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[Reply #15 on:]

I like where your heads at. When in doubt, measure it.

I dont have a scope, I do have a couple cheapo digital multimeters.
Not only is it multiplexed, but its charliplexed. 192 LEDs on 16 arduino pins.

I have a spare arduino, is there a way I could make a scope out of an arduino?

[Reply #16 on:]

See if this works for you.
http://www.negtronics.com/simplot

[Reply #17 on:]

but I have no idea how to test the theory.

You don't need to: the datasheet lays it out clearly for you.

The typical diodes have a log-linear v-i curve, and the typical leds have a logn-linear v-i curve at low current levels and a linear curve at high current levels.

The resistor really is there to provide some negative current feedback to prevent a thermal runaway on the diode.

[Reply #18 on:]

I take this as an implicit "I'm happy to field all future "my Arduino and/or LED array is knackered because I don't understand current limiting or datasheets because I'm an artist/hobbyist" type questions" from now on, dhenry.

Well volunteered.

[Reply #19 on:]

You use a lot of words that dont make a lot of sense, are you intentionally being confusing?

It sounds like you thing that someone here hasnt read a datasheet or heard of a current limiting resistor being used with an LED, but im pretty sure everyone here knows about that stuff, so this just sounds like an insult with no merit, why even post it?

Thanks for the link, im going to read up on simplot today.

[Reply #20 on:]

I dont have that equipment, nor the expertise to use them, but i have used a microscope before to examine dies. You can probably just use a regular microscope for this scale, and The LED even has a clear lens to see right into it. I still wont be bothering to do that. Instead I built the thing, and have been running it as a test to see how long it will take to fail, and it has shown no signs of failure yet.

This is not something you can do with an optical microscope. You need a scanning electron microscope to find out about the conductivity of the material. The effect of excessive current in an LED is that there is a depletion of charge carriers which reduces the light output over time. Waiting for a total failure with one sample is going to tell you absolutely nothing about your design. Most people do not understand about statistics and probabilities which is why the Casino owners are so rich and poor people continue to buy lottery tickets.

Its been months with no sign of dimming, ......

OK so lets see how you have tested that statement. I assume you have simply looked at it, I assume you have not taken any measurements of the light output. The eye's response is non linear, it is almost impossible to tell the difference in brightness of 25% over a month. That is why we have instruments to measure things. So I am not impressed with your test. If however you have conducted a real test I would be most interested.

Now dear henry:-

Engineering isn't about "you cannot do that", or "that's wrong". Engineering is about knowing why you cannot do it so you know when / where you can do it.

You may be surprised to here that I absolutely agree with that statement. However there are several points that you keep forgetting.
The biggest error you make is this forum is for beginners, they need to keep things as simple as possible because they know little and they are easily confused with esoteric arguments especially when you leave out all the caveats that your arguments normally involve to make them work.
Lets look at one of your off trotted out remarks:-
  

Leds, especially high brightness leds, behave far more like resistors (at high current levels) than like diodes,

First off here we are not talking about high brightness LEDs, and second any curve if you look at a small enough section of it looks like a straight line. It is not a matter of common sense it is a matter of knowledge of electronics, you seem to have a bit of the latter but non what so ever of the former.

the resistor really is there to provide some negative current feedback

Nearly right. In fact the current produces a negative voltage feedback to in effect reduce the applied voltage.

the datasheet lays it out clearly for you.

Indeed it does, things like internal impedance of the driver will limit the current to a non infinite value, but it is odd that you are willing to believe the data sheet over some things and totally reject the information over others.

Engineering is about being in control of what ever it is you are engineering, be it a jumbo jet, the world's highest building or a humble LED. It is about making decisions and trade offs.
With today's planned obsolescence culture then you indeed might want to design a circuit that only has a life slightly longer than the warranty you give it. You might however want to design a circuit that has a life significantly longer. In those cases you would not even drive the component at the rating given in the data sheet you would derate the device by 80% or even more.
If you want UL approval on anything then the capacitors in the power supply have to run at less that 80% of their voltage ratings even though you might make the "engineering decision" to run it at the maximum operating voltage.

