How do you add an LED and voltage limiter??

[gvdobler]

Sorry if this is old news.

If you put an LED, say 3V, in place of the headlamp then you would need a voltage limiter I assume. What parts do I need and does Radio Shack have the parts?

What does the wiring diagram look like? In line on the power side limiter first??

If you have a better way or any info or pitfalls, it would be appreciated.

[Tom The Brat]

Easiest way is to calculate a resistor.

Take the volts you usually run at, say 18. Subtract the 3 for the LED to get 15. Now divide that by the current the LED needs, which might be 20ma to get 15/0.02 = 750. Then go to Radio Shack and pick up a resistor that's close to 750 ohms. It's not very critical. That is, if you run your 20ma LED at 30ma, it will be brighter, but will only last until your grandchildren are old and grey, rather than your great grandchildren. Of course, if you put the LED straight to track power, it will last about 3 millionths of a second, which is too short to even see the flash.

Remember, an LED is polarity sensitive, so it will work while you're running one way, and not the other.

Some locos have a regulator inside them so their low voltage lights come on before the loco starts to move. Sneak in there with your volt meter and see what voltage the loco is actually putting to the headlight and use that in the formula above.

[grandpopswalt]

As far as I know LED's are like any other diode and are rated at
.6 volts. 20 ma is usually the optimal current. based on that, use Torby's formula to calculate the value of the resistor.

Walt

[Tom The Brat]

Many modern, bright LEDs are 3 volts at 30ma these days.

[piercedan]

There are 2 types of led's. Older type and the newer bright led's.

Old ones have resistors to limit current from 2 batteries which most of us call 3 volts.

The bright leds need 4 volts (3 batteries) to operate properly. with limiting current resistors.

[cacole]

If you want the LED to illuminate regardless of track polarity, get a small, 1 Amp bridge rectifier and add to the circuit. The track voltage will connect to the AC Input terminals (even though you're using DC) and the bridge outputs go through a resistor as mentioned above, to the LED. The bridge will drop an additional 1.2 Volts, so you can add that to the resistor calculation if you want to be exact. As a general rule, a 1K Ohm, 1/4 Watt resistor should be sufficient whether you're using the bridge or not; the LED will just glow a little dimmer.