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X-Mass light (LED) — Parallax Forums

X-Mass light (LED)

YoshtiYoshti Posts: 108
edited 2010-03-12 21:45 in General Discussion
Hi all,
Can someone, teach an old fart like me, how the heck with those x-mass light (NEW TYPES LED), driven from a wall plug 120ac does not burn.
I've double check for a power supply or anything. nothing. Got a 15 lights straight from the plug..

I was putting away the x-mass lights box, and it dawn on me, how can LED lights be connected directly on a 120ac plug.

What is so specials about those LED lights?

And can I use them somewhere else? If I cut the wiring...

Cheers

Yosh

Comments

  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-12 06:24
    Yoshti said...

    What is so specials about those LED lights?

    And can I use them somewhere else? If I cut the wiring...
    Each LED drops a certain amount of voltage, around 2 volts. So if you have a string of 50 that is about 100 volts. Since they are fed AC they only use half of the AC sine wave, they are only on half of the time. If you move them back and forth quickly you can see that they are blinking very fast - about 60 times a second. Maybe 50 in other parts of the world. The string that I have also has four 1.2K ohm resistors and a two rectifier diodes in series with the string. In short, there is probably nothing special about the LEDs.

    You should be able to remove each LED from the socket and use it just like any other LED.

    Rich H

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  • WBA ConsultingWBA Consulting Posts: 2,935
    edited 2010-03-12 08:30
    Good explanation, that's it exactly. A friend and I are working on a project for Christmas to use a Power Over Ethernet like setup with LED strands. POE uses 48VDC, so we are thinking of using CAT5 to supply 48VDC and a DMX (or similar protocol) signal to a bunch of segmented LED light strands that are cut to work with 48VDC instead of 120VAC. This way, no AC is on the lawn and only CAT5 is running everywhere.

    Shaking the strands to see the 60hz "blinking" was something I saw this past year when I started using LED lights and that's what sparked this idea. Once I realized the concept of LED light strands, I came up with all sorts of ways to hack them.

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  • YoshtiYoshti Posts: 108
    edited 2010-03-12 13:45
    ~S!~ W9GFO
    Are you a teacher? Jeez... I understood very well your explanation! Even I caught it... ! wink.gif
    Old fart here, and trying to learn electronics... and my lonely cell has a hard time to learn!

    Simply great !

    For WBA, I thought that AC is less dangerous than DC (voltage for voltage). Or is it a legend?

    And yes I noticed the blinking also..I was not sure why. Got it... !

    Cheers
    Old Fart Yosh! wink.gif
  • Oldbitcollector (Jeff)Oldbitcollector (Jeff) Posts: 8,091
    edited 2010-03-12 13:59
    Yep, standard LEDs...

    Hacked up a string for my parts bench..

    Once difference.. The ones I got have a bevel cut into the top casting most of the light
    to the sides of the LED with just a ring of light in the top. Interesting difference.

    OBC

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  • W9GFOW9GFO Posts: 4,010
    edited 2010-03-12 21:27
    Yoshti said...
    ~S!~ W9GFO
    Are you a teacher? Jeez... I understood very well your explanation! Even I caught it... ! wink.gif

    Thanks! Nope, not a teacher. In fact, after I hit the "submit" button I was wondering if it would make sense to anyone else.

    Rich H

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  • John R.John R. Posts: 1,376
    edited 2010-03-12 21:45
    Yoshti said...

    For WBA, I thought that AC is less dangerous than DC (voltage for voltage). Or is it a legend?

    Legend:· Yes and no...

    At 60 Hz or so, AC and DC are probably about the same danger, given identical voltage levels.

    As the frequency goes up, AC gets interesting in that less of the current goes throgh the "core" of the conductor, and more goes along the surface (one of the reasons for stranded wire in high frequency applications).

    In terms of human safety, DC will go through the body, and potentailly the heart and other vital places where you really don't want it to go.· At "normal" line frequencies, there is still enough current·(most of it)·going through the core to do dammage.·

    When the frequency gets high enough, it (the current) just passes over the surface of the skin.· This is where the legend comes into play.· Back when Edison was pushing DC, and Westinghouse and Tesla were pushing AC, either Tesla or Westinghouse (I believe Tesla) demonstrated the "safety" of AC by passing a very high voltage of very high frequency "through"·his body (actually accross the surface of his skin).· (And you thought "deceptive advertising was a recent problem:· The frequency was magnitudes higher than anything being considdred for transmission use.)

    In terms of other safety issues, AC may be slightly safer because it transits through zero volts twice a cycle, and as a result, there is a tendancy for an arc to break.· This is the primary reason you see 24 AC signal voltages with relays and other mechanical switches.· There is less of a tendancy for arcing and welding of contacts than with a DC control circuit·("less" being the operative word here).· With solid state devices this is less of an issue, and DC has the advantage of playing nice with things like diodes, transistors, etc.

    Hope that all makes sense.



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    John R.
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    Post Edited (John R.) : 3/12/2010 9:51:13 PM GMT
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