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Topic: Running high power led's without drivers. (Read 7895 times) previous topic - next topic

carl1864

I have a couple of questions about these high power LED's, such as the cree xml-t6 and some of the others.

1. Are drivers really that important?  I googled this quite a bit before asking, and everyplace recommends a using a driver, saying led life will be shortened without one, and power won't be as steady, etc.  But for something simple like a flashlight, where brightness is more important than steady light, is it really that important? 

1A. Wouldn't it be somewhat of a benefit running it without a driver, but with a good heatsink, to make it even brighter, sort of like overclocking a computer? 

1B. Even if life is reduced a bit, is it as big of a deal as people say?  If the typical LED has about a 100,000 hour lifespan, or something huge like that, even if I reduced it to 1% of its lifespan, that's still 1000 hours, by that time whatever device I'm using it on will likely be obsolete anyways, or the LED would be obsolete, replaced by a better one. 

2. I bought some other 3w led's that say they are rated at 700ma.  I also bought a driver that is supposed to put out 2800ma.  I wired up the driver, and had 4 of these led's in parallel, and using a 18650 battery (3.7 volts, 4.2 on full charge). Should total 2800ma, which is what the driver is rated at.  However, at full power, it only seems to be drawing about 700ma for all 4 of them, and when I hooked up the 4 led's directly to the battery, they are only drawing about 1 amp, when the specs say they should be drawing 2.8 amps.  Can anyone help explain why these numbers aren't adding up?

Thanks.

Grumpy_Mike

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1. Are drivers really that important? 

Yes.

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1A. Wouldn't it be somewhat of a benefit running it without a driver,

No.

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1B. Even if life is reduced a bit

How about a lot?

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Can anyone help explain why these numbers aren't adding up?

LEDs are non linear devices they don't add up like simple linear loads. You can't put LEDs directly in parallel because they will not share the current. This is because the voltage drop is not identical on each LED even of the same type.

retrolefty


I have a couple of questions about these high power LED's, such as the cree xml-t6 and some of the others.

1. Are drivers really that important?  I googled this quite a bit before asking, and everyplace recommends a using a driver, saying led life will be shortened without one, and power won't be as steady, etc.  But for something simple like a flashlight, where brightness is more important than steady light, is it really that important?

Yes, really important if you don't want to just burn up components

1A. Wouldn't it be somewhat of a benefit running it without a driver, but with a good heatsink, to make it even brighter, sort of like overclocking a computer? 

The current through the led must be controled by something external to the led. A heatsink doesn't control current.

1B. Even if life is reduced a bit, is it as big of a deal as people say?  If the typical LED has about a 100,000 hour lifespan, or something huge like that, even if I reduced it to 1% of its lifespan, that's still 1000 hours, by that time whatever device I'm using it on will likely be obsolete anyways, or the LED would be obsolete, replaced by a better one.

Simply put, operating higher power leds without some form of external current control will cause the LEDs to melt in short order. This can happen is mere seconds. Discuss how you propose to control the LED currents by means other then a constant current driver module and we can discuss the merits plus and minus. But having no control of current is off the table. 

2. I bought some other 3w led's that say they are rated at 700ma.  I also bought a driver that is supposed to put out 2800ma.  I wired up the driver, and had 4 of these led's in parallel, and using a 18650 battery (3.7 volts, 4.2 on full charge). Should total 2800ma, which is what the driver is rated at.  However, at full power, it only seems to be drawing about 700ma for all 4 of them, and when I hooked up the 4 led's directly to the battery, they are only drawing about 1 amp, when the specs say they should be drawing 2.8 amps.  Can anyone help explain why these numbers aren't adding up?

