ANOTHER resistor check!

[pillbot]

Hello there.

Not much more than another resistor confirmation (or correction...).

Using a Nano Biscotte. 3.7v li-ion battery. Blue Cree XP-E2 LED.

All items are from the store.

I'd like to stick with what is in the store and figured (well, an online resistor calculator did...) that the 1ohm 2w resistor would be my best choice.

Am I missing anything? Any and all help is appreciated.

Cheers!

[NanoRex]

You're missing that calculators like to round up to the next common resistor value. Try doing the calculation yourself and you might get different results ;)

[pillbot]

Thank you for the reply!

Now, I see this answer a lot. I can do that and, if I have to, I certainly will.

Follow up question time though; what is the benefit? What changes might I see with the different method of calculation and, as a result, a possibly different resistor recommendation?

Also, still bearing in mind that I'd like to give my business to the store and, therefore, would be limited to what is available there.

Not starting anything here, just genuinely wanting to know the end result difference between a calculator recommendation and a doing the math myself. Would a slightly different result end up forcing me to look elsewhere for my resistor? The resistor question seems to be so simple but always pops up. I am fine with the metal cutting/bashing/drilling/tapping/hoping for the best side of things. Just trying to figure out the electrical side of it all.

Thanks again!

[Generic Jedi]

You need a resistor with a great enough ohm value to protect your LED. Any extra ohms just make the LED dimmer.

[FenixFire]

Doing the math yourself you would see that the .47 ohm or .5 ohm resistor would work and give a slightly brighter blade.

The online calculators round up to the nearest commonly available resistor with a large margin of error...usually about 60% max. Others if attached to stores ie. digikey rounds up to the nearest in stock resistor.

[pillbot]

You need a resistor with a great enough ohm value to protect your LED. Any extra ohms just make the LED dimmer.

Ah ha! So, this is why folk here say to not worry too much about the wattage?

[pillbot]

Doing the math yourself you would see that the .47 ohm or .5 ohm resistor would work and give a slightly brighter blade.

The online calculators round up to the nearest commonly available resistor with a large margin of error...usually about 60% max. Others if attached to stores ie. digikey rounds up to the nearest in stock resistor.

Interesting. So, the .47 or .5 in the store would work fine, despite the higher wattage?

(I am currently reading elsewhere about higher wattage and how this is less important than not over-doing the ohms.)

Again, sorry about rehashing a common question. I am just someone who likes to know the "why" before I jump in too deep.

[Forgetful Jedi Knight]

The wattage is "minimum necessary" to dissipate heat, so you can go over, making heat dissipation less of an issue for your resistor.

[Cheradenine]

The higher wattage is fine, it just means that the resistor can handle more heat (the current resisted by the resistor is converted into heat). Just remember that a resistor with a larger wattage is physically larger!

[Mineral]

Is there a simple list of colors and best resistor values? It seems like the variables are relatively few, so there should be a "right" answer for each color of each brand of LED given a certain power source.

Telling everyone to do their own math is an excellent pedagogical philosophy, but if there's a best resistor for each color of Cree XP-E2 powered by a 3.7V source, it ought to be fairly easy to make a list and post it somewhere to stop the few "resistor check" threads every day.

[Cern]

i was thinking this myself

[Anoril]

Hello!

I jump on this thread for resistor considerations I'm having at night.

I'm making a color-changing saber using a Cree XM-L or a Tri-Cree, anyway. There are two voltages displayed in the specifications, one for 1000mA and another for 350mA. I understand the smallest is the minimum current to get the led turned on (correct please if I'm wrong ^_^ ) and the highest is for the highest brightness before the led burns...
But applying the Ohm Law for both voltage and current, I come with two (quite) different resistor values... Because I want a color changing saber, the led may come down to the lowest current or up to the highest. As read above, a high resistor implies high resp... a dimmer led.

Then, what to do? What is the "good" solution? Should I use the max resistor? A middle one? The lowest? And then I come to... What is a variable resistor? Will it Save Us All? :)

Some mathematical examples... with XM-L (RGrB-White)
Forward Voltage (@ 350mA)-Typical: Red - 2.25V, Green - 3.3V, Blue - 3.1V, White - 3.1V
Forward Voltage (@ 1000mA)-Maximum: Red - 2.6V, Green - 3.9V, Blue - 3.7V, White - 3.7V
Let's go with the red: (with 3.7V and with 4V, as I read somewhere around we may use this voltage to be sure nothing explode)
--- R = (3.7 - 2.6) / 1.000 = 1.1 Ohm --- R = (4.0 - 2.6) / 1.000 = 1.4 Ohm
--- W = (3.7 - 2.6) * 1.000 = 1.1 Watt --- R = (4.0 - 2.6) * 1.000 = 1.4 Ohm
and
--- R = (3.7 - 2.25) / 0.350 = 4.14 Ohm --- R = (4.0 - 2.25) / 0.350 = 5 Ohm
--- W = (3.7 - 2.25) * 0.350 = 0.5 Watt --- R = (4.0 - 2.25) * 0.350 = 0.62 Watt

=> 1.5Ohm, 2W or 5Ohm, 2W ? :D

(note that for the blue and green, I'm above the 3.7V from the accu, then I may not use resistors... But Erv's Law says "Always Use a Resistor, Always". What to use, then? 0.0001 Ohm? :) )

Regards

Paul

[Silver Serpent]

The Tri-Cree LEDs are pretty robust. They're not going to pop if you calculate at 3.7v instead of 4v. I always use 3.7v with my calculations.

Use the 1000mA value for your calculations. Giving these LEDs only 350mA will make for a terribly unimpressive blade.

This: http://www.thecustomsabershop.com/47ohm-5w-resistor-P947.aspx is what we generally use when the math says you don't need a resistor. It won't hurt the brightness, and it'll prevent certain wonky behaviors from the sound boards.

[Anoril]

Hi!

Thanks a lot for the advice and for the "not-so-useless-resistor" ;) I'll go with that! :D

Paul

[TwinMill]

Is there a simple list of colors and best resistor values? It seems like the variables are relatively few, so there should be a "right" answer for each color of each brand of LED given a certain power source.

Telling everyone to do their own math is an excellent pedagogical philosophy, but if there's a best resistor for each color of Cree XP-E2 powered by a 3.7V source, it ought to be fairly easy to make a list and post it somewhere to stop the few "resistor check" threads every day.

Nice in theory, to be sure, although there is no single battery solution that will be perfect for every single saber design (at least not yet). For some people just getting into the hobby, a stunt saber may be their chosen path, so if they choose a standard battery holder... just in this store that could be 2x or 4x AA, or even a 4x AAA holder. Compound that by the fact X of alkaline batteries will have a different voltage than an equal number of Ni-MH batteries... + the store sells a AA "dummy cell" ...basically a battery shaped metal rod covered in plastic (to prevent short circuits?) and that throws another variable into a saber's potential input voltage.

In the end, there's no way the store could know which version of how many volts someone plans to apply to their saber, despite how obvious it may seem to our human brains. After all, since we bought the parts, we already know what power source we're going to use.