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I'm planning on building some dimmable lighting controller for Christmas lights, but with LEDs being all the rage nowadays, I was thinking that instead of using triacs and dimming at 60/120 Hz, rectify the AC to DC and use MOSFETs to reduce the strobing that is more evident in LEDs. This might allow the controller to be smaller and/or cheaper.

This would depend on how the LED strings are actually wired up. I don't currently have any to look at, and I could imagine a few different ways they might be connected, such as diodes that only conduct for half the cycle, which would axe the DC idea if true.

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    \$\begingroup\$ I think you need to dissect one, in order to find out what it is doing... \$\endgroup\$ – Johan Nov 11 '10 at 19:47
  • \$\begingroup\$ In the past a lot of the technology behind different Christmas light displays has had patents to how they translate different independence from failure and lifetime/safety features. I know this may not be the case with most simple designs, but just dropping a warning from what I have seen. \$\endgroup\$ – Kortuk Nov 11 '10 at 21:53
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LED Christmas lights are wired in series to "gobble" up the 110V applied to them. A great site to read up on this is here.

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  • \$\begingroup\$ Nice read. Was very surprised about the breakdown rate though! \$\endgroup\$ – Vincent Van Den Berghe Nov 17 '10 at 13:35
  • \$\begingroup\$ So an antiparallel pair of LED strings--guess powering them off rectified AC won't work that well; only half will ever be on. \$\endgroup\$ – Nick T Nov 17 '10 at 18:25
  • \$\begingroup\$ @Nick T: Over the last couple years, I've seen "color-changing" LED lights which have anti-parallel LED strings in different colors. The supply selects one or other polarity to light a particular string. I've been thinking it would be fun to have some processor-controlled TRIAC outputs so I could nicely sequence the lights in different colors, but I don't know of any off-the-shelf controllers that would allow one to dim the hot-positive and hot-negative phases separately. \$\endgroup\$ – supercat Mar 11 '11 at 18:15
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I have been doing the following:

  1. Most standard LED strings work normally if I load the controller with one incandescent load. I used two 7 watt nightlight bulbs wired in series for a load (total load 3.5 watts). It works on most controllers. Increase the wattage if it doesn't work. I have one controller with two 15 watt bulbs in series. I put the lights in series so they don't burn out.

  2. A few controllers I have produce a rectified DC output. When used with my incandescent load, This lights half of the lamps in some strings and all of the lamps in some other strings. The strings that light only half the lamps have two series of lights, one connected in each direction.

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