Calculating current(I) possible given power source voltage and the resistor?

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Lets say we have a 3.7V power source(Vs) and a blue XP-E2 cree LED with a Foward Voltage(Vf) of 3.4 and current of 1Amp.

Using Ohm's law[R=(Vs-Vf)/I] we have

.3 = (3.7-3.4)/1

So the best resistor available in TCSS is the .47ohm/.5W resistor

If we use the mentioned resistor are we able to calculate the actual current that the LED will get?
Using Ohm's Law again (I=V/R)
Current = Voltage/Resistance
I = V/R
3.7/.47 =~ 7.9
So current is 7.9? that doesn't make sense. Would someone be able to explain this and show how to calculate the actual current?

One more question:
If we have a 3.4V power source we can plug it directly to the LED and the current flow would be 1A correct?
So if the power source is 3.7 instead, what current would the LED draw? Would it be proportional?
3.4V*1.1=~3.7V
1A*1.1 = 1100mA
Is that right?

[FenixFire]

It would be .3/.47= .638A or 638mA. You can get lower ohms at any electronics or ham radio shop. I know my local shop has a .33 1/2W and .33 ohm 1W.

That would give you .909A or 909mA

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If that were the case wouldn't .638A be way too little current to power the LED?
The LED wants to run at 1A so .638A seems pretty underpowered yet I hear that this setup works just fine... (Blue Cree with .47 Ohm resistor and 3.7 battery)

[FenixFire]

No the normal power ratings for the Cree XP-Es are 350-700mA. 1000mA is technically overdriving them, but within reason when using quality heat sinks like provided with the led holders. It will be slightly dimmer but probably not noticeable unless you actually have a lux meter to measure it.

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So if I use a 1 ohm resistor in this case the current would be
.3/1 = .3A or 300 milliamps? That doesn't seem right..

I was under the impression we can use a 1 ohm resistor for any of the cree xp-e2's and they would still be plenty bright.

[FenixFire]

So if I use a 1 ohm resistor in this case the current would be
.3/1 = .3A or 300 milliamps? That doesn't seem right..

I was under the impression we can use a 1 ohm resistor for any of the cree xp-e2's and they would still be plenty bright.

Here is the recently updated spec sheet for the X lamps. The colored variants are on pages 10-14.

http://www.cree.com/~/media/Files/Cr.../XLampXPE2.pdf

edit: Posted link to my network file not the web access file.

[FenixFire]

Technically they will light around 175mA, amber will light around 150mA. And yes it does make sense. I have a Blue on a 1.1 ohm paired with a Cool White on a .33 ohm to make a nice silver blade. The blue FoC is on a .33 ohm.

I think what is confusing is that the current and voltage draws are not linear, nor are they as simple as Ohms law makes them appear. As the Current reduces so does the FV that the die is wanting to draw. I have a long explanation from the engineer as to how and why but I dont feel like copy and pasting a 40+ page Word document on the electrical and thermal properties and design requirements of the Xlamps. There are graphs in the document detailing the plots of the current/voltage curves.

[blur]

I see. Thanks for the info FenixFire.
Any chance you would be able to answer what the current would be if we used no resistor from the 3.7V source to the blue cree xp-e2?
I've read that people use this type of setup with no issues. I've also read that some people have over driven these crees to 1500ma for at least 1 hour without them blowing up.
http://www.fx-sabers.com/forum/index.php?topic=47222.0

[FenixFire]

I see. Thanks for the info FenixFire.
Any chance you would be able to answer what the current would be if we used no resistor from the 3.7V source to the blue cree xp-e2?
I've read that people use this type of setup with no issues. I've also read that some people have over driven these crees to 1500ma for at least 1 hour without them blowing up.
http://www.fx-sabers.com/forum/index.php?topic=47222.0

To be honest, I personally would not push them to the 1500mA. I now work with a former engineer from Cree, who worked on the development and testing of the Xlamps. He was very surprised that the colored ones were holding up to 1000mA for an extended time. That was what I was referring to by the "Updated" spec sheet. They actually only tested them out to @900mA during testing and originally had 750mA listed as their MAX. Knowing how these companies work posting the max spec at 60%-80% what it is actually capable they might actually be pushed to the 1500mA, but you will want to use a very good heat sink. Heat is the main failure, not necessarily the current...the heat is just a byproduct of the current. Same principle in the Liquid cooling GPUs to overclock the chips.

[FenixFire]

Most of the comments about running with no resistor are also coupled with using the LED power setting in the config file to set the max current to the LED. But general rule of thumb is to put a resistor on all LED dice.

Just looked at the linked post. His heatsink is a 2"+ long solid chunk of solid aluminum...that is why he is able to significantly overdrive the LED dice.