choosing the right resistor

[supertrogdor]

So most of us have the chart Tim offered for what resistor to use, and others of us use a resistor calculator that can be found here
http://led.linear1.org/1led.wiz
But supposing you got the wrong resistor. Last night after i finished soldering some powerleads to the LEDs for my next build i connected it to power to test it out and it was EXTREEMLY brighter than my other saber. I tried fresh batteries on both counts because i thought that the difference was abnormally high. Now don't get me wrong, i am pleased about this, hoever it makes me think that i royally screwed up the resistor on my other saber and it should be substantially brighter.
So here is what i did. I looked at the technical datasheet from http://www.lumileds.com/pdfs/DS46.PDF and it has a variance of 3.03min to 4.47max with a 3.7typical for forward voltage for the white-royal blue, so i plugged in all the available options into the aforementioned resistor calculator, and came up with the following results.

@ 6volts and 700mA remaining constant
3.03 forward voltage ==>4.7ohm 2.3 watt resistor
3.7 ==>3.3ohm 1.6 watt resistor
4.47 ==>2.2ohm 2 watt resistor

Now obviously tim sells a 2.2 ohm 5 watt because if you cant find the right resistor then you need the next higher number as i understand it. So obviously we want our sabers to be as bright as possible and would therefore shoot for the highest output possible, right?
So i have a couple of questions for those more intelligent than myself with regard to electronics.
as the resistance gets lower(from 4.7 to 2.2 ohms) i understand puts greater strain on the circuit, but lets more current through, right? And if that is so, is there any potential danger to your LED by running the lower resistance circuit? I also noticed that while pluggin in info on the lux 5 and even some dimebag LEDs (blue) from radioshack that it gives a suspicious warning about the forward voltage numbers being too high, double check before wiring the circuit but we will calculate it anyway. Does anyone know why that is? I will admit to being rather ignorrant with regard to electronic circuits, but i am anxious to gather more information. Any assistance here would be greatly appreciated, and i will be getting a new resistor to test my theory as well.
cheers

[Do-Clo]

The important thing is what is important is the total current through the circuit not so much as the voltage, the brightness of the led is based on the amount of current. First you want to find the maximum current rating for your led. Then decide on what amount of current you want your led to operate at, Once you decide that then look at the suggested forward voltage of the led. Take the forward voltage of the led and subtract that from the total voltage supplied to the led example, if your voltage is 6 volts and the led's recommended voltage is 3.7 then you subtract the 3.7 from the the 6 volts giving you 2.3 volts this is the voltage drop across the resister. Now you take the current that you decided you want to run your led at and divide the voltage across the resistor 2.3 volts by your current: example 2.3 volts divided by 700 ma this gives you the value of the resistor need for this circuit 3.29 ohms.

I hope this helps

Do-Clo
Sith Lord
Brotherhood of the Sith
Give yourself to the Dark Side!!!

[james3]

Dang I go to reply and Do-Clo beat me to it[]

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[Do-Clo]

James that is why you are the apprentence[}]

Do-Clo
Sith Lord
Brotherhood of the Sith
Give yourself to the Dark Side!!!

[Strydur]

Well for starters we dont want to run the white - royal blue 3W at 700mA we want to run them at 1000mA. So here is what it would look like.

[img]../scripts/resistor.jpg[/img]

Tim
The Custom Saber Shop

[supertrogdor]

thanks tim, i think that will make the largest difference in what i have been trying to accomplish
cheers

[gundamaniac]

Sorry, I know this is off topic, but I didn't want to start another thread. Does it matter what guage wire we use? Or would something generic be good enough?

A Jedi gains power through understanding; a Sith gains understanding through power.

[james3]

For everything we do it is low current enough that you don't really have to worry although 22 or 24 stranded is very nice to work with.

Stranded is your friend when there is a lot of bends or constant moves such as pulling the stuff in and out.

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