weird LED phenomenon?

Discussion in 'FAQs' started by nachoman, May 1, 2007.

  1. nachoman

    nachoman Guest

    I installed a white LED as a locomotive headlight. I used a 1000 ohm resistor in series with the LED, and am running standard DC. because the LED is a diode, I would expect it to light in one direction only. I was careful to wire it such that it lights in the forward direction but not in reverse. The headlight is parallel to the motor. Well, it lights in forward, but also in reverse! In reverse, the light does not come on constantly. It flashes irregularly, and does not apear to be full brightness.

    Is this normal? I can't think of why it would be doing this. Could something be wrong with my power pack?

  2. Ray Marinaccio

    Ray Marinaccio Active Member

    Hi Kevin
    The LED is getting back EMF from the motor.
    There was a thread that showed installing a diode and a capacitor to eliminate the back emf. (Can't find it now)
    I burnt out a few LEDs untill I started using a full bridge rectifier, capacitor and the resistor.
    I've experimented with a few other curcuits.
    Current Regulators
  3. nachoman

    nachoman Guest

    Interesting. I thought about that, but didn't think that would be enough to light the LED. This is an HOn3 loco, and I barely have room for the resistor as it is! Hopefully, it will be okay as I don't go in reverse all that much!

  4. Russ Bellinis

    Russ Bellinis Active Member

    Since it is hon3 am I safe in presuming it is a steam engine? Steam engines did not generally run lights during the daylight hours. At night they would have the lights on, but they wouldn't shut of the headlight when doing a backing move and if regularly used for switching duty they would have a headlight on the tender and would probably have both lights turned on all of the time after dark when switching. I'm afraid one of my pet peeves with locomotive manufacturers is this fetish they have with directional headlight. If a locomotive is being used in a consist with other locomotives, you don't want the headlight in back to light at all. If a locomotive is being used for switching, the engineer would not keep flipping lights on and off everytime he changed directions, he would turn them all on and leave them on.
  5. BigJim

    BigJim Member

    I suspect the LED is "breaking down" with the reverse voltage. Yes - LEDs are diodes BUT they are very low voltage diodes. If you have a variable voltage DC power supply try connecting the LED (with a resistor of course) and turning up the voltage. I will guess you will see the same thing at about 8 or 9 voltes.

    Two ways to fix: 1 - a higher voltage diode in series wired in the same direction - stop the reverse current. Drops the voltage to the LED a little but shouldn't be noticeable. 2 - any diode in parallel wired the other way. This will "sink" the reverse voltage current so the LED never sees reverse voltage. If this second diode was another white LED and was on the back of the loco it would only come on when you were in reverse.

    1 ----->|---/\/\/\/---->|----

    2 --->|-----

  6. Travellar

    Travellar Member

    I'm betting on the reverse current generated by the motor. With a 1000 Ohm resistor, there shouldn't be any problems of burning out your LED unless you've got way more voltage running through your system than I suspect. Still, I'd think you'd want some kind of voltage regulation in there, which would solve both problems.

    p.s. if it starts glowing in an other-than-specified color, you're overdriving the LED, and it will burn out soon.
  7. kf4jqd

    kf4jqd Active Member

    I think I wrote that article about adding a capasitor! Use a .01 microfarad cap. I had the same problem until I installed a "cap".


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