How About A LED Primer?

quote:

Ok, I give....what are wallwarts?

Good question (not that there's ever a bad one)!

Basically slang for a power supply that plugs directly into the wall. See the following for a pic.

http://en.wikipedia.org/wiki/AC_adapter

I have a couple that go from 1.5 to 12 vdc:

VARIABLE VOLTAGE WALL WART

I have some Radio Shack wall warts that have multiple voltages, they also came with multiple tips.

You can power several hundred LED's off the larger output wall warts.The wife was going to toss several that came with some rechargeable sweepers she no longer had.
They're now under my work bench waiting to be used.

I have all my lights wired to a switch at the control panel.Just cut the recharge jack off the wall wart and wire the hot one to the switch and the ground to the lights.

David

If this site hasn't been mentioned already, try: http://ledcalc.com/ Here you can put in your parameters and the table will compute your resistor value for the circuit with a click of the mouse. It will also present you with a recommended circuit diagram showing your leds connected with the resistor and the power source.

quote:

Originally posted by DPC:
You can power several hundred LED's off the larger output wall warts.The wife was going to toss several that came with some rechargeable sweepers she no longer had.
They're now under my work bench waiting to be used.

I have a pretty large box with retired wall-warts, whenever we retire a device, I save the supply. I've used a number of them for various things, and a number of times, I've had the exact supply for a missing one for some device.

Please note that most wallwarts are unregulated. The voltage on the label will be present when the device is supplying the current listed on the label. For lighter loads, the voltage will usually be higher - in some cases a few volts higher.

Dale is right most of the wall warts are unregulated letting the voltage fluctuate up and down a little but if you construct your LED's so they'll operate up to say 18V using a 12 volt wall wart won't be a problem.

believe me the LED's will be plenty bright.

David

Here's the second installment of "Fun with LEDs" with, I think, the simplest possible way to light an LED.

Fun with LEDs II

A final installment will be here after the Thanksgiving Weekend.

TT

There's a lot of good ideas in this thread already. I hope we can keep this going or at least make it a sticky and add to it as we go along.

TT, good work!!! That's about as easy an explanation as anyone would want.

Thanks Bob. if people are getting something out of this I'll keep it up.

TT

What about those LEDs that are pre-wired, with resistor already? You can buy them off eBay, in bundles... ?

Take a look at this post:

DOWNTOWN BUILDING LEDs

I'm trying to use a Christmas light string of 30 LEDs, cutting them off to suit each building.

Here's the third installment of "Fun With LEDs". Hooking LEDs up to DCC decoders.

Fun With LEDs III

TT

Demo;

There are some issues with the pre-wired LEDs that you may want to consider:

1) The wire that they use is rather fat and stiff, that may prevent you from stuffing the LED where you want it to go.
2) They are set-up for 12V. Depending on the LED itself and the resistor used it may or may not work on the higher voltages sometimes encountered on model railroads. This is not a consideration if you are sure that you'll never exceed 12-14V.
3) Without a rectifier, you cannot run these LEDs on AC, DCC or reversing DC (like for your headlight or passenger car/caboose lighting).

Not insurmountable obstacles, but you should be aware of them.

TT

Some prewired LEDs have a bridge rectifier in them and will run off AC. In fact any LED will run off AC but the reverse voltage half of the cycle puts an unnecessary strain on the LED.

Pre wired LEDs usually are directional. These are OK for headlamps but for general lighting there is little light distribution. Better to use warm white LEDs with a wide 120 to 140 degree view angle to light a building or platform for example..

Individual pre wired LEDs are expensive compared to the LED itself purchased separately and the resistor is sized usually for 12 volts input. Track voltage can peak at 24 volts. Typical LEDs cost 35 cents each warm white with a wide view angle. Prewired maybe $2.

4 pre wired LEDs driven off 12 volts input in parallel would use 4 times the power as as 4 LEDs wired in series off the 12 volts. Series wiring 4 is easier to do than 4 pre wired ones also. Wiring would actually be neater running only 2 wires instead of 4 pair of wires from a power source.

