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Using LED as Indicator if DC Control is On

Koot models in HO and queries:

“Do I install the LED in series or across the two power leads going to the track to indicate power on?”

10 Responses to Using LED as Indicator if DC Control is On

  • David Stokessays:

    My understanding is that the resistor goes inline on the + side but the supplier of your streetlights should have something to say on the subject.

  • Kevin Chingsays:

    be very careful how you wire them they are 3 volts and your line voltage is 0 to 12 volts you will need a resistor to limit the voltage on the full 12 volts but they may not light until the voltage reaches about 5volts

  • Geoffsays:

    If it is installed in series, then it will only light up when the circuit is complete, ie when the locomotive is on the track. If you want a visual cue that power is going to the track when no locomotive is present, then it should be across the two power leads.

    • Kootsays:

      Thanks Geoff. Across is what I am going for,

    • Phill Laceysays:

      Depends on the polarity for loco direction movement. If you use only one it will only light in one direction.
      Use two LEDS, for example one “red” and one “green”.
      So the green can light up if in the normal direct and red if reversing.
      Or one LED and a bridge rectifier, it’s just 4 diodes, you can find on the internet how to assemble a bridge rectifier.

  • Kootsays:

    Thank guys
    Across the leads is my way to go.

  • Sheldon Clarksays:

    Does wiring something across the leads of another component (i.e. “in parallel” rather than “in series”) have an effect on the voltages passing through the two components?

    • roy carslakesays:

      The answer is yes as the LED will drop 1.7 volts as will any diode used in a circuit

  • Jay Rosssays:

    To answer to your question, it would help to know if you are using DC, with changing polarity, or DCC. Also, is the power source variable or constant? Off hand, I’d say to wire the LED parallel (across the leads to the track). If you use a two-lead bi-color LED, then it will show either green or red, depending upon the polarity of the power. If it’s a variable power source, then I suggest that you select the current limiting resistor according to the maximum power setting. Of course the LED will be off, of very dim at the lowest power setting. The resistor has to be wired in series with the LED, either side of the LED, it don’t matter. On may layout, I have installed signals with Bi-color LED’s on blocks in several locations. The signal is Yellow if I’m running DCC on that block. If running DC, the signal is green or red, depending on the direction of the trains. Not exactly prototypical, but it works for me. The signals are wired across the rails (Parallel) in each block, a simple installation.

  • David Broadsays:

    You can do it but it won’t work well. If the resistor chosen for the LED to drop its forward voltage to about typically 3 volts from a typical controllers off load voltage of 16 -22 volts the LED won’t really light on small controller settings with a loco on the track. Ideally you want two LEDs wired in parallel in opposite polarities to show which direction the train s to move but you may also need different resistors as Red LEDs typically have a lower max and operating voltage than Green/ Yellow and Blue/ clear typically have a very much higher voltage, often 4.5 volts against 2volts of the Reds. Apply 12 volts to any standard LED and its life can be measured in nanoseconds as it glows brilliantly and dies.

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