Electronic – Electrical properties of LEDs when overdriven

led

I've found in a batch of LEDs that they are very variable in the current and/or voltage that they can withstand. Over a period of 24 hours, a proportion will become significantly dimmer, and even slightly change colour. I expect this is because of poor manufacturing.

I would like to quantify the results of such tests so I can determine a voltage/current that for example 99% of specimens can be reliably driven at. However, optical measuring equipment is very expensive, and I am wondering:

  • Would the electrical characteristics for the partially damaged LEDs change after being overdriven? And if so, what would the changes be?
  • If electrical characteristics could pre-determine if an LED is more susceptible to blowing?

I am considering plotting forward voltage vs forward current to investigate, (either using an oscilloscope and signal generator, or making some hardware that would facilitate collecting the results for analysis) but statistically quantifying the results would be hard if I don't know what I'm looking for.

Best Answer

You seem to have some basic misconceptions.

First, the voltage spec of LEDs doesn't tell you the fixed voltage you should run them at. It tells you the voltage range you will get when running the LED at some current. LEDs are diodes, so at the normal operating point the voltage varies little as a function of current. Flip this around and you see that the current will vary a lot with a small change in voltage. LEDs are so sensitive to voltage variations, and the exact voltage to achieve full brightness can vary enough, so that you can't usefully drive a LED with a fixed voltage because the current will be too unpredictable. Think of these devices as being driven with specified current, and the voltage just comes along for the ride. The voltage spec is there only to give you some idea what that voltage will be.

Second, when you exceed maximum specs on any device, it is grossly unfair to blame its resulting failure on bad manufacturing. The point of a spec is to tell you what you can do to the device so that it will continue to meet its other specs and not fail.

You left out the all-important information of what size and type of LEDs these are and the actual voltage and current you are driving them with. If they are losing brightness and changing color from only 24 hours of use, then you are overdriving them. That is exactly the kind of failures to be expected from moderate overdriving.

If you have typical T1-3/4 LEDs, then assume they are rated for 20 mA. Most discrete LEDs are rated for 20 mA, some more. Some very small suface mount LEDs are rated for less. The voltage at full rated current is usually a bit over 3 V for blue, about 2.1 V for green, and around 1.8 V for red. Again, that is not a spec to drive the LED at, but what you are likely to end up at when driven properly at full brightness.

LEDs intended for lighting are a different matter. These come in more expensive packages designed to allow for heat removal. As a result, they can be driven harder for the same size. There are many proprietary packages, and units that look similar can actually be quite different. Some can take 100s of mA or more if kept at the right temperature, but you really need a datasheet to know how to use them effectively but still not damage them. These kind of high performance LEDs are tuned closer to the edge with less wiggle room past the specs.

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