LEDs without resistors

[quinn]

So I made some incorrect assumptions and didn't order all the correct stuff. When I was reading through the diy blade thread, a couple guys said that the 0805 LEDs could be wired directly to a 1s lipo with the exception of red. This is apparently not the case since I just burned up a few during testing. Is this just not true at all? Or do some 0805's have a higher forward voltage that can be run directly without a resistor? I just figured since I have to order more stuff anyway, it would be nice to not have to deal with resistors if thats possible. Thanks!

Btw, I bought white and green, both with forward voltage of 3.0 to 3.2

[JWeber]

My experience is that some can be wired that way, but they do not last very long that way and will need to be replaced. What seems to happen is that some of them end up taking more current than the others and burn out very prematurely. The whites seemed to go first for me and the greens and blues lasted longer. Anything that was yellow, pink or orange just didn't work at all.

I'm sure that this all depends on which LEDs you buy and their manufacturing specifications as well.

[quinn]

Ok thanks, so I guess it's just right at that limit where some work for a while and others don't at all, so probably better to just do it right with resistors. No big deal, its just that today is my first day of my 10 day holiday break from work and I was planning on entertaining myself building night blades and flying them. Is it likely I would find the proper resistors at a common electronics store? Or most likely need to be ordered?

[fastflyer20]

I also did not have the right resistors when I went to build mine. A single radio shack resistor was soldered in line then trial and error for LED placement to yield proper intensity.

[myxiplx]

I read a write up about this, from memory it was along the lines that without a resistor to limit the maximum current you will typically see them blow. The problem is that as the LEDs run, they heat up, and as they heat their internal resistance drops, so more current flows through them, so they heat up more, etc.

The end result is a runaway effect which will ultimately break the LED. And if you have several LEDs connected, after the first goes, the effect begins again with the next.

The gist of it as I understood was that you need a resistor in the circuit. And so I plan to keep soldering 0805 resistors to each LED.

[quinn]

So here's an idea, what about life for the batteries? You could get a 200mah 2s and split the cells. That would give a 200mah 3.3 volt battery for each blade. That should work perfect for LEDs with 3.2 forward voltage right?

[zzyzx_cm]

i used nte 30024 and 30019 bought at a local electronics shop. both 0603 package white 4.2 forward voltage max and blue 4v max respectively. I charge 160mah lipos to 4v. seem to be ok, ran them for a few hours on my bench supply before using them as well. I'd say they have 30 or so flights on them.