Need assiatnce in Resistance Computation

[spooky3369]

Can some one please help me double check my computation for the RESISTOR.

I'm planning to make a rebuild, instead of having one LED I'll be using Tri Cree XP-E2 CopperNova - Blue/Blue/White for FoC effect. I'm thinking of putting resistors for each of the LEDs.

Main Power source is 7.4v Li-ion 2600mAh 18650

Tri-Cree Details:
Blue Cree Part# XPEBBL-L1-0000-00301
45.7 Minimum Luminous Flux @ 350mA
Forward Voltage @1000mA = 3.47v

White Cree Part# XPEBWT-L1-0000-00F51
266 Minimum Luminous Flux @ 1000mA
Forward Voltage @1000mA = 3.15v


Computation:

BLUE LED:
Power:7.4v - 3.47v = 3.93v
Resistor: 3.93v / 350mA = 11.22 ohm with 1.37 watts

WHITE LED:
Power: 7.4v - 3.15v = 4.25v
Resistor: 4.25v / 1000mA = 4.25ohm with 4.25 watts


I was also wondering if I would just put 1 resistor (more powerful) for the 2 BLUE LEDs? I'm open to better suggestions, thank you!

[darth_chasm]

Can some one please help me double check my computation for the RESISTOR.

I'm planning to make a rebuild, instead of having one LED I'll be using Tri Cree XP-E2 CopperNova - Blue/Blue/White for FoC effect. I'm thinking of putting resistors for each of the LEDs.

Main Power source is 7.4v Li-ion 2600mAh 18650

Tri-Cree Details:
Blue Cree Part# XPEBBL-L1-0000-00301
45.7 Minimum Luminous Flux @ 350mA
Forward Voltage @1000mA = 3.47v

White Cree Part# XPEBWT-L1-0000-00F51
266 Minimum Luminous Flux @ 1000mA
Forward Voltage @1000mA = 3.15v


Computation:

BLUE LED:
Power:7.4v - 3.47v = 3.93v
Resistor: 3.93v / 350mA = 11.22 ohm with 1.37 watts

WHITE LED:
Power: 7.4v - 3.15v = 4.25v
Resistor: 4.25v / 1000mA = 4.25ohm with 4.25 watts


I was also wondering if I would just put 1 resistor (more powerful) for the 2 BLUE LEDs? I'm open to better suggestions, thank you!

Your calculation for the blue is wrong. You're using the test current from the luminous flux measurement. While you could do this, your led wouldn't be very bright. You need to use the forward voltage test current. I've highlighted the lines of importance above.

It is recommended to use one resistor per each led.

[spooky3369]

What I did here is subtract power source voltage (7.4v) to the forward voltage of the LED (3.47v for the blue)

[darth_chasm]

What I did here is subtract power source voltage (7.4v) to the forward voltage of the LED (3.47v for the blue)

Yes, but for the second part of the calculation you used 350mA rather than 1000mA.

[spooky3369]

oh, i see what you mean. thanks!