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AC polarity via LED?

  1. Feb 26, 2007 #1


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    We routinely test "continuity" in our phone lines by holding a red/green LED to our "tip" (hot) and "ring" (cold) which lights up bright red if all is well with the AC signal.

    A bridge rectifier added to the mix lets the "green" shine; reversing the LED wires on + and - will make the "true" red light come on. All pretty simple.

    There must be also be a fairly simple way to get such an LED to show proper AC polarity as well, i.e "green" if good, and "red" if "tip/ring" is reversed?
  2. jcsd
  3. Feb 27, 2007 #2
    The fact that it is AC means that there is no Polarity as current flows in both directions.
    The LED is acting like a halfwave rectifier so that on one half cycle it flows through the RED LED and on the other it is blocked. If you are using a two pin bi-colour LED then I can only assume that the green is swamped by the red cycle.
  4. Feb 27, 2007 #3


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    I was looking at the principle behind the diode bridge here:


    To my understanding there is one "hot" wire in your AC wall outlet you should not touch, the "tip" as it were, that can be idenified by using a simple mains tester:


    There are more "complicated" AC outlet analysers that dislpay any possbile combination of wiring scenarios, but all I need is a bridge diode/rectifier setup that would send the ACs "hot" through a red/green LED the proper way - "green" if its on tip "red" if its on ring.
  5. Feb 27, 2007 #4


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    If that makes sense ;)
  6. Feb 27, 2007 #5
    Electricity needs to be in a circuit to work. Current flows round all parts of the circuit, in a DC circuit it flows in one direction, in an AC circuit it flows in both directions. The Hot/Live convention does not mean that when a circuit is active current is only flowing in one conductor.

    It would be possible to divert the current through the Earth connection, I believe in the US the Neutral is tied to Earth on the 110V side, In the UK it is seperate and any Earth leakage has to be kept very low. You could use a low current device with suitable current limiting resistor to create a circuit between Live and Earth and Neutral and Earth. Current should flow in the former but not the latter.
    This of course should not be attempted unless you have Earth Leakage protection on the supply and at least believe you know what you are doing and the implications of getting it wrong. Relying on the 100A supply fuse can be career limiting.
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