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Topic: Running high power led's without drivers. (Read 7 times) previous topic - next topic

retrolefty


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You've damaged your LEDs with all the 'experimenting'?


Most of the high power LEDs actually have a v-i curve that looks far more like a resistor than a diode, once they start to conducting.



The curve just posted here doesn't seem to show a linear V/I relationship to me?

fungus

#11
Oct 05, 2012, 09:25 pm Last Edit: Oct 05, 2012, 09:36 pm by fungus Reason: 1
While I'm posting pictures, here's the graph of light output vs. current for the same LED.



To get to 20% extra output you'd have to push about 1100mA though it. That's 60% more current than it's rated for.

...except you wouldn't ever get 20% more light because it'll run burning hot and light output goes down as temperature rises. According to the datasheet you get 30% less light at 100 degrees C.



This is why trying to 'overclock' LEDs is a doomed idea.

(unless you're dunking them in liquid nitrogen - cooling them down actually gives more extra light than increasing the current)
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Grumpy_Mike

Here is the voltage / current curve from the LED that the OP mentioned in the first post.
You will see that the current goes from 2.2A to 3A with only a 0.1V increase in forward voltage.


dhenry

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0.3V though?


Sure. Your point is still valid that a small change in Vfwd can cause a large change in If. It is just that the magnitude was a little bit too much, and too "exponential".

That "exponential" relationship holds true for regular diodes - if you look at their v-i curve, you will see that they are not just very vertical, they are progressively vertical.

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You will see that the current goes from 2.2A to 3A with only a 0.1V increase in forward voltage.


3/2.2 = ?

fungus


Here is the voltage / current curve from the LED that the OP mentioned in the first post.
You will see that the current goes from 2.2A to 3A with only a 0.1V increase in forward voltage.


Another thing: The voltage range for (eg.) my LED at 700mA is:

Min: 2.79   Typical: 3.55   Max:3.99

Q: Given the above curves and the variance in voltage, what value resistor should you pick...?   A: Don't even try.


This is also why you don't commit sin number (2) of the original list, ie. connect LEDs in parallel.

Q: What happens if you pair a 2.79V LED with a 3.99V LED?    A: The 2.79V LED doesn't last very long.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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