Choosing LED's

[William Brillinger]

Hi everyone,

I have a New DCC Ready Kato SD45 that I want to finish up and weather. I've installed an LM-3S into it using an 8 to 9 pin TCS wiring harness. The LM-3S is not driving the lights. The existing LED's do still work under DC.

I'm not sure why they don't work with the LM.

I figure I'm ready to pull the board from this unit and do a complete hard wire, and I thought I might try my first LED install, but I'm new at working with LED's. I've pretty much ignored lighting until now.

Kevin P indicates that he uses SMD LED's from Evans Design.
http://www.modeltrainsoftware.com/smd-chip-leds.html

My questions are:
Which voltage?
What resistors?

Any tips or suggestions?

[nodcc4me]

Bill, before you do a hard wire, try reversing the 8-pin plug in the socket. I have had to do that to get the lights to work on more than one installation.

[William Brillinger]

Bill, before you do a hard wire, try reversing the 8-pin plug in the socket. I have had to do that to get the lights to work on more than one installation.

Well, that's embarrassing! Thanks Jacob that worked.
(and so much for using the tidy little 1" harness - the longer one is now required.)

I'm still interested in the answers to my questions about LED's though.

[nodcc4me]

Don't be embarrassed. That tip is not written in any instructions that I've ever seen. You can be embarrassed for calling me Jacob though. 

[William Brillinger]

Don't be embarrassed. That tip is not written in any instructions that I've ever seen. You can be embarrassed for calling me Jacob though. 

Crap. I'm going to bed. Clearly I've been awake too long today.  Sorry Al!     

[KPack]

Bill - I use two sizes of LED's for nearly all of my installs.  I use the "chip" size for head and rear lights, and the "nano" for ditchlights. http://www.modeltrainsoftware.com/smd-chip-leds.html.  I use the chip because I can get it to sit flat up against the back of the rear and front lights.  The larger round LEDs would be perhaps brighter, but they are rather long. 

Get the ones without any resistors on them...just bare wire.  You'll want to install your own resistors where you want them.  I use SMD resistors, 1K ohm 1/4 watt.  I'll see if I find the exact ones I bought from Digikey.  I bought a roll of 100 or so....it'll last me forever.

One trick I learned is to use liquid electrical tape to cover the back of the LEDs after they are installed to prevent any light leakage into the shell.  Easy to do, and works very well.  Removable in case a LED ever burns out, heaven forbid.

-Kevin

[William Brillinger]

Thanks Kevin, one key question is still unanswered: Which voltage?

 Select Voltage
 3 volt DC
 5-12 volt DC
 7-19 volt AC/DC/DCC(+1.00)

OH, Wait, I maybe you did answer that...
Get the ones without any resistors on them...just bare wire.

Looking at the pictures on the Evan's site, that would be the 3 volt ones.

Thanks Kevin.

[Alan]

Bill, voltage is an odd specification when talking LEDs. They don't have an "operating" voltage per se, at least not like you are accustomed to with other devices. LEDs have a voltage drop across them, typically 1.7V - 3.5V. Termed forward voltage drop it is specified in the data sheet as V_f. The voltage applied must exceed the minimum voltage drop of the specific LED for the LED to light. Once the minimum voltage has been applied an LED becomes a current regulated device, not a voltage regulated device. The series resistor sets the current level flowing through the LED which dictates how bright the LED emits.

So, when working with LEDs the variable that is important to you is the current, or more precisely, forward current. The maximum forward current value for a given LED will be specified in the data sheet as I_f. If you want the LED to be less than full brightness then decrease the forward current by increasing the value of the series resistor. Most LEDs will light with as little as 1mA of current or less. Here is the ultra handy formula you should use when working with LEDs:



R = resistor value
V_s = power supply voltage
V_f = LED forward voltage
I = LED forward current

Or even easier... http://www.ohmslawcalculator.com/led-resistor-calculator

You know a typical LED will light anywhere between 1mA and 20mA. Use the formula to determine the resistor value for 1mA and then the value for 20mA. Those two resistor values now become the min and max for whatever voltage you are using. Choose your brightness level by selecting a value somewhere between min and max. Notice that, other than as an operand in the formula, voltage doesn't matter to the LED. Hence, why voltage is an odd specification for an LED. 

