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mains rectified voltage for christmas lights

Discussion in 'LEDs and Optoelectronics' started by cjdelphi, Dec 18, 2016.

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  1. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Using a capacitor to drop directly from mains can be used on a low AC voltage too!

    So why not use a capacitor 24ac to get it down to closer to 12v and then a zener to regulate it (low current)
     
  2. cjdelphi

    cjdelphi

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    Oct 26, 2011
    OK - i just designed this simple circuit..

    24ac to 12vac

    Replace the zener for a 12v linear regulator, the same way a cap is used to drop 240vac can drop 24ac to 12v using a 100uf electrolytic cap (50v rated to be safe)

    Out pops 15v ish which can be regulated
     

    Attached Files:

  3. AnalogKid

    AnalogKid

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    Because a capacitor is a fixed impedance at a fixed frequency, the "output" voltage will vary inversely with load current. At 60 Hz, if the load is switched off, the current through the zener will jump to over 800 mA, for a zener diode power dissipation of almost 10 W. I've used zener diodes in a TO-3 power transistor package. Not cheap.

    ak
     
  4. cjdelphi

    cjdelphi

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    Only used a zener because the Sim has no 7812 :p

    I added an inseries resistor to limit the current as well, 1ohm could be swapped for a 10 or higher
     
  5. BobK

    BobK

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    Jan 5, 2010
    Non-polarized, I hope.

    Bob
     
  6. cjdelphi

    cjdelphi

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    Polarized or not, it should not matter for 40vac not that i would but for 3 years now i've got a chinese star light for the tree which uses an electrolytic! lol
     
  7. BobK

    BobK

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    If you are using it to pass AC, it must be non-polarized. A polarized electrolytic can handle only less than 1V in reverse.

    Bob
     
  8. cjdelphi

    cjdelphi

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    If that was true, chinese Christmas lights would be small bombs, but they work, i was surprised as well since safety film caps are used
     
  9. BobK

    BobK

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    The Chinese XMAS lights I have seen simply use enough LEDs whose forward voltages add up. Not seen one with a capacitor, much less a polarized electrolytic. BTW: you know there are non-polarized electrolytics, right?

    Bob
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    My understanding is that some LEDs are made with more bulk silicon which adds an effective series resistance to them.

    It would be interesting to trace the curves of some of the devices. Maybe something to do with Christmas lights after they're taken down...
     
  11. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Normally yes... i'll go abd grab the star from the tree and show you.
     
  12. cjdelphi

    cjdelphi

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    Oct 26, 2011
    400v electrolytic capacitor, 3rd year running.. the leds are 5mm self changing leds, they are NOT being stepped down by an areay of leds
    ..


    They do work, however how reliable is another question !... 3rd year running though from mains 240vac

    And yes i have non polarized caps in my stock thank you, now can you please stop telling me i'm wrong
     

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  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Not connected to AC either.
     
  14. cjdelphi

    cjdelphi

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    Don't make me wire it up (they pulled off when i grabbed it)

    I can assure you this is how they do it, and electrolytics do work, don't shoot the messenger
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's DC going to the capacitor. See the 4 diodes?
     
  16. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Yup, 240 in through the diode rectifying to 300 odd volts

    However, there are only 10 self changing diodes, and not in series...

    I was convinced it had to be a dropper, but there's no extra circuitry in the star to explain the voltage drop for 10 self changing leds...

    I've cracked it open
     

    Attached Files:

    Last edited: Dec 21, 2016
  17. cjdelphi

    cjdelphi

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    After ripping into it, i found 10 x 2.9k resistors from a rectified 300v

    Explains why each resistor shows signs of burning

    And yes, fair enough my mistake i never traced the circuit at the time to discover that as it was glued in..
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah I've seen Christmas lights like that where the resistors drop the bulk of the voltage and dissipate comparatively large amounts of power. :-(
     
  19. cjdelphi

    cjdelphi

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    Yeah i am pretty shocked, i'd have put money on it dropping via the cap, but yeah i should have realized when i saw it 3 years ago it was not a safety film capacitor..

    Live and learn (incidently, if i was going to do it, i would have used a film mk2? I have a bag or 2 or 3 full of them, never used lol)
     
  20. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Film caps at that size are way more expensive than aluminum electrolytics. Can you spell china?

    BTW, if it were a series cap, the peak current would be 235 mA and the LEDs would have a noticeable flicker as you walked by.

    ak
     
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