

This is the information I have for for the LEDs.
I did some calculations, but I was way off.

So when I do this. I attach a resistor to each die and then intertwine the two blues and wire up to the NBv3 right? and then take the remaining white led and take to FoC on the NB.

Currently also checking out your "What Resistor Do I Need" post.

[darth_chasm]

Ignore all the superfluous info like luminous flux etc. it's only going to confuse you. Check out the tri Crees in the store. Their product pages have the info you need to make it easy. Basically, all Crees are safely run at 1000mA.

[darth_chasm]

One wire from board, split into a Y. Resistors on the branches of the Y, then LEDs. You can do that in reverse order as well.

[brthegreat117]

Sweet, I also posted this on Fx and believe you replied as well. The help was much appreciated.
So will all LED diodes have their own individual resistor rating according to my calculations? The blues will probably have the same resistors, and the white because it differs in setting will have another different resistor?
3.7-3.1=.60 ohms/watts for blue
3.7-3.5=.20 ohms/watt for white

Will definitely check out the cree site.

[NanoRex]

You are using the wrong voltage. You need the LED forward voltage when run at 1000mA (you are using the one for 350mA), which can be found in the store description for the LED.

[darth_chasm]

You're struggling and I know you're doing the work to try and figure it out so I will help you.

Go here http://www.thecustomsabershop.com/BlueBlueWhite-Cree-XP-E2-CopperNova-P1013.aspx
Look at the LAST line of info for the blues and the LAST line of info for the white. Those are the numbers for your calculations. Subtract those forward voltages from your battery's voltage and you have your answer.

One calculation for the blues and they will use the same value resistor. Remember one each. Anoth r calculation for the white.

[brthegreat117]

I can't find anything on the voltage from the site for 1000mA.
I am now more confused.

[brthegreat117]

Ahhhhhh I see it now. I posted this before refreshing.
Thank you so much Darth Chasm.
The LED is no different that the one offered by SaberBay, since both are XPE-2 to, but one is copper nova.

[darth_chasm]

No problem. The copper nova is a slightly better MPCB that manages heat a little better, but you should be fine with what you have.

[brthegreat117]

I believe I done did it.
Blue- 3.7-3.47=.23 watt/ohms
White- 3.7-3.15=.55 watt/ohms

So those are my values for the resistors, now to find the watts I need.

[darth_chasm]

Thats it! You'll have to round up to the next most common value as those values will be difficult to find.

[brthegreat117]

So .23 should go to .5 resistor.
.55 should go 1.0 resistor.

Now my finally idiotic question. Using the second formula, how do I find the wattage for the resistor. Or does it coincide with the power output of the LEDs?

[brthegreat117]

I just solved it I believe.
I think that if I purchase 3 of the 1 Ohm, 2 Watt resistors then I will be good to go on my merry little way

[brthegreat117]

I received a couple PMs on FX regarding this. I received some info that the LED doesn't need any resistors because the Fv value is close to my battery voltage. Since each led runs 1 amp and I wire the blue in parallel and the white led is only on for a bit. Is this info correct. It's from Darth Alice.

[NanoRex]

Technically he is correct, but he is a very experienced sabersmith and as a best practice we recommend that newer saber builders always use resistors unless they really know what they're doing. I would use a resistor just to be safe.

[Silver Serpent]

If you leave out the resistors, you run the risk of the LEDs drawing too much current and causing quirky behavior with the NB.

[brthegreat117]

I also messaged the builder of the LED that I am using and he mentioned to use .5 ohm 1 watt if I have issues with false swings, and leave the white open with no resistor.
Now I am confusing on getting 1 ohm 2 watt or .5 ohm 1 watt or just going no resistors.

Thank you for your responses

[Silver Serpent]

Both resistor options are fine. You're unlikely to notice a difference in brightness between the two resistors.

[brthegreat117]

Sweet!

[PhoenixPyros]

Just trying to figure this out myself, I am converting a 2007 Hasbro A-RotS measureing 4.5v-2.65v (Red Cree XP-E2 CopperNova) =1.85 ohms so I should use a 2ohm resistor?
I have ordered a Ceramic Cement Resistor 2 Ohm 10W hoping that I was doing this right, but figured I should play it safe and ask, please let me know if i'm doing this wright and set me in the correct direction if I'm not.
Thanks for listening.

[darth_chasm]

Welcome aboard. The calculation is correct; however, 10 watts is extremely large and much more than you would need (2 watts). Fine if you have room for it, just want to make sure you are aware.

[Calculon]

This was very helpful! Cheers!