LED Current Ratings

[tom_pay]

Hi All,

I have decided to make a LED torch. I have an old high powered Star LED and wanted to use it. It is rated for a maximum current of 500mA, and delivers 80 Lumen. I wanted just a little bit more light power.

I would like to know what governs the LED's current rating. That way I might be able to get around it and make it just a bit brighter.

Is it possible to get more than the maximum light output of this LED, or is the maximum value the maximum value?

Thanks

Tom.

 

[Reloadron]

Is it possible to get more than the maximum light output of this LED, or is the maximum value the maximum value?

Absolutely! For a very, very short period of time which can be milli-seconds. At that point the LED is very dark and will remain so forever.

LEDs work on current. Running a LED a little below it's rated current will produce less light. Running the LED at its rated normal forward current will produce its rated light and assure a long prosperous life for it. Running a LED above its maximum ratings will destroy it. It may run a little above rated current for a short time but that is about it.

The solution for more light from a LED is a larger higher rated LED.

Without a circuit of exactly what you have that is about it.

Ron

 

[Boncuk]

Agreed with Ron,

better use the maximum forward current (If) instead of increasing the max forward voltage (Uf).

LEDs are current sensitive and you should find a way not to exceed that current. The forward voltage (Uf) will adjust itself if the correect forward current (If) is supplied.

There are simple ways to make a constant current source like the LM37xx.

Regards

Boncuk

 

[tom_pay]

Hi

I was going to use a LM3405A, which is a specialist LED driver and wire it up as per the data sheet. It runs at about 1.6Mhz.

Do you reckon I would be able to get it to go a bit more out of it?

Thanks

Tom

 

[MikeMl]

Not unless you figure out how to put a more efficient heatsink on the backside of the LED, like liquid cooling or liquid nitrogen.


Even the existing rating is based on having the Luxeon bolted to a HUGE heatsink, which you likely don't have (or even know that you need). Running without a heatsink, you will vaporize your Luxeon unless your cut the power input to 100mA or less.

 

[cowana]

You might well find that you can't run it at 500mA - at that value, it will heat up pretty quickly. Unless you have a large heatsink/forced cooling to get rid of that heat, you'll have to reduce the current going to the LED to stop it burning up.

 

[Reloadron]

Hi Tom

OK, lets look at the below quote:

LED ratings are specified by current, not voltage. For longest life, we recommend you run them at 20-25 milliamps (ma). HOWEVER, in our LED flashlight conversions (and many commercial LED flashlights), the LEDs are run at 50-60ma, twice the rated current. One of our test LEDs ran at 98ma for over 200 hours without damage or appreciable light loss. So go ahead and experiment with running them at over rated current if you are willing to take the risk of a shorter life. In my opinion, a flashlight bulb that lasts 100 hours is a huge improvement and cost saver over the incandescent alternative which gives only 15-20 hours before it dies.

That quote was taken from here.

Now let's look at your original post:

I have decided to make a LED torch. I have an old high powered Star LED and wanted to use it. It is rated for a maximum current of 500mA, and delivers 80 Lumen. I wanted just a little bit more light power.

I would like to know what governs the LED's current rating. That way I might be able to get around it and make it just a bit brighter.

Is it possible to get more than the maximum light output of this LED, or is the maximum value the maximum value?

This comes down to what you see is what you get. Now using your constant current buck driver the LM3405A you can adjust the value of R1 to drive as high as 1 amp. So what will happen if we adjust that driver for 750 mA? The LEDs will get brighter. I would venture for not very long though. Run it up to 1 amp. They will get much brighter, for how long? Beats me. The point is you have LEDs with a maximum current rating of 500 mA. You exceed that and bad things can happen. With a maximum rating of 500 mA it would be unwise to run them at the maximum to begin with, let alone exceed it. The driver has nothing to do with it. The end result will be the same.

What you are being told is it would be unwise to exceed the maximum ratings. Now if you choose to do so, cool with me but you were advised against it. They are after all your LEDs to use or burn up as you choose. Go for it! When they fail you will have a lesson learned.

