120VAC Dimmer for LED christmas light strings

Discussion in 'The Projects Forum' started by tootalltechie, Apr 9, 2008.

  1. tootalltechie

    Thread Starter New Member

    Apr 9, 2008
    2
    0
    I am thinking of making a set of dimmers for Christmas lights. They operate at 120 VAC 0.03A (35 LEDs per string). I have tried to use a normal 300 watt wall dimmer to control the brightness but with out a small incandescent lamp in parallel with the LEDs. The string of Christmas lights will not light until the dimmer is above some unknown voltage (I have not measured). Does anyone have a dimmer circuit that I could make that would not need the small incandescent lamp in parallel with the LEDs?

    thanks for the info
    Gordon
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    If you use the "Search" link in the blue bar above, and look up "PWM" or "Pulse Width Modulation" you will be well on your way to achieving your goal!;)
     
  3. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    The LEDs have a forward voltage that must be overcome, assuming they're in series, something like: 35 x 2V = 70V, plus some amount of drop over the ballast resistor in order to generate enough current for them to appear as lit.
     
  4. tootalltechie

    Thread Starter New Member

    Apr 9, 2008
    2
    0
    Thank you for understanding my issues. If I am correct a square wave PWM out put would deal with problem. correct?
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    That is correct. The LEDs will be brighter with a longer duty cycle and dimmer with a shorter duty cycle.
     
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