What's acceptable LED overdrive?

What\'s acceptable LED overdrive?

My Lightwave 4000 drives 10 LEDs at 26 milliamps per bulb.
My Trek drives its 7 LEDs at 8.7mA each.
My LED reading lights vary from 3.2mA to 11.5mA per bulb (separate posting in General Lights).

So is there a good way to determine what these various currents will do to the life of the bulbs. Some of these bulbs are rated a 100,000 hours, but of course they will lose a lot of their brightness over that period. Will overdriving LEDs accelerate that dimming?

Re: What\'s acceptable LED overdrive?

Heat is what kills LEDs more than the current fed to them.
Some lights overdrive their LEDs to 100mA apiece; when that is done to a typical white LED, it will emit that characteristic "pissed off blue color" (a distinct shade of turquoise blue), and probably won't live very long. If the heat generated by that current is not sinked away, the LED will last approximately 90 seconds to 3 minutes. But if the cathode (the lead that goes to the LED's die cup) is well heatsunk, the LED lifetime could be extended to many hundreds of hours, instead of a hundred or so seconds.

Lights that drive their LEDs to 50mA apiece or less won't have these kinds of problems. Most modern 5mm LEDs are rated at 20mA in order to reach that 100,000 hours to half intensity mark. Driving them harder will decrease that life, but to what degree depends largely on the amount of overdrive you apply, and what kind of heatsinking the LED has.

Re: What\'s acceptable LED overdrive?

yep
i think leddie has sum it all up.
typical rated current for led is about 20 mA, but most folks can get away with 50 mA as long as it is properly sunk.
when led reach the 100,000 hrs mark (or less depending on overdrive) that means their brightness will be approximately half of the previous brightness.

Re: What\'s acceptable LED overdrive?

Thanks The LED Museum and Shiftd. The bulbs on the Trek (7 LEDs at 8.7mA each) are running about 65 degrees F in a cool, unheated cabin, which is about 55F. The Lightwave 4000 (10 LEDs at about 26mA each)is running at 100F.

Bulb temperatures on my 36-LED reading light vary from 145F to 215F. This bulb has no heat sinking, and seems to me to be running way too hot. Josey

Re: What\'s acceptable LED overdrive?

Josey, I always heard that Tek-Tite (Trek) overdrove the heck out of its leds. Did they wise up and add some resistors? It's possible your meter added some resistance.

I do think it's time for Lumileds or some other led manufacturer to make some "high power" 5mm leds rated to be run at the 70 mA level. The blue tint of the standard white-led Arc AAA and CMG Ultra, as well as of 2-cell coin lights, seems caused by overdriving. The color from a recent CMG Infinity that I bought, and from a white coin light running on 3 volts, doesn't have that tint.

Re: What\'s acceptable LED overdrive?

One of the problems with my mA/LED calculations is that I don't know if the LEDs are hooked up in parallel (would would split the current) or if they are in series (in which case the current would be constant through every series bulb). It would be nice if we had at least a simple
schematic for these lights.

I doubt that my Fluke affected the amperage, since I hooked it up in series and series current does not change.

The 36-LED light with 12 62 ohm resistors seems to be 12 parallel circuits (3 bulbs in series per circuit) driving each bulb at about 34 mA, but the 200+ temperature has me worried.

On what I called my Trek (CC Crane 7LED): It was running quite a bit cooler than my 10-LED Lightwave 4000, so I doubt that the Trek is overdriven, at least relaive to the LW. Josey