Led voltage question

Discussion in 'FAQs' started by YmeBP, Mar 24, 2007.

  1. Gary S.

    Gary S. Senior Member

    One thing that we always seem to forget is that a diode is essentially a switch, not a load. This is true regardless of whether it is a standard diode, or a light emitting diode.
  2. Torpedo

    Torpedo Member

    Switch implies a selection of one thing versus another. While it may be technically accurate to refer to a diode as a switch, I think it will prove to be confusing to the average modeller who does not have an electronics background. I think it may be better to refer to a diode as a one way valve for electric current, and leave it at that.
  3. Ralph

    Ralph Remember...it's for fun!

    OK, here's a hopefully related question, that maybe got answered in some form already but I'm an electrical idiot: If I wanted to wire a two color LED (red and green) to the track for power so that the LED could be a signal indicating track polarity (hence direction) would I need a reistor on both legs of the LED?

  4. baldwinjl

    baldwinjl Member

    If you fill in the blanks a little I can give a good answer. DC? DCC? Frog? The quick answer is one resistor will do, but if I know a bit more I can give you a more detailed response.

  5. Ralph

    Ralph Remember...it's for fun!

    Ah! DC. Not on a frog, just a single track that will help me remember to be sure to set the direction on my power pack the right way to get a train out of a hidden staging track. Thanks for the advice!
  6. steamhead

    steamhead Active Member

    While we are asking questions...That set-up I had with the 12V, lamps for my reversing loop (on DC), will it work with DCC-since DCC doesn't have a "polarity" as such..??
  7. baldwinjl

    baldwinjl Member

    The problem you'll have is that when you are sitting still there's no power on the track, hence no light. By the time the LED lights, you might well already be moving.

  8. baldwinjl

    baldwinjl Member

    While DCC does not have polarity, as such, the rails are always opposites. So, an indicator between to rails will be off if they are the same, and on if they are different. So a setup like that can work. 12V lamps might be a bit taxed, but you could work with it.

  9. Gary S.

    Gary S. Senior Member

    It does select one thing or another. It selects "on" or "off" depending on the polarity. The diode either lets current flow, or it doesn't, like an on-off switch in your wall that turns your lights on in your living room. But instead of being controlled by a person, the diode is controlled by polarity.

    I think we just said the same thing:)

    My point was that a diode does not limit the current like a load does... That is why we always need a resistor to act as the load for the circuit when using an LED.
  10. Torpedo

    Torpedo Member

    Absolutely. I agree fully with that part, but I still don't like referring to a diode as a switch in a simple DC power model railroading context. It adds needless complexity to the discussion.

    My $.02. :D
  11. Gary S.

    Gary S. Senior Member

    How about if we just say:


    and leave it at that?

  12. Torpedo

    Torpedo Member

    Works for me. :wave:
  13. YmeBP

    YmeBP Member

    I'd like to post to say thanks! It all worked great. I have a red led for each section's turnout.


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