If the supply to the incandescent headlight is a 3-volt sine wave, that means that the instantaneous voltage varies between plus and minus 4.2 volts. An LED with a forward voltage of 3 volts will draw a very large current when presented with 4.2 volts.
The ballast resistor usually used in series with an LED is not there to reduce the voltage but rather to regulate the current. To be effective in doing that, the supply voltage needs to be substantially higher than the forward voltage, so that the current is not very sensitive to variations in the supply voltage nor in the LED's forward voltage. If the headlight voltage were high enough to allow use of a ballast resistor, it would also exceed the usual 5-volt specification for reverse voltage and would damage the LED on the negative half cycles. This is why an LED circuit for AC voltage usually includes a diode, either in series or in anti-parallel with the LED.
The headlight voltage may not be AC. The purpose of using a low-voltage lamp, if that's what you have, is likely to allow regulation of either the headlight's voltage or current, for constant brightness, since incandescent lamps' light output varies enormously with voltage variation (unlike LEDs'). This regulation would be much easier to do with DC. If it is the voltage that is regulated, it is unlikely that it will play well with an LED, because of its sensitivity to voltage. If it is the current that is regulated, it is probably regulated at a much higher current than the LED can stand, since they usually can stand no more than about one-tenth the current of a miniature lamp.
Whether or not your locomotive's present headlight gets 3 volts, AC or DC, regulated or not, I think you would be better off operating an LED headlight from the track voltage, with a suitable ballast and effective reverse-voltage protection. The light output will vary with track voltage; but so much less than with an incandescent that it is very difficult to notice and not worth the trouble of regulating it. The color temperature does not vary at all. I recommend using white LEDs that have been tinted to imitate the color of incandescents, like these http://richmondcontrols.com/ . I have found that LEDs imitating incandescents are a lot more convincing if used with some capacitance, just enough to blunt their very fast turn-on and turn-off times.
Bob Nelson
I have replaced all my diesel headlights with LED's but I heard that the 49er may use 3 volt lights in which case I wouldn't need to add a resistor. I will check it out to see where the power is coming from.
Thanks,
Kevin
Can't stop working on the railroad!
LEDs are very different from incandescent lamps. You can buy LED assemblies built into lamp sockets and meant to replace incandescent lamps. They contain circuits that provide current regulation and reverse-voltage protection, since LEDs are exquisitely sensitive to voltage and are easily destroyed by reverse voltage.
But I doubt that that is what you have, since it would be very difficult to build a white LED circuit to operate at that low a voltage. I suspect that you have actual LEDs, which would require some engineering to serve as headlights.
Can you trace the wiring? It is very possible that the headlight is simply connected to the track voltage, in which case we can probably work something out.
I am trying to figure out what voltage this MTH 49er's headlight is so I can replace it with a LED. http://www.mthtrains.com/sites/default/files/download/instruction/30st12360i.pdf. I have 3 Volt LED's but also have the resistors in case they are needed. Any help would be great.
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