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Author Topic: So what (grin) resistor should i use for this LED then? :D  (Read 2259 times)
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What you reckon? 1/4watt resistor? LMAO...

http://www.ebay.com.au/itm/Cree-XPG-100W-High-Power-LED-100-Watt-Cool-White-Warm-White-Light-13000-Lumens-/160885000696?pt=AU_B_I_Electrical_Test_Equipment&hash=item25757e49f8

This is were I stick a voltage regulator, dial in a voltage knowing my LED wont pull more than X amount of Amps, but I always get the Ammeter out, and Mesasure the Current as I slowly increase the voltage, once i hit, 1amp, 2amp, 3amp, i stop...

But I can't get over how defensive people get over using Resistors With LED's...  you going to make me use one? smiley-grin


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voltage is not your problem, especially on high powered leds, bonus for arrays

as they heat up your current draw will continue to rise, great set them for a voltage, its going to be the wrong current 10 min later, its going to be way off an hour later, you going to sit there fiddling with a knob adjusting hundreds of milivolts as currents continue to increase?

my advice, a constant current driver

what current are you trying to keep the panel at?
« Last Edit: January 19, 2013, 11:43:20 pm by Osgeld » Logged


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I don't have a problem.... or a question.

I've had a 10watt LED running at 1amp (no current regulator) quite nicely, nice stead 9.7v for the past 2 hours, it's hot to touch but it's just fine, i'm using a 30watt CPU cooler, working just fine.


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I don't have a problem.... or a question.

I've had a 10watt LED running at 1amp (no current regulator) quite nicely, nice stead 9.7v for the past 2 hours, it's hot to touch but it's just fine, i'm using a 30watt CPU cooler, working just fine.




That's cool. As long as other people understand that driving such high power leds 'correctly' is via a constant current driver, not a constant (or even adjustable) voltage regulator. As you have found out that is not a fixed requirement just the recommended best engineering solution, and that is all that needs to be said on the matter.

Lefty
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nice stead 9.7v for the past 2 hour

I have had LEDS run at all sorts of steady voltages, but again voltage is not the issue, great you have a 9.7 volt supply, whats the current when you first turn it on, whats the current after 2 hours

does it exceed the heat or current ratings?

you dont know or else you would have provided numbers

I am not going to bash you, but "its works and I can touch it" is useless in the real world, watch them dim 40% in a month, I dont care
« Last Edit: January 20, 2013, 01:27:05 am by Osgeld » Logged


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I'm not entirely sure how it would dim?...

The LED's max current exceeds anything the cheap switching regulators can provide, the regulator would kill itself before this 100watt LED,  you might peak at 30watts but the regulator will shut down
before these LED's will, I should know i have several "Direct Drive" which run directly from 4.2v lithium batteries, the SSC P7 and Cree Q5, both perfectly fine after thousands of hours...


if your regulator can handle a max of 50 watts, just how would it kill an LED that can handle 100watts, even if you put your regulator to the same voltage as your input 12v+ more, you'll find the regulator struggling like hell because the LED can go much higher providing it's adequately heat sinked.... 
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well theres no such thing as a 100 watt LED, you have a panel of led's

and all led's dim over time, but hey you got it figured out, so why argue right

heck why did you post?
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You're avoiding the 'how' it will dim....

but that's ok.... we're not living in 1999 anymore providing the voltage is constant so will the current draw .... it maches the datasheets data.





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we're not living in 1999 anymore providing the voltage is constant so will the current draw .... it maches the datasheets data.

Yes, that's right. 
Congratulations for having exposed the "Resistor Conspiracy", a hoax manufactured and perpetrated by the hide-bound "EE" Theocrats - Create Your Own Reality!
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The Op is basing  this on the total leds looking like 10 ohm load 30 volt 3 amp 10 ohm.

You set it as the leds came on  And it worked it will for now but it may not on the next set.

I played with current driving leds and the voltage idea your using.

I did it like this I hooked 10 red leds in series I supplied 1.5 to 25 volts as I turn the pot led 1 came on
the supply used  lm317   turn it up led 9 came on went for led 10 and on it came.

I sat there and said cool till they popped like popcorn but my supply could put out 3 amps your is limiting the current.  
« Last Edit: January 20, 2013, 11:07:55 am by be80be » Logged

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You're avoiding the 'how' it will dim....

they dim just by being lit up, all led's will, go look at a 20 year old alarm clock

Quote
but that's ok.... we're not living in 1999 anymore providing the voltage is constant so will the current draw .... it maches the datasheets data.

Here is a little snippit from a chart I did for work, these were not cree led's but it should illustrate what I am saying. As the heat in the led's increased so did current, after 87 hours or so it was over 50ma more. Is this significant in your design and parts? I cant tell you that, but I can tell you that it was a constant 3.3 volts and currents were not constant for many hours. It did eventually reach a point where the line went more or less flat, but that was days after first being lit, nearly about 80 ma more than where they started.



* chart.jpg (39.29 KB, 332x335 - viewed 21 times.)
« Last Edit: January 20, 2013, 03:32:43 pm by Osgeld » Logged


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Hey, he reached equilibrium before disaster.

I've read that new leds and new solar cells lose about 10% in less than a year with normal use but the loss rate slows down after that.

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You're avoiding the 'how' it will dim....

they dim just by being lit up, all led's will, go look at a 20 year old alarm clock

Quote
but that's ok.... we're not living in 1999 anymore providing the voltage is constant so will the current draw .... it maches the datasheets data.

Here is a little snippit from a chart I did for work, these were not cree led's but it should illustrate what I am saying. As the heat in the led's increased so did current, after 87 hours or so it was over 50ma more. Is this significant in your design and parts? I cant tell you that, but I can tell you that it was a constant 3.3 volts and currents were not constant for many hours. It did eventually reach a point where the line went more or less flat, but that was days after first being lit, nearly about 80 ma more than where they started.



80ma?

80ma is barely enough to read a book with...


these take 8000ma  eaay
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80ma increase, is that 80ma MORE than the led can handle? In my case no

the point is the led was fed a constant voltage and its current consumption increased over time, you would see that if you bothered to read what I had said instead of bragging about your led's

« Last Edit: January 20, 2013, 06:42:35 pm by Osgeld » Logged


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its current consumption increased over time,

It is less a function of time, but a function of rising temperature and being driven by a constant voltage source. Even if driven by a voltage source, the rise in temperature will eventually stop, as the device reaches its thermal equilibrium. The question really is if that equilibrium is still within the device's performance envelope.
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