Wiring LEDs in a DC loco

[Blind Bruce]

How can I prevent overvoltage to my LED headlight when the track voltage goes beyond 1.2V? If I install a series resistor, the LED wont conduct at low speeds.
Thanks, BB

[cacole]

If you're talking about straight DC block control, you must have a bridge rectifier to keep the polarity the same on the LED regardless of changes in track polarity; and a 1.2 Volt regulator.

With DCC, the decoder takes care of the polarity and you only need an appropriate resistor, usually 1K Ohm, in the circuit because the decoder puts out a constant voltage.

[nfmisso]

BB;

A LM317, a bridge rectifier, a cap and a pair of resistors, and you are in business....will also work for DCC.

A good supplier: http://www.allelectronics.com/

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[dinwitty]

for DC this gets directional, but an LED is already directional,
I solder 3 mini-diodes in series and another 3 in opposite direction, soldering the ends together.
a 1.5v lamp or LED with resistor can be placed in between any 2 diodes of the 3 series to get 1.5 volts constant lighting directional. I blacktape it up to prevent shorts.
then tape or goo it in the loco somewhere. I use wirewrap wire for wiring, its tiny.
The motor wont run till it gets enough voltage, by this time the lamp is already lit.

[rrinker]

Yes LEDs are directional already - but excessive voltage in the reverse direction will kill them. You get away with it because the way you wired them, the diodes you connect the LED across take the reverse voltage.

What you described is the exact circuit used by Life Like in most of the Proto2K locos.

--Randy

[Virginian]

Interesting. I run DC. I thought I had found an easy solution so I just wired in LEDs in series with resistors and I had nice constant directional lighting. Now I'm hearing other stuff. I haven't lost one yet. Doesn't the resistor that knocks down the voltage do the same thing in forward and reverse? I believe it do if I remember my circuits class.
At low voltage, you get current flow, you just get lower amperage, and an LED doesn't take much.

[cwclark]

i use 580 ohm resistors to one leg of the LED and haven't had one burn out yet..chuck