Electronic – LED properties for throwies

Here is some interesting info on throwies from evilmadscientist.com

[Connecting a 1.7 V LED directly to a CR2032]

Wait-- 107 mA?!--‽

Yes, this is reproducible. (That is to say, we wasted used up another battery just because we didn't believe it either.) But holy cow anyway.

"And they said this was safe?" There are a couple of legitimate concerns here. Lithium coin cells aren't designed to source nearly that much power-- and aren't lithium batteries a fire hazard? And why does my LED-- rated for 25 mA continuous current survive this? I've certainly seen enough LEDs destroyed by overcurrent, and this one was over 25 mA for ten minutes solid. But, and perhaps against my better judgement, I do believe that this actually is safe in practice. With all of the throwies and similar things out there -- don't forget the keychain flashlights -- they just don't seem to be exploding or catching on fire. (Breaking, falling apart, running out of photons, yes-- but those modes of failure are usually not as dangerous.)

[...]

So.... Do you need a resistor? No, not really. As we said, it seems to be reasonably safe without one. Should you use a resistor? Yeah you should, if you want a red, yellow, or orange LED to last more than a day or so. So you can solder a resistor in place, but that somewhat defeats the purpose of easy to assemble throwies.

So to make it not a total pain-in-the rear to add resistors, here's a way to do it without soldering: just twist it.

Source: http://www.evilmadscientist.com/article.php/throw

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.