Is there an LED tutorial somewhere on the internet? Thank you
Yes, there are tutorials.
Just Google, LED tutorial.
I will assume, for DCC so search for led dcc tutorial.
Just in case you don’t search, the below fellow has lots of info.
http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm
Generally a 1k resistor is used for a single 20 ma LED’s but with high output LED’s the resistance has to be quite higher but for most, 1k, 1/4 watt is normal.
Store links in Favorites.
Rich
This has always been a very handy picture for me when wiring LEDs up to decoders:
The blue (common) wire is always attached to the anode (+); the yellow & white wires are always attached to the cathode (-). If you look inside the LED bulb the cathode leadframe is always the larger of the two. Another identification mark is the flat spot on the cathode side.
While an LED is a diode and current can only travel through it in one direction, a needed resistor can be soldered to either the anode or cathode.
Tom
Do you have a particular application for LED’s? They are used everywhere.
All the light bulbs are LED types in my house.
I built a bicolor red/green signal system for our club some years ago that used five VDC logic and a lower resistance.
The NCE Power Cab uses a red LED and 1k resistor to monitor the DCC output. No diode.
Christmas trees loaded with LED’s and operate off of mains power.
Rich
ndbprr:
As was suggested, tell us what you want to do with the LEDs and we can give you more focused responses.
Dave
An LED is NOT a light bulb. It is more like a rectifyer. It does not consume electricity, it just passes it through albeit with a minute drop in voltage.
But it is very sensitive to current. You need a resistor to cut it down to a safe level. With the correct resistor a common LED will operate on 3 volts to 20 volts or more.
LIONS simply use 1K Ohm resistors on 12 volts, signals operate on 16 volts.
Do not wire LEDs in series. They will work (without a resistor) but not for long.
ROAR
There are two basic things to know about LEDs.
-
They are polarized. They only work hooked up the right way. + and - matter, unlike a light bulb.
-
LEDs are current devices. The voltage does not matter, the current does. A light bulb gets brighter as the voltage rises, until you exceed the rating and it burns out. An LED tends to get brighter as the current rises, until you exceed the maximum and it burns out - though an LED burning out can be literally explosive as it blows a small chunk out of the epoxy coating. Light bulbs have a rated current they draw, voltage is what varies. LEDs have a rated voltage they drop, the current is what varies.
Combine those two facts with Ohm’s Laws and Kirchoff’s Laws (many tutorials on this exist although the Kirchoff ones may get overcomplicated as they discuss nodal analysis (forget I said that, you don’t need to know what that means) - so search one of my old posts where I’ve explained it (I need to copy that and just put it on my web page) for a basic circuit of a power source, LED, and dropping resistor.
And also apply the one thing common to most anything electrical./electronic - unless you have a really darn good reason, never run any component at its maximum rating. The numbers on the datasheet are typical (typ) or maximim (max). The variable parameter (in the case of an LED, the current) is usually specified as a max. For most common white LEDs, this is often 20ma. Following what I said - you do NOT want to design the circuit so the LED runs at 20ma. Just like you do not want to load a 12V 1 amp power supply to a full 1 amp constantly. Leave a 75-80% margin - when it comes to LEDs, often even more of a margin to get a pleasing light level - remember, more current = brighter, less current = dimmer. For regular size white LEDs used as headlights, the oft-quotes 1K resistor connected to a typical decoder results in about 9ma through the LED - that’s less than HALF the rating, but is plenty bright. Bigger resi
For many years I have usually run electronics at about fifty percent. With diodes, that is about 10 ma.
Having worked in electronics for many years I have a 2.5 k pot, assortment of resistors, 12 vdc supply and clip leads for figuring values.
If you are good at math, you probably will not need test equipment such as a multimeter.
Rich
A couple years ago some fellows in a couple Yahoo DCC forums picked up LED testers shipped from China free. As I recall, the device was a few dollars and worked quite well.
On ebay. Most stuff we buy today is from China.
Current and brighness determination.
Rich
Simple LED tester…
Get one of those 2-battery AA or AAA battery holders. Radio Shack MAY still carry them, but are commonly available. Attach a black lead to the negative output. Build a red lead with a piece of wire and a 1k ohm resistor, attach to positive output.
Touch to the LED, + to + and - to - to test.
A set of clips on the leads is somewhat fancier and may be useful. I just touch them to the LED in question.
My approach is that you need about 570 to 600 ohms to protect the LED on 12 volts, then anything extra is to dim to suit the application. I use over 100 K ohm sometimes, but for loco headlights 2k is good unless you have a modern diesel, which tend to be more intense.
