They’re really easy to figure if you know just 1 thing about electricity.
E = IR
For an explanation, see http://www.barefootelectronics.com/led001.aspx
Awesome website. I bookmarked it. I had to take a few electronics classes in college but since I dont use that stuff all the time I forget. This website makes things extremely easy and practical.
Thanks Tom.
Nice write up Tom. It made for an enjoyable read while you took the mystery out of a confusing Electronic part. I’ll bookmark it also
I also bookmarked it (peer pressure, ya know- everyones doing it). Great site, great explanations. Thanks, Tom.
-Kevin.
Good information in the link, but one problem I have is with the CL2 comment.
The CL2 does limit the maximum current to 20ma but you can use a variable resistor in series to dim the LED’s the CL2 would just be your upper limit of current then.
Why bookmark it? Put it in the Articles section here and it will always be backed up and available.
'Cause it’s SUPPOSED to attract you to my web site 
I’ve almost given up using resistors, using CL2s instead. That’s because I power from track power (which by definition varies) and often can’t plan how many LEDs I’ll use in a circuit.
I can power from one to five passenger cars with track power and a CL2.
You’re right, they are not available from the Shack, but Mouser sells them for about $.30 each. Buy a lot, since the shipping is the same for one or one or two dozen!
I’ve even made a pc board with has a rectifier (to get the current flowing in only one direction, regardless of where the train is headed, a capacitor to store current for the inevitable dirt on the track, and the CL2.
In my shop I’ve got one LED hooked up to one of these, and a strip of a dozen LEDs attached to another.
I’ve also found out that you can’t use a CL2 if you want directional lighting using LEDs. The don’t like current coming in from the back. Don’t ask how I know this.
Dick - you can use the CL2’s for directional lighting - just use two additional diodes to protect the CL2’s - I have notes and a circuit on my web page at:
http://www.trainelectronics.com/LED_Articles_2007/LED_104/index.htm
See the section at the very bottom of the page.
dave
Thanks, Dave. I remember talking to you about this, and I’m glad it came up on this list as well. It just might save someone from burning up one or two CL2s.
I’m wondering if the same thing could be accomplished by using the LEDs in their diode function and two resistors to control the current draw. Then save the CL2 for something else.
I’m thinking of a switching version of a CL2. But then, that wouldn’t sell for 30 cents.
Of course you can use a CL2 for directional lighting. Just add another diode. Counterintuitively, put it across the CL2 backwards, so it conducts when you want the LEDs off. It will protect the CL2 from reverse voltage and the LED’s will keep current from flowing.
(It might work fine without the extra diode anyhow. Worth 30 cents to give it a try?)
Dave, why do you need the additional diodes if you have leds in series with teh cl2? Is it to keep the max reverse voltage to the leds within spec?
Greg
Greg - in theory you don’t need them as LEDs are, themselves, diodes - it is just double protection - belt & suspenders if you will!
dave
Actually the piv of leds can be pretty low so an electronics engineer would say bravo, just not many people listen to them ha ha!
Greg - If you’re going to use TLAs (Three Letter Acronyms), at least capitalize them, and better yet, decode them.Of course that would defeat the whole purpose for you. 
PIV = Peak Inverse Voltage for those of you playing at home.
No, I’m not doing that for the reason you are accusing me of (thanks for the unwarranted dig)…
I replied on a cell phone and it’s a pain in the butt to capitalize and just even enter characters… and besides, it’s probably only germane to David, since to most people it seems unnecessary…
Sorry I participated.
Greg
Greg and Del,
If you guys have a personal thing going on, that;s OK - lets not let it get in the way of the informative nature of this board/thread.
When it comes to electronics, I for the most part know just enough to be dangerous. In this case I happen to know what PIV stood for. There have been many treads I have read that took sometimes pages before I got enough information between the lines to go find what the acronyms meant (and then sometimes still didn’t fully understand). Greg, I appreciated the reply and would not like to see you stop posting, especially on this subect - I have learned a good bit by just reading. If the cell phone is that much of a problem to use, wait til ya get home, this is only a forum and I am sure a couple of hours lag won’t mean a thing.
Off my soap box.
Bob C
If folks would edit their posts so that the word “you” was left out of them - yes it is possible - things might then run a little smoother. Nobody here is out to piss on anyone else’s cornflakes; it’s usually just a misunderstanding.
But that little word “you” can be absolute poison. It can feel like someone’s personally pointing a finger your way.
I challenge YOU to try my suggestion. (Smileyface here)
My fault all the way. It was uncalled for. I’ll just leave it at that.
Ok, so if the following is TMI (too much information) I apologize in advance, but this is an explanation for how LEDs can fail when the voltage is reversed.
So, if you hook up a LED backwards, or run it from an AC source (like DCC or off an AC transformer) you could burn out a LED quite easily.
It all has to do with the rating of Reverse Voltage on the “diode part” of the LED. Most of us are used to thinking that if we have 25 volts AC and use a 50 volt diode to “switch” the voltage, everything is fine. The 50 volt rating is indeed the peak reverse voltage it will stand (although notice the 35v rms rating)
Well, here’s a typical diode: 1N4001… http://www.diodes.com/datasheets/ds28002.pdf
OK, cool, what we expect.
But what happens when we put an LED on AC and just add a current limiting resistor?
Well, it turns out that the peak reverse voltage rating of the LED is WAY lower:
Here’s a representative LED: http://www.datasheetarchive.com/indexer.php?file=DSA00271545.pdf&dir=Datasheet-016&keywords=LED1200-35M32&database=user-highscore# (you might have to cut and paste the link)
Notice that the peak reverse voltage is ONLY 3 volts?
Even though you have a current limiting resistor, at low current (like reverse voltage) the resistor does not “lower” the supply voltage, so on 12 volts AC, you have exceeded the peak reverse voltage capability of the LED…
So you could, theoretically, easily nuke the LED on only 12v ac.
Here’s where the “belt and suspenders” comes in… adding a normal diode with a 50 volt PIV rating in series with the LED protects against such a situation where the voltage could be applied backwards.
So be careful when testing an LED and you have a supply over 5 volts and you might connect it backwards. If you run LEDs on DCC, you should add this extra diode.
There’s also circuits where a high reverse voltage can appear, like powering a relay coil or motor.
Sorry for the long winded post, but I do run DCC and I have friends that run AC on the rails of their trains (O scale and MTH) and we were perplexed by strange LED failures.
Greg