L.E.Ds with AC???

Discussion in 'General Electronics Chat' started by electronewb, Aug 4, 2012.

  1. electronewb

    Thread Starter Member

    Apr 24, 2012
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  2. DerStrom8

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    Feb 20, 2011
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    Don't forget that LEDs are diodes. If you took a high-speed shot of that LED, you'd actually find that it's flashing on and off--it allows current through it on one half of the AC cycle, and it blocks it for the second half. It's flashing so fast that, to your eyes, it looks like it's on.


    EDIT: I just watched the video, and there are a few things you missed. One, that's not an LED. It appears to be a simple Christmas Light bulb. Second, when you push a magnet through a coil of wire, it does not create AC unless you manually spin it one direction, then the other. It's actually generating DC.

    EDIT EDIT: I watched more of the video. See my original statement :D
     
    Last edited: Aug 4, 2012
  3. Austin Clark

    Member

    Dec 28, 2011
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    Perhaps the frequency is high enough that we can't tell that's it's blinking. LEDs will light up with AC, but only during half of the wave cycle.

    EDIT: DerStrom8 snaked it from me :)
     
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  4. DerStrom8

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    Pretty sure I said that. And he did in the video, too ;):D
     
  5. Austin Clark

    Member

    Dec 28, 2011
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    Yeah, I was slow to post and didn't have audio in that video (I was working, I'm employed at my local libraries computer lab), you snuck yours in before me. :p
     
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  6. DerStrom8

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    Oops, sorry :D Benefit of working the morning shift--I'm home in time to connect to my own internet and sneak posts out from under those who are at work for the afternoon shift ;):p
     
  7. electronewb

    Thread Starter Member

    Apr 24, 2012
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    The first light is tungsten but the green ones I believe are LEDs
     
  8. DerStrom8

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    Yes, that is why I corrected myself and told you to go ahead and read my original statement ;)
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You can also put two LEDs in inverse parallel, back-to-back so each one lights on the alternate half cycle.
     
  10. electronewb

    Thread Starter Member

    Apr 24, 2012
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    In theory since LEDs are diodes would it be possible to build a bridge rectifier with LEDs???
     
    Last edited: Aug 4, 2012
  11. DerStrom8

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    Sure. I've seen it done. It's just not very effective because the current would have to be very low.
     
  12. #12

    Expert

    Nov 30, 2010
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    Yes, and the PRV is very low, too.
     
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  13. WBahn

    Moderator

    Mar 31, 2012
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    Sure, and doing so can make for a good educational demonstrator because you can not only see which diodes are conducting when, but also have a rough visual indication of how much they are conducting. To do so, of course, you have to have a low frequency (1Hz or less, preferably about 0.2Hz). You also have to keep in mind that they won't take too much current or too much reverse voltage (which, combined with the large forward voltage drop, make them lousy choices for an actual bridge rectified).

    When you put a resistor across the output, you will see the LEDs smoothly get bright and then go out on their respective half cycles. If you put a well-chosen capacitor across the load, you see the fraction of time that each LED is on shorten and the intensity peak much more quickly. You can put an LED in the load circuit and get a feel for the ratio of the average current to the peak current, as well.
     
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  14. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I have Christmas light LEDs that run right off the AC line.

    I can see them blink, a non-zero percent of the population can perceive this.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Something no one has mentioned, so I will. Powering LEDs off of mains is a direct violation of AAC's Terms of Service. Look up our text book and it has an example of how it is done, but the point is it can not be done safely. The only reason the circuit discription is there (and I argued in favor of keeping it) is you will see commercial projects that use this.

    It is directly equivalent to making a power supply off of mains without a transformer, which also can not be done safely.

    There is no reason what-so-ever to not use a wall wart or some other DC power supply to power them directly, and it is much safer.

    For the sake of argument let's keep this discussion to low voltage AC, otherwise I will have to close this thread.

    One major clue though, never let an LED face AC by itself, always have a LED facing the other way for the other half of the cycle.
     
  16. electronewb

    Thread Starter Member

    Apr 24, 2012
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    I was thinking along those lines just as well for educational purposes What
    would I need to make this work? What rating power supply would I need?



     
  17. DerStrom8

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    I'd suggest using a signal generator, instead. Set the output voltage to about 5 volts, and don't forget the resistors!

    A signal generator would also have an adjustable frequency, which would be ideal.
     
  18. Wendy

    Moderator

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    If you want to experiment with the concept use a 25.2VAC transformer instead.
     
  19. electronewb

    Thread Starter Member

    Apr 24, 2012
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    I have a 16VAC transformer Would it work?
     
  20. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The 16 Volts transformer will work for your experiments.
    Take care that the reverse voltage on the leds is not to high.
    A reverse voltage of 5 - 7 Volts (depending on the type of led) will damage the led.
    You can prevent the high reverse voltage by putting a diode (or other led) anti parallel.
    With leds anti parallel one led will be lit on the positive cycle and the other led on the negative cycle.

    Bertus
     
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