confusing CREE XP-G datasheet

[jureivis]

Hello,

so i decided to build a led setup. Checked data sheet of Cree XP-G to find out Vf at 1000mA and that is what i found:

in data sheet of XP-G called "CLD -DS 20 REV 12A" it is 3.15V at 1A, but at the same time i found data sheet of XP-G called "CLD-DS20 REV 4" where voltage goes up to 3.3V at 1A.

As i need to calculate voltage in my led string - i need to be exact. How can i find out which product (with which voltage at 1A) did i buy? Purchase was done on January 2013 (if it helps).


Thanks in advance.

 

[mcgyvr]

just use 3.3V and call it a day.. Its close enough for your purpose and highest number is always the safest to use..
If you really want to be anal you must realize each color will have a different Vf. Then depending on heat sinking/temp the Vf will change.

 

[TropTrea]

Hello,

so i decided to build a led setup. Checked data sheet of Cree XP-G to find out Vf at 1000mA and that is what i found:

in data sheet of XP-G called "CLD -DS 20 REV 12A" it is 3.15V at 1A, but at the same time i found data sheet of XP-G called "CLD-DS20 REV 4" where voltage goes up to 3.3V at 1A.

As i need to calculate voltage in my led string - i need to be exact. How can i find out which product (with which voltage at 1A) did i buy? Purchase was done on January 2013 (if it helps).


Thanks in advance.

This is why you need to run Current drivers rather than voltage. At 1.0 Amp you could have one that is using 3.2 Voltts and anotther usung 3.3 volts. But the real important thing is if you run them at 3.3 volts one might be using 1 Amp of current and another might be running 1.6 Amps of power.

It is the nature with High intensity led's that a very small change in voltage can create a very drastic change in current. A few years back I did some testing as I wanted to run them on voltage supplies as well. While it is a different LED I tested these are my results.

350ma = 2.85 Volts = 0.99 Watts
500 ma = 2.95 Volts =1.47 Watts
700 ma =3.05 volts = 2.13 Watts
1000 ma = 3.10 volts = 3.01 Watts
1,200ma = 3.14 volts= 3.77 Watts
1,500ma = 3.20 Volts = 4.8 watts.
2000ma = 3.23 Volts = 6.46 watts (burnt out at after about 5 minutes of operating time.)

Now with a low cost power supply it is possible to get a 5% voltage difference with line fluctuations. So if you were to tune this say 3.10 volts the voltage could varry between 2.94 volts and 3.25 volts. Thius could result in a varience of running the LED between 1.4Watts and over 6 watts of power. While with a 5% variants of current and a goal of 1000ma the range would be 2.85 Watts to 3.15 Watts.

 

[jureivis]

Thanks for information.

I will try briefly describe what i am up to. I am going to use a 48v mean well power supply which will feed a string of 10 XP-G and 2 RB. According to my calculation all 12 will going to use 40.4V (10x 3.3 of XPG + 2x 3.7 of RB). to cut off the rest of 7.6V i am going to use an adjustable resistor which i will set to 7.6 Ohm. Does the adjusted resistor not make sure that the current stays at 1Amp??

 

[nemosworld]

Thanks for information.

I will try briefly describe what i am up to. I am going to use a 48v mean well power supply which will feed a string of 10 XP-G and 2 RB. According to my calculation all 12 will going to use 40.4V (10x 3.3 of XPG + 2x 3.7 of RB). to cut off the rest of 7.6V i am going to use an adjustable resistor which i will set to 7.6 Ohm. Does the adjusted resistor not make sure that the current stays at 1Amp??

assuming you mean one of these:

you will need to add some drivers to your build, you cannot add leds directly to that power supply. the preferred choice of drivers today are LDD led drivers:
http://www.onlinecomponents.com/mean-well-ldd1000h.html?p=43544930
needs to be soldered to a custom designed pcb board:

or

wired version, no pcb needed.
http://www.onlinecomponents.com/mean-well-ldd1000hw.html?p=43920484

 

[TropTrea]

Thanks for information.

I will try briefly describe what i am up to. I am going to use a 48v mean well power supply which will feed a string of 10 XP-G and 2 RB. According to my calculation all 12 will going to use 40.4V (10x 3.3 of XPG + 2x 3.7 of RB). to cut off the rest of 7.6V i am going to use an adjustable resistor which i will set to 7.6 Ohm. Does the adjusted resistor not make sure that the current stays at 1Amp??

What is the nomenclature on the meanwell driver? With your combination of Chips I would recommend an LPC-60-700 or LPC=60-1000. These are current driver not voltage drivers.

The problem with a voltage driver is that the resistance of the LED's is not a constant. At different currents they will produce different resistances. Simularly when the warm up there resistance changes. So when you have them running at 3.3 volts When they warm they will pulling under 3.3 Ohms each for a total of 3.3 Watts. But if your voltage drops to 3.2 Volts there resistance might move up to 5 ohms resulting in 640 ma draw or 2 Watts. But the real danger hits if you get a small spike to even 3.4 Volts resulting in the resistance dropping to 2.5 Ohms meaning you will be drawing 1.28 Amps or getting 4.35 Watts which could eventualy burn out the LED.

While the XP-G's will be more forgiving as long as they don't go over 5 Watts the Royal Blues may not be. Then you need to keep in mind that if one LED shorts out for you it will cause the others to get more voltage which will cause a chain reaction. And istead of just one LED going bad for you now you burnt out the entire string.