LEDs and AC power

Discussion in 'General Electronics Chat' started by gramps, Aug 19, 2016.

  1. gramps

    Thread Starter New Member

    Dec 8, 2014
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    I have a need to use LEDs as power indicators on an AC circuit. If it were DC, I'd have no problem. I don't have a clue as how to calculate the limiting resistor value. Is it the same as DC? Is it 1/2 the value, since the LED will be conducting only 1/2 of the cycle? Is the value calculated based on the peak (vs RMS) voltage? The circuit in question is 24VAC, so will this even be possible since the reverse voltage of the LED is typically about 5v?

    Any insight is greatly appreciated.
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    886
    I would use roughly 1/2 the resistance you would use if it were DC.

    RMS.

    Just put an ordinary silicon diode, like a 1N4148 or a 1N4001, across the LED in the reverse direction to limit the LED reverse voltage to ≈0.7 V.
     
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  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    .... or you could use a second LED instead of the 1N4148/1N4001, to make use of both half-cycles.
     
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  4. OBW0549

    Well-Known Member

    Mar 2, 2015
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    .... or you could connect the LED to the (+) and (-) terminals of a small full-wave bridge rectifier, and connect the resistor in series with one of the AC terminals.
     
  5. gramps

    Thread Starter New Member

    Dec 8, 2014
    11
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    Thanks for the replies. I'm going to go with the 1N4001 solution, since I've got lots of them on hand and that seems like the simplest way to go. The circuit includes 5 bi-color (red/green) LEDs.
     
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