Help with Resistor Value(s) for sound saber build

[JigSaw73]

Hello All,

So now that I have officially confused myself, I thought it was time to post and ask for help...

I have built two stunt sabers so far and they've worked very well. I had built the first two w/ a Cree XP-E2 Blue LED, 1000mA Buckpuck, latching switch, and a 4xAA Battery pack. I have the LED on the switch wired so it lights up when I power on the saber (w/ a 150ohm, 1/8watt resistor on the + wire).

This next build will be w/ a NBv2 for sound and a 3.7V li-on battery for juice. I don't understand what size resistor I will need for the blade LED. Conversely, can I still use a Buckpuck or does it HAVE to be a resistor? I've been combing through these forums and have yet to find anyone using a buckpuck instead of a resistor.

I calculate the resistor I need at:

R=(3.7-3.47)/1.0 = 0.23 where R=(3.7v Li-On Battery - 3.47 Blue Cree LED) / 1.0 (1000mA desired current for LED) = 0.23
P=0.23*1.0 = 0.23 where P=0.23 (value of "R") * 1.0 (1000mA desired current for LED squared (1.0A*1.0A=1.0)) = 0.23

So I get a 1/4ohm, 1/4watt resistor value... does that round up to a 0.47ohm, 0.5w resistor?.
When I use the Resistor Calculator online from Silver Serpent's sig, it says to use a 1ohm, 2watt resistor.

Why such a huge discrepancy? I wouldn't mind a short answer (Use the Buckpuck or get a __ohm, __watt Resistor Value) so I can buy it for my build, and a long answer so I can better understand how to figure this out for future builds.

Please and Thank You.

[bigkevin61]

First off, I use this led resistor calculator: http://www.hobby-hour.com/electronics/ledcalc.php, and it confirms your Ohm Calc of 0.23, but it gives a resistor wattage of 0.5.

As for using a resistor different then the recommended calculated:
1) you can always "go higher" on the wattage, as that is just the amount of power they can disapate. So more capacity doesn't hurt. The larger the wattage, the physically larger the resistor (the 5 watt resistor is appropriately nicknamed "the big honking resistor" for a reason). Too little wattage, and you can blow the resistor.
2) for the Ohms, it is better to "round up". This is what some calculators do, to give you a "common" resistor you can actually find in a store. I believe this is the case with the one in Silver Serpents's link. More Ohms is just more resistance, which means less current to the LED. "Going up a little" won't make that big of a difference. Rather be safe then sorry, as LED's aren't that cheap.

Now, you CAN push some LED's and "overdrive" them. If you do so within a proper range, you can get more light, but at a cost of shortening the LED's lifespan. People go this route, as sabers aren't "on" for a lot of hours over its life, and the LEDs usually have pretty robust expected life.

There is a bit of "wiggle room" in calculating a LED resistor. You usually use the Battery Pack "normal" voltage. But that can be higher, after charging. You usually use a "typical" forward voltage for your LED, but variances in manufacturing makes each LED different. That is why some say to measure your own LED's forward voltage (especially if you will play with overdriving). Even resistors themselves have a margin of accuracy, with common resistors at 5% (but there are 2% and 1% tollerances also, guess which are more expensive).

As for a buck puck with a NB, you don't see people use it, because it can't go together. The NB can't handle more then 5.5 volts, and the buck puck need at least 5-7 volts (depending on which model you get) to work. And you don't get battery solutions in the 5-5.5 volt range.

[JigSaw73]

Ah, OK. Thanks for the quick reply and all the information on ohms, watts, LEDs, and resistors. I'm understanding now and it is extremely helpful. Especially the part about the Buckpuck usage (or, more to the point, not using it!).

I guess ordering the 1ohm 2watt resistor will be the way to go. I tried to get some today at the local Radio Shack to save on shipping but they only have 1/8, 1/4, and some 1/2 watt resistors in stock. Oh well.

Thanks again. Appreciate it.

[Bark]

That original calculator doesn't seem to like anything under 1 ohm, 2 watts. Your math looks good to me (except you need to square the current value in the second formula, not that it matters when it's one ).

[Silver Serpent]

The .47 ohm .5 watt resistor in the store is ideal for your setup. As others have said, the calculator in my signature rounds up, and doesn't give values lower than 1 ohm.

[JigSaw73]

Thanks for the replies. At least I feel confident I'm understanding how to calculate the resistor values correctly!