LED Constant Headlight Question

Okay, let me stick my neck out a little. I think it was David Starr's recent comments on LED headlights that inspired me to attempt constant
lighting headlights. I tried one headlight using the bridge rectified (Radio Shack), a white LED and resistor and the thing worked fine. I model the Espee and now I want to try constant lighting on four headlights (2 front, 2 rear). My question is how do I wire the four LEDs coming off the positive/negative rectifier leads, serial or parallel? I think parallel with the resistor on either of the rectifier voltage leads, but not sure.
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Byron Lane spake thus:

Optimally, you should wire them in parallel but with each LED with its own resistor. If you want to share resistors--that is, have more than one LED in parallel, all with a single resistor in series--you'll have to recalculate the resistor value, since now you have more current going through the circuit. (See previous thread to see how to do this.) Also, when sharing a resistor, you may exceed its 1/4 or 1/2 watt capacity. If it's only by a little, no problemo; the resistor will get a little warm. Otherwise, you'll need a bigger (higher power) resistor.
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Byron Lane skriver:

No matter the direction ?

Do you run conventional or digital ?
Klaus
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On Mon, 18 Sep 2006 16:48:45 +0200, "Klaus D. Mikkelsen"
Yes, I want all four headlights on all the time no matter which direction the locomotives are moving. Also, I have a conventional block layout. I think David answered my question and it looks like a one watt resistor on the voltage lead in series with the rectifier and four parallel LEDs will work (trying to keep it simple). If not, I will put smaller resistors on each parallel LED circuit like Dave suggested.

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Byron Lane skriver:

Okay, may I ask why ?

Okay, so you need the LED's to be on from as low voltage as possible.

A LED is an avalanche diode, it starts to conduct at a certain voltage. If the start voltae of your LED's is slightly different, on LED will consume all the power and perhaps one will not light at all.
I'll recommend that you skip the resistor before the rectifier and then have one resistor for each LED.
Try to calculate the maximum current to the LED with ohms law.
Klaus
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From a slightly different perspective but since we are talking LEDs, one of the red marker lights on my 3-rail Lionel loco is "out". Lionel manual talks about replacing headlight and firebox light but regarding marker lights it simply states marker lights are LEDs and should not need replacement. Well, one of mine is out!
Any suggestions what red LED I might get without paying Lionel prices plus a minimum of $5 S&H?
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Byron Lane wrote:

It will work fine if you just add LEDs AND a current limiting resistor for each LED, in parallel with the LED and resistor that you have working now. Each LED needs it OWN current limiting resistor. If you share the current limiting resistor (one resistor to four LEDs) bad things can happen, like one LED going super bright and some of the others not lighting up at all. With four LEDs you have to expect each LED to have a slightly different forward voltage drop. For example the four LEDs happen to have forward voltage drops of 1.95 2.10, 2.15 and 2.47. In this case the 1.95 volt LED will hog all the current and light very brightly, and the other three LEDs will be dim-to-dark. Give each LED its own current limiting resistor and then things work fine. I would not put the LEDS in series, at least on DC. If you series up four LEDS, each with a nominal two volt forward voltage drop, then you need 8 volts on the track (nearly full speed on a analog (DC) system) before any of the LEDs begin to glow. On DCC, with a constant track voltage, series connection with a single current limiting resistor chosen to limit current to 20 mA working with DCC voltage (more than 12 volts, not sure just how much) less 9.4 volts (2 volts per LED plus 1.4 volts for the rectifying diode bridge) can work.
David Starr
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The only problem that putting multiple lights on a loco are that it can happen that the motor won't pass enough current to light the bulbs. In such a case, you'll have to consider the loco to be a car and wire it the way a car would be wired with a resistor or bulb in addition to the motor for a load. Been there and done that with a micromotor powered loco once! Motor drew 15ma just trucking down the road while the light bulbs took 18ma. I added a small auto light bulb parallel with the motor in the loco and all was well
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