You would have to show me a wiring diagram to see what is what. But general rules using a constant current driver are:

1. You can't drive power leds in parallel with one driver as there is no means to ensure equal balanced sharing of the current.
2. You can use a 700ma driver to power more then one 700ma led if wired up in series, but how many is determined by the maximum output voltage the driver can go to in maintaining the 700ma constant current drive.
3. A constant current driver rated at 2800ma output is not useful for 700ma rated LEDS without lots of parallel resistors to waste the difference of 2800 to 700 in heat dissapation, a real waste of power unless you need a light/heater appliance.

Lefty

Thanks.

dhenry

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1. Are drivers really that important?


It depends. Drivers are useful in terms of efficiency, and longivity / thermal management. But they are not absolutely necessary in all situations.

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1A. Wouldn't it be somewhat of a benefit running it without a driver, but with a good heatsink, to make it even brighter, sort of like overclocking a computer? 


heatsinking doesn't fundamentally solve the problem. Drivers are necessary / desirable because they solve a very important problem: LED's forward voltage drop goes down with temperature and a little change in voltage can generate tremendous amount of change in current. So a little bit of heat reduces Vfwd. That leads to more current -> more power dissipation from the led -> higher led temperature -> even lower Vfwd -> ...

[quot]1B. Even if life is reduced a bit, is it as big of a deal as people say?  [/quote]

Your reasoning is sound.

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Can anyone help explain why these numbers aren't adding up?


Too many possibilities: your battery may not be delivering the voltage specified to generate the current, etc.



fungus


I have a couple of questions about these high power LED's, such as the cree xml-t6 and some of the others.

1. Are drivers really that important?  I googled this quite a bit before asking, and everyplace recommends a using a driver, saying led life will be shortened without one, and power won't be as steady, etc.  But for something simple like a flashlight, where brightness is more important than steady light, is it really that important? 


Yes. LEDs require a certain current, not a certain voltage.


1A. Wouldn't it be somewhat of a benefit running it without a driver, but with a good heatsink, to make it even brighter, sort of like overclocking a computer? 


It won't get much brighter. LED light output follows a curve and when you get close to the max rating you start to get more heat than light.


1B. Even if life is reduced a bit, is it as big of a deal as people say?  If the typical LED has about a 100,000 hour lifespan, or something huge like that, even if I reduced it to 1% of its lifespan, that's still 1000 hours, by that time whatever device I'm using it on will likely be obsolete anyways, or the LED would be obsolete, replaced by a better one. 


Why don't you just get a higher power LED?


2. I bought some other 3w led's that say they are rated at 700ma.  I also bought a driver that is supposed to put out 2800ma.  I wired up the driver, and had 4 of these led's in parallel, and using a 18650 battery (3.7 volts, 4.2 on full charge). Should total 2800ma, which is what the driver is rated at.  However, at full power, it only seems to be drawing about 700ma for all 4 of them, and when I hooked up the 4 led's directly to the battery, they are only drawing about 1 amp, when the specs say they should be drawing 2.8 amps.  Can anyone help explain why these numbers aren't adding up?


You've damaged your LEDs with all the 'experimenting'?
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

dhenry

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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.

fungus


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1. Are drivers really that important?


It depends. Drivers are useful in terms of efficiency, and longivity / thermal management. But they are not absolutely necessary in all situations.


If you want to work near the maximum light output you need a driver.

A resistor simply isn't good enough when you're on part of the curve where a variation of 0.1V can double the current.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

dhenry

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a variation of 0.1V can double the current.


I trust you on that. But it would be helpful if you can show us all a high power LED where a variation of 0.1V can double the current near its rated output levels.

Just one example would be sufficient.

fungus


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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.



Sure, but it's almost vertical. Here's the curve for the 3W LEDs I installed in my hallway:



It doesn't take much extra voltage to overheat one.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

fungus


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a variation of 0.1V can double the current.


I trust you on that. But it would be helpful if you can show us all a high power LED where a variation of 0.1V can double the current near its rated output levels.


Well, OK... 0.1V might be hard to find. ;-)

0.3V though? No problem. See above...
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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)
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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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