Engineering is about making those informed decisions. So yes by all means run an LED with no current limiting resistor after all they do cost $0.001 each. The LED and driver will be in land fill sooner but hey the economy ticks over faster because people have to buy more stuff. But people have to know this stuff. Like simple Hippynerd, he has absolutely no idea what he is doing and is happy if his one sample hasn't stopped working after a year, but you can't say he is making an engineering decision, he is being fooled into a simplistic way of thinking. Someone has done a good job on him too as he clings to his misguided beliefs.

So dear henry, while you like to think you are much cleverer than most people here confusion and obfuscation are no substitute for real engineering.

This link tells about the fundamentals of reliability:-
http://www.ece.cmu.edu/~koopman/des_s99/electronic_electrical/

This tells a little about how LEDs fail:-
http://en.wikipedia.org/wiki/List_of_LED_failure_modes

[Reply #21 on:]

but im pretty sure everyone here knows about that stuff,

I wish.

[Reply #22 on:]

Oh poor misguided mike, he just doesnt understand the real world, and clings to his tired old beliefs, insulting people that disagree with him  (see how lame and unproductive those kinds statements like that are?)

I guess you forgot(or you chose to ignore that...) that I had mentioned that Im running 2 cubes 24/7 and I have a 3rd that I run occasionally, and that they have been running for months (since the beginning of november I think.) If you dont like the 2 as a small sample size, consider that each cube has 192 LEDs maybe that size is big enough for you?

This cube isnt really my design, I found it on the internet while struggling to get my other cubes working (which by the way, are still unfinished, and not working, but yet these cubes are running strong, so one advantage to this design is that its working now, not some mysterious point in the future. This design has some advantages over other designs, its easier to build, has less parts which means less places for mistakes. Those may be unimportant to you, but to someone that wants to program an LED cube, it is important.

It may turn out that it takes 5 years before this cube fails, a cube with resistors may only last 1 year more, or it may last 10 years more, but one thing we know, they will all fail eventually, resistors will likely delay it, but can not prevent it.

Regarding dimming, clearly there is no way I will have expensive equipment to try to view any damage, the closest I can get is optical magnification, which may not be good enough, but what I do have is a fat sack of LEDs and I can compare the brightness of a new LED to one that has been running for months.

Where do you get resistors for less than one cent each? I get them cheap on the internet, and they are 2-5 cents each, in packs of 100 or more. LEDs arent very expensive either. My whole cube and arduino costs about $25 each, its a fairly inexpensive test.

It may turn out that I can extend the life of the cube with a few resistors, but as it is, its looking pretty good without them.

The reason casino owners and lotteries make money is because people are greedy, not uninformed.

[Reply #23 on:]

The reason casino owners and lotteries make money is because people are greedy, not uninformed.

I disagree - greedy AND misinformed.

[Reply #24 on:]

Hippynerd - I am truly sorry for knowing stuff that you don't know. I will try and forget it all so I will be as good as you. Please forgive me. I will now try and get drunk in an attempt to kill of more brain cells in order to stop me being old, but I might be some time.

[Reply #25 on:]

Ha ha, good luck with that. I never asked for an apology, but its nice that you offer one. I forgive you, but please do try to make valid points without resorting to insults.

[Reply #26 on:]

The reason casino owners and lotteries make money is because people are greedy, not uninformed.

Lets see.
You can't win if you don't play.
You lose if you play.

I guess there is no way out of this delima. 
 

[Reply #27 on:]

Where do you get resistors for less than one cent each? I get them cheap on the internet, and they are 2-5 cents each, in packs of 100 or more.

Pretty much anywhere. Here's some 10K resistors from DigiKey for 0.2 cents each at qty 100:

http://www.digikey.com/product-detail/en/1622826-4/A106045CT-ND/3477682

As an aside, this is easily my favorite thread of the new year.

[Reply #28 on:]

As an aside, this is easily my favorite thread of the new year.

Not one mention of Sparkfun's place in all of this yet (OP's example was written by a Sparkfun employee).

Sparkfun did sell a no-resistor-multiplexed-led display, their 7 segment serial display. They updated it. Spot the difference

[Reply #29 on:]

Oh poor misguided

Your goal is noble.

However, at some point, you have to recognize an individual's right to remained challenged.

It makes you happier, and it makes the people who refused your help happier too.

Win-win.