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Yorginsnarff
07-22-2015, 07:04 PM
I've looked over FJK's "Down and Dirty" Ohm's Law tutorial, and I want confirmation on whether or not these resistors would fry my LED

(I'm using a tri-cree, G/G/W, Greens in parallel).

For the greens: 1 ohm, 1 watt resistor

For the white: 1 ohm, .5 watt resistor

Forgetful Jedi Knight
07-22-2015, 07:17 PM
Telling everyone the battery solution would also help them determine whether you are "wrong" or not. ;)

Yorginsnarff
07-23-2015, 11:09 AM
yeah a 3.7V lithium ion battery. 1 ohm, 2 watt resistors (for both the green and the single white die) would keep them from frying, right? That's what I got from the "Down and Dirty" thing...

Forgetful Jedi Knight
07-23-2015, 11:17 AM
Those aren't the ideal resistors, though they will work.

Yorginsnarff
07-23-2015, 11:25 AM
hm... well for example I got 0.12 for the resistance, so should I round that to a .48 ohm 1/2W resistor for the greens and the same for the white?

Forgetful Jedi Knight
07-23-2015, 11:28 AM
The White is different, because it has a different Vf from the Greens, but you essentially have it right, now.

Yorginsnarff
07-23-2015, 12:00 PM
okay, so I re-did the math.

For greens: (I will be wiring the two in parallel, so I only use one resistor where they splice, right?) I will need one .48 ohm, 1/2W resistor.

For the white: I will need one 1 ohm, 1W resistor.

Correct now?

Forgetful Jedi Knight
07-23-2015, 12:37 PM
okay, so I re-did the math.

For greens: (I will be wiring the two in parallel, so I only use one resistor where they splice, right?) I will need one .48 ohm, 1/2W resistor.
Correct now?

You will need 2 resistors. One for each Green LED.

Yorginsnarff
07-23-2015, 12:49 PM
oh? In Madcow's video, he twisted the two positives and negatives together and just added one resistor...

so, then I attach one resistor to where the two wires splice, then put the other on the "bridge"of the PeX?

Forgetful Jedi Knight
07-23-2015, 12:53 PM
The way Madcow does it is not a way that I recommend. If you are still intent on doing it that way, you would need to double the size of the resistor.

Yorginsnarff
07-23-2015, 12:55 PM
I see. So it'd just be better to wire two .48 ohm 1/2W resistors on each respective wire, THEN splice then off to the board.

Yeah Madcow's method seems a bit more professional. I'm still a youngling after all ^_^

Bark
07-24-2015, 05:44 AM
Agreeing with FJK, one resistor on a splice for two LEDs in parallel is problematic for this reason. If there is a slight difference in the LED dies, from the manufacturing for example, they won't draw current equally. One will draw more than the other. If it draws so much that it pops, all of the current then goes through the remaining LED die. Since that's twice the current it can handle, it will shortly pop too.

Some of us here, like FJK and me, think it's just safer to dedicate a resistor for each LED die. That's the only way to make sure the LED die isn't drawing more than what you intend.

However, I personally don't like to splice off the board if I can just splice wire closer to where the junction physically has to be. For example, one wire on the board going to the splice near the heat sink. If you put the resistors on the positive side, you can still do a Madcow setup for the negative side wiring if you want to save real estate and have only one negative wire for both LED dies.

Yorginsnarff
07-24-2015, 09:09 AM
ooh, so I found a .22 ohm, 2W resistor. Can I use these for the greens, since I calculated 0.12 would be the resistance value and still not blow the LED? (The 2 watts should take care of any heat problems I have)

Equation for the greens: ( 3.7-3.58 ) = 0.12/1A = 0.12 ohms is Resistance <-- Rounding up to .22 ohm 2 watt resistor? Is this ok?

Equation for the white: ( 3.7-3.15 ) = 0.55/1A = 0.55 ohms is Resistance <-- I'm rounding this up to 1 ohm 2 watt resistor

Yorginsnarff
07-24-2015, 09:39 AM
nvm I remembered that resistors are always safer the more you go up. My brain had a stupid moment lawl.