How long will an overdriven LED last?

[EWorkshop1708]

I made a small flashlight, and it uses 2 bright white 7000mcd 5mm LEDs.

It says they are rated for 3.3V nominal and 3.6V max. That's a bit low for a white LED, most are 3.5-4.5V. It says the FW current is 25mA.

I'm using an in-line resistor for each to limit the current due to the fluctuating battery voltage and 3.6V limit of the LEDs.

However, the LED seems to use more current than specs say. When using a 15 ohm resistor from 4.1V I get 3.3 on the LED, I get 3.1 on the LED at 3.8V To bring the voltage up, I went to 10 ohm instead. At at full 4.15V battery, I get 3.52V on the LED, and at a lower battery of 3.7V, I get 3.3V. Despite them running at lower than max voltage, they get a little warm, but are blindingly bright!

Even though I'm not driving them to the max of 3.6V, I'm getting 55mA of current in each LED by using 10 ohm from the battery. That's a bit higher than the rated 25mA @ 3.3V Each LED is in a small metal LED holder mounted in the case, that has a small reflector, and acts like a heatsink, because they get warm when the LEDs are on for a while. I've used them for several hours now, at least 48 or so, and they are still working so far.

I'm powering them from a 3.7V Li-Ion battery that goes as high as 4.15V at full charge, and I want the LED's to run bright down to 3.7V but don't want to burn them out. If they are fine, however, I'll leave them the way they are if they are able to take the power. They run forever on the Li-Ion battery

Any input on this? Have you folks overdriven LEDs and had them last? Why would some LEDs pull more current at nominal voltage than they are rated for?

[multi-volti]

I don't think the behavior vs voltage is meant to be precise. As with other semiconductor parameters, there is often a temperature dependence, batch variation etc. A good example is the forward voltage drop of a rectifier - textbook nominal (some say 0.6, some say 0.7V )is at a given forward currrent. If you look at a diode datasheet, it will usually plot VF vs current and temperature. It is far less than 0.6-0.7 at low current and on many diodes can exceed 1 V at high current.

Back to LED's...datasheets usually have a power dissipation for the LED junction also. Given the forward voltage increases with forward current, and you are over 100% beyond rated current, by the time you read this, , your LED may be unhappy. I wouldn't expect it to last long at all.

Some datasheets for some LED's used in pulsed applications can have a much higher forward current, but for short durations.

[The Peasant]

I've worked with quite a few LEDs in the last few years and I've never encountered a white LED rated for anything but 3.3 - 3.6 volts. However, it's the current spec that is the important one here. The more that you overdrive a LED with DC, the sooner that it will fail, but if you keep within rated current they last virtually forever.

When I design battery powered LED equipment I choose the series resistor to keep the LEDs within their ratings at the highest possible voltage that the LED will ever see. If I am using a rechargeable battery then I either set the resistor at the maximum charging voltage or I lock out the LED circuit so that it can't be energised when the battery is being charged.

Take care,
Doug

[EWorkshop1708]

I have it like that, the LED gets the highest voltage near its rating at battery full charge, then it goes down as the battery goes down.

Since there is a resistor in series with each LED, I notice the forward voltage across the LED goes down with higher temperature, and the current rises with temperature as well, and it stabilizes after a little while.

It seems like the LEDs don't get as warm on a medium or low battery charge, compared to a full charge, so the LEDs are probably fine. The LEDs only get warm and not hot when running for over 30 mins on a full battery.

[The Peasant]

I don't think that 7000mcd LEDs should produce any significant heat at all, I have 25,000mcd LEDs that do not get noticeably warm when used within spec. I would be surprised if you get much life out of them at double the maximum current and producing that much heat.

Take care,
Doug

[EWorkshop1708]

Thanks for all your replies.

So it comes down to heat. Either lower the current, pulse them, or keep them cool so they last.

I think it's helping that these LEDs have some heatsinking, but under normal circumstances, these LEDs probably can't take that much power for a long time. They have to be on for a while before they get warm, so I hope that helps with temp.

I had the LEDs on for over an hour constant last night while working on a project, and they still work just fine.

Without a heatsinked reflector, and the same power, these 5mm leds get just slightly hot after 10mins time.

For this flashlight, I'll keep them running extra bright, but I have plans for more of these bright 7000mcd LEDs to be run from a 3.7V battery. I'll probably use a 22ohm series resistor on the next projects to ensure LED reliability. If I use more LEDs, I can get by without running them so bright

*BTW, what about those 25000mcd LEDs? Are they standard 5mm or a different size? What kind of power ratings does such an LED have?

[peufeu]

If you want some bright LEDs, try Seoul P4 3 Watts, or 1 Watt.

[The Peasant]

*BTW, what about those 25000mcd LEDs? Are they standard 5mm or a different size? What kind of power ratings does such an LED have?

They are 5mm and they use about the same amount of power as your 7000mc LEDs, they are just more efficient.

Take care,
Doug

[TheMG]

LEDs don't even have to become warm to fail prematurely. Now, when I say fail, understand that LEDs don't usually go out like light bulbs, they will slowly get dimmer as they wear out, and you can wear out an LED faster by putting too much current through it even if it is kept ice cold.

When using LEDs I usually go by the current alone, and pay no attention to the voltage. I usually keep the current to about 75% of the LED's rating. Note that LED brightness near the maximum current rating does not have a linear relationship, once you've reached about 75% of the current rating any current increase after that will only increase the brightness very little.

[peufeu]

HelmetLight !

http://audio.peufeu.com/node/67