overdrive LEDs?

How much overdrive is safe for an LED?

Accepting that you will shorten life, can you overdrive an LED rated at T to 9 volts? If so, how much does it shorten life and change to color?

Bill

First of all, LEDs are current driven devices. You never talk in terms of driving an LED at a given voltage. Second, no LED can be operated at 9V. This is instant disaster.

As for overdrive, for 5mm LEDs the maximum continuous current is usually 30 mA. If heat build up isn't a big problem then you can go to 50 or 60 mA for short periods. There is little point going beyond that though since any further increases result in only a little more (<10%) light. Past about 80 mA you actually get less light due to the die heating up. As for power LEDs like Luxeons the best way to determine the maximum drive current is to mount it on whatever heat sink you plan to use, and increase the drive current in small steps. Measure the output with a light meter after the whole assembly has had time to come up to temperature (this might take a few minutes or more between steps). When you reach a point where there is no further light increase back off current to the previous step. That is the maximum value you should use. While there is sometimes a gain in output by overdriving Luxeons, there seems to be little point in overdriving Luxeon IIIs or Vs. With most heat sink assemblies they put out the most light they're ever going to well before even maximum rated current. Indeed, even with an infinite heat sink a Luxeon III doesn't put out any additional light past about 1.3A.

Lifetime? Let's just say that it decreases exponentially with current. Regular 5mm whites will dim to half brightness in 6000 to 10000 hours even at 20 mA. If you run one at 60 mA you can expect it to last only a few hundred hours before dimming to 50%.

i just fried my LED lenser V2 1.2W CR123 (3V-3.5V) by putting a rechargable lithium in it(4v-4.2). it was greenish tint so i thought i'd give it a better tint with a little overdriving, which by the way it did (perfect white) but it only lasted about 20 seconds before it gave up on me.

so 9V will defenitly fry your led in a flash.

oh well i guess ill put a 3watter in and a small resistor to be safe. ill even have a better light after im done, i guess it's good to look on the bright side of life.

well i think all depends on the quality of the led... for example:

i have blow few cheap chinese led-keychain in few minutes drivig them with the original batteries...

other on this forum have blow their MXDL 3W after few hours with the standard batteries...

but i have drive the key-mate at more than 120ma with any problem

20000mcd white leds at 90ma for hours with only a bit of heat

lux-I at 800ma

lux-III at 1600ma

lux-V at 1500ma (for short period of course /ubbthreads/images/graemlins/grin.gif) but it still working well

Bill,

LEDs work as current-driven devices, and the voltage-current curve is VERY steep. Along the lines of 0.1V per 100ma in-spec. Also, as they heat up, their resistance drops so unless there is a current regulator circuit, you could go into thermal runaway and fry the Lux.

Incans work more as voltage-driven devices. You supply the right voltage and the current that flows through the bulb is the result of that voltage.

Yes you can overdrive leds, but not to the same degree as incans. Also as mentioned above, there is a rapid drop-off in rate of return past the spec current. With spec current of 700 ma, you can usually drive a LuxIII orV to 1000ma or maybe 1300ma with good heatsinking. Past that and you're just heating up the led with no real gain in lumens. Remember that that is still 15% to 85% overdrive!!!!

All that said, 9V supplied to a Lux will be almost instant death, except for maybe a W-bin-Vf LuxV at the high end of the Vf scale.

Did I read this post from Amonra right?: "it was greenish tint so i thought i'd give it a better tint with a little overdriving, which by the way it did (perfect white) but it only lasted about 20 seconds before it gave up on me."

Is it possible to alter the tint of an LED by overdriving it? Does it only work from greenish to whitish? The reason I ask, besides idle curiousity, is that I have a Nuwai Q3 that was on the greenish side, but when I started driving it with 3.7v rechargable Li-ion batteries I swear I thought the tint got slightly whiter. It was not a huge difference, though, so I thought my eyes might be playing tricks on me. Did I just experience happiness?! /ubbthreads/images/graemlins/ooo.gif

Yes, white LEDs become slightly bluer as you overdrive them but the change isn't that profound unless you reach levels which blow out the LED in short order.

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davidefromitaly said:
but i have drive the key-mate at more than 120ma with any problem

20000mcd white leds at 90ma for hours with only a bit of heat

lux-I at 800ma

lux-III at 1600ma

lux-V at 1500ma (for short period of course /ubbthreads/images/graemlins/grin.gif) but it still working well

[/ QUOTE ]
Why? In all of those examples except for the Luxeon I you're not gaining any more light by driving at the levels you mentioned. For 5mm LEDs you typically don't get any more lumens past 60 mA. In fact 40 mA is usually a good place to stop as anything past that is well into the curve of diminishing returns. The Luxeon III only gives more light up to about 1.3A or 1.4A, and then only on an infinite heat sink. In the real world you probably gain nothing going past the rated current of 1A. The situation is even worse for Luxeon Vs since they produce more heat. Also, since they already have a short lifetime even at rated current overdriving makes absolutely no sense from any standpoint.

I'm not criticizing the fact that you overdrive LEDs but rather the levels you overdrive them to. The only purpose of overdriving is to get more light. When you overdrive to levels where you get the same or even less light then all you're doing is making heat and depleting batteries. I've heard of people here overdriving a Luxeon III to 2.4A and I cringe. There is absoluely no logic behind it. They would get the same amount of light at probably 700 mA and have about 4 times the runtime (and a few hundred times the life) . Please people, let's have some sanity checks on the overdriving. /ubbthreads/images/graemlins/banghead.gif

this topic interests me as well because im interested in implanting one of the MJLED's(lambda) into the rayovac 3-in-1 headlamp. $12 headlamp with white led, red leds, then a bi-pin based xenon. headlamp uses 3 AAA batteries(4.5 volts) and the MJLED is supposed to only be used for up to 3.6volts /ubbthreads/images/graemlins/frown.gif and i don't really one to be stuck with having to use Nimh's.

jtr1962: i haven't a luxmeter but at my eyes when i overdrive a led i obtain more light.

of course isn't like a incandescent (that output about 100% of light with only 20% more voltage) but when i drive a lux-III at 1600ma is much more bright than at 800ma

and a lux-V VV1T at 1500ma is bright like a TL-3 xenon /ubbthreads/images/graemlins/cool.gif

ok the life of the lux-V will be very short but isn't a problem for a flashaholic /ubbthreads/images/graemlins/wink.gif i hope to replace with a WV0T /ubbthreads/images/graemlins/mad.gif

I'm sure you do get more light at 1600 mA than you do at 800 mA but my point is that you would probably get the same amount of light at maybe 1100 mA or 1200 mA. With overdriving it's best to increase in small steps, and then stop when it no longer keeps getting brighter. As for overdriving the Luxeon V, the life of this part at 700 mA is already only 500 hours. At 1500 mA it's probably a few tens of hours. I personally don't consider lifetime like that acceptable even in a flashlight but to each his own. I doubt you'll be able to find a WV0T either. It looks likely to me that Lumileds will discontinue production of the Luxeon V once efficiency of the Luxeon III is increased enough to put it in the same lumen range. That is likely to happen this year. In any case I consider the Luxeon V an overpriced dinosaur that I wouldn't touch with a ten foot pole for any application.

It seems to me Lamina Ceramics has much better overdriving possibilities than anything from Lumileds. I was still getting increases in output overdriving their BL-2000 white at nearly 20W (nominal is 4.7W). Of course, you need a monster heat sink for this.