Ron

 

[MikeMl]

But the main point is: to get even 500mA, the LED has to be bolted to a BIG heatsink.

 

[cowana]

Very tue. Very roughly, from a hobbiest point of view, it's heat that kills LEDs, not current. Current causes the heat.

 

[Reloadron]

But the main point is: to get even 500mA, the LED has to be bolted to a BIG heatsink.

Absolutely agree Mike. The higher the current, the greater the heat and the sooner the end. Going to need a big heat sink just for the LEDs to run at their maximum current. Sans cooling they would have a very short life I would venture running at their rated maximum.

You want more light? You going to need bigger, brighter LEDs and those LEDs need to be properly heat sinked. Failing that they will be real bright for a short period and darkness will prevail.

Ron

 

[Russ Hensel]

Very tue. Very roughly, from a hobbiest point of view, it's heat that kills LEDs, not current. Current causes the heat.

Sort of, actually current x voltage, less light output.

 

[tom_pay]

Ok Guys,

Thanks very much.

You say BIG heat sink, what defines big?

I have glued a 10mm x 20mm piece of old CPU heat sink to the back of it. Will this be enough to run it at 400mA?

Thanks Heaps for your help, I really appreciate it.

Tom

 

[Mosaic]

Tom....here's the deal. 500mA at a 2V pd. at 50% efficiency u gonna get .5 Watt in heat and the rest as light.
Do u know the thermal resistance of the LED....? If not u can't calc the temp rise. So u gotta play it safe.

Slap the baby on a real CPU heatsink including fan. Use thermal grease/glue and I'd say you have a bright future.

 

[RMMM]

Glue is not what you want to use to attach the LED to the headsink. You need thermal paste/grease.

The glue will work as an insulator, keeping the heat IN THE led,

Thermal grease gets the heat AWAY from the led and into the heatsink.

 

[#12]

How long will it last if you do it right? I just had my first LED failure under normal operating conditions. The "on" light on my Onkyo audio amplifier died about a week ago. I bought that amp in 1977 and used it as my primary home stereo amp for all that time. It has been continuously on, 24/7, for the last 3 years.

The LED in the temperature controller I built in 1978 has not failed, but I only use that for 3 months a year.

I'm not saying I haven't smoked a few LEDs in the course of experimenting. I'm saying that if you do it right, you will have a very old and happy LED, many years from now.

 

[audioguru]

I have used many name-brand LEDs for many years and have never had one burn out. After being on continuously for 20 years they just look a little dim.
But my son buys cheap Chinese LED flashlights and their no name-brand LEDs burn out in a couple of hours.

 

[Sceadwian]

Actually smaller value LED's like 20ma LED's don't need any heat sinking even at their max current, but that's obviously because the current going through them is so low the ohmic heating isn't of concern, but that is pretty much where the cutoff is, anything over 20ma's is a power LED and I've seen very few 'midrange' devices, I seem to see going from 20ma to 500 or more, not a whole lot in between.

 

[audioguru]

My "ordinary" 5mm red LEDs are diffused MV8190 made by Fairchild. Their max allowed continuous current is 40mA at an ambient temperature of 25 degrees C. I use them at 25mA and they are very bright and last "forever".

My Luxeon SuperFlux red LEDs are in a 4 pins case because the extra pins are for cooling. The max continuous current is 70mA at room temperature and I operate 36 of them at "only" 53mA each in a well-vented case. They got so hot they turned yellow but still work. The plastic case became crazed from the heat.

 

[tom_pay]

Hi

I have been doing some small bench tests with the LED. First of all I set it up to run at 450mA, just with a current limiting resistor. Whilst it didn't get too hot, it did get warm. I then connected it up to a 555 running at 30kHz with a 50% duty cycle. This was no where near as hot as the first test, though it was a little bit dimmer.

I think that I will use the 555 to run the LED at 450mA. It was only slightly dimmer and colder than the non pulsed LED.

Is this a good solution.

Thanks

Tom

 

[audioguru]

The max allowed output current of a 555 is 200mA so you won't be able to get 450mA without adding a current booster transistor.
At a duty cycle of 50% the average current is half the peak current and the brightness is half.