For a head lamp run off track voltage the prewired LED would dim in conventional when the throttle is lowered. If circuited by the user with a CL2-n3 chip,the LED can be made constant voltage.

here is a basic single LED circuit for DYI.

http://www.jcstudiosinc.com/Bl...d=619&categoryId=426

here is a basic series circuit

http://www.jcstudiosinc.com/Bl...d=407&categoryId=426

And here are various circuits installed in a caboose

http://www.jcstudiosinc.com/Bl...d=487&categoryId=426

Dale H

There just has to be an easier way. Years ago, I saw an article in ANOTHER train magazine, and the author had a way to use a resistor and a diode on the anode and cathode of the LED. The resistor reduced the voltage and the diode made the AC flow only in (one) the positive direction (half wave rectifier). This unit was small, cheap, and simple, and it has been working as an ETD on a caboose for more than five years with PW ZW. If I could only remember the year and month of the article. Or even better, if I could decipher the values off of the one that is currently working.

quote:

Originally posted by lionelbob:
There just has to be an easier way. Years ago, I saw an article in ANOTHER train magazine, and the author had a way to use a resistor and a diode on the anode and cathode of the LED. The resistor reduced the voltage and the diode made the AC flow only in (one) the positive direction (half wave rectifier). This unit was small, cheap, and simple, and it has been working as an ETD on a caboose for more than five years with PW ZW. If I could only remember the year and month of the article. Or even better, if I could decipher the values off of the one that is currently working.

I'm not sure I understand what the problem is. Virtually all the LED's I install have a resistor and a diode, that's pretty standard practice and mandatory for AC operation if you want the LED's to live a long life.

As far as resistor values, a 1k resistor is generally good for almost any voltage you'll find on a standard O-scale layout. Note that for blinking LED's like you'd use in an ETD, you'll need a capacitor to maintain the DC to the blinker during the negative half-cycle.

quote:

Originally posted by lionelbob:
There just has to be an easier way. Years ago, I saw an article in ANOTHER train magazine, and the author had a way to use a resistor and a diode on the anode and cathode of the LED. The resistor reduced the voltage and the diode made the AC flow only in (one) the positive direction (half wave rectifier). This unit was small, cheap, and simple, and it has been working as an ETD on a caboose for more than five years with PW ZW. If I could only remember the year and month of the article. Or even better, if I could decipher the values off of the one that is currently working.

Half waving works but does not achieve maximum brightness as it only lights up 60 times per second rather than 120 times with a full bridge.. If used in lighted cars without a capacitor it will flicker badly. With a capacitor a single diode will work as well as a bridge rectifier but would require more capacitance as it is less efficient.

Dale H

I guess I don't spend a lot of time worrying about the efficiency of an LED drawing 10-12ma, it doesn't seem to be much of an issue. As for flicker on a half-wave circuit, I don't see that at all. I just grabbed an LED, resistor, and a diode and connected it to my transformer, steady as a rock.

I use a capacitor for the LED light strips, usually a 470uf. It makes a difference in the flicker from poor track connections, though a really large power outage still blinks the lights. I also added a second pickup to the 2400 series cars I had to give the lighting better performance.

quote:

Originally posted by Bob Delbridge:
Alan I agree, that's why I selected those links, it (the guesswork) is mostly already done for you.

About the hardest thing I see with LEDs is telling the polarity (easy if you know what to look for) and type (if you happen to have some loose ones laying around and don't know where they came from).

I had put 3 LEDs in an Ameritowne 3-story building a while back. I wired them and slid them inside a length of clear plastic tubing so each floor would have a bulb. I drilled holes in the interior floors I made and slid the tubing up from the bottom. I then connected the LED string to a wallwart.

It worked fine, but was a PITA to get in and out of the building. Next time I'll drill holes in the rear walls for the LEDs and run the wiring up the back,maybe put the wires in something to keep them covered.

Are there pop-on covers that can be placed on LEDs so the light is diffused? If I put them in from the back of the building anyone looking into it will be blinded by the intense light they emit, not to mention how unreal it would look. A diffuser, even a small plastic shield, would eliminate the spot light effect you get when looking at them from the top.

Bob, Try #HLDCL-C holders from All Electronics. They're 3 for 90 cents.

Thanks! They look perfect

Took me several days to plow through the greybeards of this thread on the bow.

 

Thanks to everyone for a wonderful wealth of information. I think I will rest a day or two because head hurts. =)

Bob Delbridge, there's only one thing you need to know about LED's, and that is Evan Designs. I'm an electronics guy, but, their products are so good, and reasonably priced, I wouldn't bother doing it myself.