I just do it now based on experience. Used to be a little more uptight about calculating, but observed the results and pretty much do it by eye and hunch now.
Also, run them on less than 12 volts and you need even less. I have lots of buildings on 3.5 to 4.5 volt supplies. I often use a single 10 ohm resistor for a building with multiple LEDs. No failures. I recently experiemented by doing a structure with a dozen LEDs without resistance except for what was in the wiring itself on one of the 3.5 v circuits. It’s a little brighter than others nearby and intentionally so, which is why I decided to experiment. Still running fine after a month,on 12 hrs/day average on time.
As OP I have recently converted to dcc. Leds go hand in hand and there is no sense in using light bulbs on any railroad application since leds will probably last longer then I will. Just looking for generic info at this time much of which has been answered. Thank you
I have 12v DC feeding my buildings and lights on my garden railroad. I use CL2N3-G LED drivers which are about 40 cents each from Mouser.com. No resistors, it regulates the current ‘automagically’. I can usually drive 3-4 LEDs with one of these. 12v in one side, into the LEDs which you wire in series and out to ground. Couldn’t be simpler. The LEDs are nice and bright without burning them out. http://trainelectronics.com/LED_Articles_2007/LED_104/index.htm
http://trainelectronics.com/LED_Articles_2007/LED_104/index.htm
Made the link clickable.
I didn’t know such things existed!
Thanks
Dave
Constant current drivers of all sorts have been around for years now, single chips and slightly more complex ones made up with individual transistors. Such things are critical where the voltage can and does vary - say a good quality flashlight, thus the light stays at the same brightness throughout the life of the battery. Or in a car, where your voltage can drop to 12V or less with a weak battery but once the engine is running you can get close to 14V. Or to use an LED with DC powered trains - there really isn’t a decent resistor that keeps the LED at a reasonable current from 3 to 12 volts, but a constant current circuit can keep the LED at the proper level no matter how fast you go. Or as a couple of posts up - a fixed voltage driving all accessories and building lights, some with 2 LEDs, some with 5 LEDs or more - just throw that inexpensive constant current chip in there and you won’t have to worry about calculating resistors depending on how many LEDs you equip a building with.
–Randy
Randy:
Thanks for the explanation. I just ordered some.
Dave
Randy:
I have what may be a dumb question. The output from the LED driver is 20 ma which will make the LEDs quite bright. Can I use a resistor in series to dim the LED(s)? I’m thinking of cases where I’m using three or four LEDs in series in structure lighting.
Thanks
Dave
For a 20ma current source use 2 LEDs in parallel for 10ma each, or 4 in parallel for 5ma each. All must be the same type though so they split the current approximately evenly - don’t mix and match a regular 3mm LED with a high efficiency SMT LED, for example. A small resistor, like 10 ohms, on each one can help even out the current distribution among the parallel LEDs if it is an issue and some are much brighter than others.
A series resistor won;t work like it will with a voltage source. Kirchoff’s Laws, 2 devices in series have the same current flowing through them, so you put say a 1K resistor in series with an LED powered by a 20ma constant current source, the resistor will pass 20ma and the LED will pass 20ma. The resistor will have a voltage drop E=IR, or 1000 x .020 = 20V so unless the voltage is > 20V + Vf of the LED, it will just not light up. Even with a smaller resistor, it just drops less voltage, it does not reduce the current through the LED. That is the downside of the contant current sources for LEDs, often they are made for applications like flashlights or LED replacement lights where you want the maximum brightness, thus the common 20ma output - you’ll find them in the better LED replacement bulbs (cheap ones use a capacitor dropper (for AC, a capacitor can drop voltage like a resistor does for DC), a rectifier, current limit resistor, and a whole bunch of LEDs in series to add up to say 100V worth, with the resistor handling the extra). Check out bigclivedotcom on eBay, he does teardowns of various LED repalcement lights among other things so you can see what’s inside a lot of that stuff. Plus his day job is theatrical lighting/sfx and he often has behind the scenes footage of the various lighting and effects devices. And some downright silly stuff, like the time he and his crew made popcorn in a PAR can - a theatrical spot gets MORE than hot enough to boil oil in which to pop popcorn!
Randy:
So it wasn’t a dumb question! Just the person asking it[swg][(-D].
Thanks for the explanation. I think I understand the issue with using resistors and LED drivers together. Putting two or more LEDs in parallel is certainly easy to do.
Your knowledge is much appreciated![bow]
Cheers!
Dave