LED and resistor specs/sources?


P

PIN DR

Member
I have read dozens of posts about on here about LED lighting and resistor values for them, etc., but I'm dense I guess...

I have a nice 5volt power supply feeding an auxiliary line all around my layout. I want to light buildings, street lights, signals, etc. down the road. For now, my primary concentration is to hook up LED lights for buildings and the like.

What I am looking for is a source(s) and the actual specs or part numbers for LEDs and resistors that would be appropriate for HO scale. I know the size of the resistor determines brightness, but I'd like to get to a starting point so I can at least start experimenting with values and what works best.

So, can someone tell me what I should be looking to get for LEDs and resistors? Maybe an idea of an assortment? Does anyone sell something in the way of a "kit" that covers lots of options? I go to Mouser and the LED choices alone are in the thousands! I am in HO scale, so I'm not looking for giant bulbs.

Jeff
 
PIN DR said:
I'd like to get to a starting point so I can at least start experimenting with values and what works best.
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I think the best place to start is a string of Christmas LED lights. I got 3 strings of GE soft white a few years ago for headlights. And a string of red/green for signals, and "fire". Christmas lamps come in 3 and 5mm size. The 5mm work great for F unit headlamps, the 3mm work better for signals. For structural lighting it is going to depend on if you are just making the windows glow or want to do fancy stuff like the Motel in the other thread. Anyway, Rip the LEDs out of the strings and go (It is more work than it sounds, but it is a cheap source of LEDS). For resistors buy a pack of 1K ohm and 10K ohm resistors and start from there. You can combine two 10K resistors in parallel to get the equivalent of 5K, two 1K in series to get 2K, etc.

Yes the Mouser catalog is overwhelming even for people who are experienced in electronics.
 
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Jeff,

I have just gone through exactly what you are now contemplating. What I ended up with were 360 degree 3 mm LEDS from http://www.moreleds.com/whiteinverted.htm They came with "free" 470 ohm resistors which were no where "strong enough". I ended up using one 20K (20,000 ohm) resistor and, for me, that looks pretty good for building lighting. I quickly realized that you have to start BIG on the resistor side of things. As Iron Horseman has said, you can add more resistors if one size isn't "strong enough".

I have since dipped one LED into a bottle of Tamiya Clear Orange and that has given more of a warm glow but isn't perfect.

Forgot to mention that I am also running my building lighting from a 12 volt power pack.
 
Interesting that you provided that link. I have been web surfing looking for options and came upon that site and it looked like what I am looking for. Thanks too for the advice on the resistor size. I liked the earlier advice too to strip out the LEDS from a line of Christmas lights. I might even have a set of those sitting at home somewhere.

Jeff
 
What ever you choose, good luck on this. I know I had heaps of frustration initially understanding the relationship between LEDS and resistors.
 
wombat457 said:
What ever you choose, good luck on this. I know I had heaps of frustration initially understanding the relationship between LEDS and resistors.[/QUOTE

Part 1. For Mr. Wombat.

LED's have a forward voltage value below which they will not illuminate. In most cases, at least with the standard LED's that I use, this amounts to 1.7 VDC. That is, below that value, the LED will not light. Above that value, it will light up.

We need to limit the current to approximately 20 ma. (.020 amp) with a resistor on one leg of the LED. When solving for resistance with a known/desired current and a known voltage, Ohm's law is: I = E/R. I (current) = E (voltage) divided by R (resistance). To take account of the forward voltage, we reduce the known voltage, say 12 VDC by the forward voltage. (1.7 VDC).

Thus: 12-1.7 = 10.3 VDC So .020 = 10-3 divided by our unknown resistance.(1)

We simply divide the known voltage by the known current to solve for resistance. So 10.3 over .020 gets our resistance value of 515 ohms. Resistance can vary a bit with no damage to our LED. For example, I recently converted my LGB 2060 Schoema Diesel to LED's on the forward and rear lights. I had some 560 ohm 1/2 Watt resistors which puts the current at a little more than 18 ma. They are still quite bright due to the white paint inside of the lamps but it's quite convincing, especially in low light. I run analog DC track voltage and they light up nicely at 4-5 VDC even with 560 ohms in place. Continued on next post.
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Part 2. Now, which leg of the LED is which? Well, the long leg is the anode and the short leg is the cathode. Some folks say that you MUST connect the resistor to the cathode and some say the anode but in practice, there doesn't seem to be any difference. I do always check to see if there is any heat generated by the resistor before I put the LED into service. Usually, there is none, but if you have multiple LEDS, there might be. I don't use multiple LEDS myself and there are difference calculations for that issue so I'll let someone else speak to that. If there is heat, you can simply substitute for a resistor with a larger wattage. (1/2 W say, instead of 1/4 W, etc.

I hope this helps Mr. Wombat. :)
 
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I'll paint the specs with a really broad brush ....

Colored LEDs (red / green / yellow) require about 2.2 volts and the entire white family (cool white / warm white / etc.) require 3.4 volts. On average, for maximum brightness from a 12 volt power supply, use 470 ohms for colored LEDs and 1000 ohms for white LEDs. These are safe values for the resistors without running the LEDs at their maximum potential. If you want the LEDs dimmer, you can always add a higher value resistor to suit your taste.

Also consider the capacity of your power supply. Most LEDs draw 20 milli-amps at their full brightness. You could run up to 50 of them on a 1 amp power supply.

The number of variations in LED shape and color is almost mind numbing .... but that's a good thing for us ! As I'm sure you'll see on ebay, you can get them for dirt cheap, so it doesn't cost much to get a variety to play with. I've purchased many lots of 100 for $5.00 with free shipping from China. At those prices, you really don't care if you damage some in the process.

Mark.
 
Mark,

Can you point me to an ebay seller that you have had good luck with? I like the idea of buying a huge lot of different types and experimenting.

Jeff
 
I'll buy from many different ebay sellers - who-ever has the cheapest price / shipping and doesn't have disasterous feed-back.

These are my last few purchases ....

http://www.ebay.com/itm/280924255035?var=580116032322&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 100 LEDs for $3.29

http://www.ebay.com/itm/A144-SMD-LED-0402-Available-white-warm-white-red-blue-yellow-orange-green-/141048340657?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item667c0621fd 25 0402 warm white SMD LEDs for $6.70

With the hundreds of thousands of LEDs on ebay for sale, you really need to have a rough idea of what you are looking for to at least narrow your search down a bit.

I've ordered all kinds of LEDs from these China sellers over the years and have never had a problem with any of them.

Mark.
 
Cool, thanks Mark. For what they cost, it's no biggie to take a chance, even if there is a problem, it's not like I'd be out big bucks.

Jeff
 
The thing you sometimes find with buying the cheap bulk lots is that the color may not always be consistent. I get the 3mm warm whites for headlights, but you need to check the color first before you use them. On average, over half of them have the correct warm color I want. The rest may be too white, too yellow or even have an ever so slight greenish or pinkish hue to them - not offensive, I just don't use them for headlights. I set those aside and use them for structures where the color shift isn't as noticeable.

Still - even if only half of them are the color I want, they are still dirt cheap. I don't doubt they are indeed seconds, not because of quality, but because of color variation. If you want quality LEDs with a guarantee of uniform color, you'd probably pay that same amount for 10 ! You could throw half of these in the garbage and still be way ahead of the game.

I've literally bought thousands of these bulk LEDs over the years, and while I have packages of them left over with color I don't care for - I've never had one that just plain didn't work out of the package.

Mark.
 
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