When suppliers advertise LEDs for different voltages (Evans) what they are really saying is "This LED has a series resistor of sufficient value to permit 20mA of current to flow at x voltage." The LEDs themselves are not different voltage LEDs because there is no such thing.

[KPack]

Yes Bill, the ones labeled as 3V are what you want. 

Thanks Alan for the deeper explanation.

-Kevin

[PatP]

Bill,

If it helps, I use an iPhone app called "Electronics Toolbox Pro" to figure out resistor size for LEDs (among other things). As Alan described, the voltage drop is color dependent while the current is fairly constant at 20ma with a peak of 30ma to 50ma (the light that burns twice as bright burns half as long), again depending upon color, clarity, etc... In this case, size does not matter, with the smd style being a bare chip and the "bulbs" being the chip embedded in an acrylic shell.

My brother-in-law (a true electronics Guru, he writes the manuals for military radar, radio, sat, etc..) uses an app called "Electrodroid Pro" for his Android. Says it saves the grey cells from overuse.

Either app lets you pick the color and source voltage and will calculate the resistor for you.

This is another of those things that I know just enough to be a nuisance.

PatP

[William Brillinger]

Thanks Everybody.
Time to order some LED's.

[William Brillinger]

I use SMD resistors, 1K ohm 1/4 watt.  I'll see if I find the exact ones I bought from Digikey.  I bought a roll of 100 or so....it'll last me forever.

Kevin, do you have a product number for these?

[Alan]

Digikey that Kevin mentions is a good electronics house however they are generally one of the most expensive. For those looking to source electronics parts you may also want to look at the following. I have dealt with every one and they are all reputable.

Allied Electronics: http://www.alliedelec.com/
Arrow: http://www.arrow.com/
Mouser: http://www.mouser.com/
Online Components: http://www.onlinecomponents.com
Verical: http://www.verical.com

And don't discount eBay from China. The vast majority of my electronics inventory came through eBay from China vendors. In the electronics business there is not good, better, best but rather tight spec, loose spec. It is called "binning". Same assembly line, components that are well within spec go in one bin while components that drift from spec go in another. The domestic electronic houses sell tight spec as their customers are primarily manufacturers of devices where in spec is really important. Most of the Ebay China stuff is loose spec. However, for hobbyist use such as ours loose spec is just dandy. You may have to wait 30 days for delivery but there is often amazing savings.

With respect to LEDs, binning tight spec means all LEDs of that part number will be exactly the same color and brightness. Binning loose spec means there may be slight variation. Other than by lining them up side-by-side I challenge you to see the difference. IMHO buying tight spec for hobbyist use is a waste of your money.

[KPack]

https://www.digikey.com/products/en?keywords=RMCF1206JT1K00CT-ND

I spent $1.00 for 100 of these.  They are small and you can fit them anywhere.  With some installs you may need to get creative on where to put things.  On my recent Scaletrains SD40-2 install, I CA'd one of them to the side of the my front speaker and ran the wire around to it to avoid bringing the wire over the top where space was scarce. 

Keep in mind that each LED will use a resistor on the output wire.  The blue common has no resistor.  For the Evan Designs LEDs, the green wire is the one you solder to the resistor, the red wire is what connects to the common blue.

Alan, thanks for the links to other vendors.  I'll have to go check them out.

-Kevin

[Alan]

I spent $1.00 for 100 of these.

Kevin, I did not mean to imply you paid too much by shopping at Digikey. Just wanted to point out there other suppliers some of which may be less expensive than Digikey. My implied point was more directed at folks that buy electronics components such as LEDs from traditional hobby suppliers (Bill referencing Evans). Doing so is a guarantee you will pay through the nose.

I couldn't help but do a quick eBay search. The first listing that appeared has resistors priced at $0.003 each. That's 30 cents for 100. Free shipping! If you can use 3400 of them they can be had for $0.0017 each or 17 cents for 100. And that my friends is why I like eBay China for electronics pieces.