Resistor calculation math?

[Noyl Wendor]

Howdy,
I'm setting up a Tri-cree GGW (white FOC) with a nano 2.0.
I came up with a 1.2ohm 3watt resistor.
3.58-3.7 = .12 /? . mA? or round up to 1A? my resistor is waaaay to big, I see green but both dice are really dim. Same as the guy trying to figure out his wrong turn;)

[Generic Jedi]

Resitor = (BatteryVoltage - LEDforwardVoltage) / Current (measured in amps). Your battery is 3.7 volts. The forward voltage for the green LED is 3.58 volts, and the white is 3.15 volts. You can run the Cree LEDs at 1 Amp.

Power (watts) = (BatteryVoltage - LEDforwardVoltage) x Current.

You'll want to put resistors on each green.

[Greenie]

I've also used a GGW Cree, White FOC I used a 0.47ohm 1w just for safety's sake. The greens in parallel sharing a 0.5ohm 3w. Saber works just fine and they are almost brighter than the sun :cool:

[Generic Jedi]

What's the equation for one resistor on two die in parallel? Isn't it safer to use one resistor for each LED?

[Noyl Wendor]

I figured my calculation at 1amp. that's were I ended up with the 1.2ohm resistor. I didn't figure I would have to resistor each green, they are in parallel. so the one resistor on the "+" should do the trick for both. I'm just wondering if I'm horribly wrong somewhere else now

[Greenie]

With greens in parallel at 1000mA each you need to calculate at 2amps
Yes, everyone will say resistor on each die is best practice but I've watched Madcow's vid for wiring a nano with pex and he 'showed us' two dies in parallel with one resistor and let's be honest he is an expert saber smith and it is tutorial for those who want to know how to do it.
If it works for Rob then it works for me :mrgreen:

[Noyl Wendor]

With greens in parallel at 1000mA each you need to calculate at 2amps
Yes, everyone will say resistor on each die is best practice but I've watched Madcow's vid for wiring a nano with pex and he 'showed us' two dies in parallel with one resistor and let's be honest he is an expert saber smith and it is tutorial for those who want to know how to do it.
If it works for Rob then it works for me :mrgreen:
Yep I watched the same vid. So I'm really sure its my math for the resistor. Or there's a hiccup I'm not seeing

[Forgetful Jedi Knight]

What's the equation for one resistor on two die in parallel? Isn't it safer to use one resistor for each LED?

It is safer to use one for each die, and that is how I teach it. And the tutorial that I wrote is in the LED Wiring Section. ;)

[Forgetful Jedi Knight]

Resitor = (BatteryVoltage - LEDforwardVoltage) / Current (measured in amps). Your battery is 3.7 volts. The forward voltage for the green LED is 3.58 volts, and the white is 3.15 volts. You can run the Cree LEDs at 1 Amp.

Power (watts) = (BatteryVoltage - LEDforwardVoltage) x Current.

You'll want to put resistors on each green.

Not quite accurate, you might want to check out my tutorial for the proper "power" calculations.

[Bark]

It is safer to use one for each die, and that is how I teach it. And the tutorial that I wrote is in the LED Wiring Section. ;)

We have said this elsewhere: if one die pops, the other one is GUARANTEED to pop too if you combine into one resistor. A die popping is more likely too, as a manufacturing defect/imperfection could cause it to draw more current than designed, and you would be happily giving it twice the current it's designed to take.

Resistors are cheap. Wiring one for each isn't that bad. The question is whether the risk is worth the reward. The reward is that you'll save yourself five minutes, a couple of cents, and a little room. The risk is that you'll have to remake your entire LED module later.

[Noyl Wendor]

It doesn't seem from the manual that I should need two resistors for the main led. I went off the idea that the single resistor would be sufficient. I didnt account for running at 2 amps. So the resistor I am using is a bit too much.

[Jay-gon Jinn]

We have said this elsewhere: if one die pops, the other one is GUARANTEED to pop too if you combine into one resistor. A die popping is more likely too, as a manufacturing defect/imperfection could cause it to draw more current than designed, and you would be happily giving it twice the current it's designed to take.

Resistors are cheap. Wiring one for each isn't that bad. The question is whether the risk is worth the reward. The reward is that you'll save yourself five minutes, a couple of cents, and a little room. The risk is that you'll have to remake your entire LED module later.I've used the single resistor method ever since we started using mutli-die leds, and have never had one of the led's "pop," but if you feel safer doing it that way, that's your perogative.

It's your saber Noyl, wire it up whichever way you prefer.

[Noyl Wendor]

Im going with a single resistor. But I'm sure lowering the resistor value. So that should fix the issue

[Bark]

I've used the single resistor method ever since we started using mutli-die leds, and have never had one of the led's "pop," but if you feel safer doing it that way, that's your prerogative.

It's your saber Noyl, wire it up whichever way you prefer.

It's not a "feeling." Link: Why exactly can't a single resistor be used for many parallel LEDs? (http://electronics.stackexchange.com/questions/22291/why-exactly-cant-a-single-resistor-be-used-for-many-parallel-leds)

[Silver Serpent]

Does it work? Yes, if the LEDs are perfectly (or very closely) matched. Is it something I'd recommend to a beginner as normal practice? No.

The risk of thermal runaway goes up as the number of LEDs in parallel increases. If you've just got two LEDs in parallel, the risk is minimal. It goes up from there. For a budding sabersmith, I'd always advise to err on the side of caution. There's enough ways to burn out an expensive LED without adding more risk to the mix. Especially when resistors are just inexpensive insurance.

Once you've got a handful of sabers under your belt, you're confident in your abilities, and don't mind the possibility of replacing the LEDs, then by all means try it out.

[Noyl Wendor]

Well I'm pretty confident on the single resistor. Mainly I was looking for confirmation on the math LM not at home ATM. But what info I do have tells me this resistor I listed at the beginning of the thread is the one to use. As far as wiring resistors to each die I would like to save the room if I can. Both LEDs are the same (ggw tried cree) so in my mind one resistor makes sense. From everything I've read there's not really a down to using one resistor.