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  • Ot christmas lights

    I decided I couldn't let Steve's arc-light christmas tree go unmatched. Therefore I have 20 50w halogen bulbs that I want to run from one of my led strings. I have some colored gels and it will be easy enough to put this together. The control box has a variety of blinking modes, and all I need do is put one opto isolator in series with each string to get a control for a triac to run the halogens. So I look around for some triacs, and all I have are some 25 and 40 amp ones, kind of overkill for running about 250 watts of lights each. I have several scrs, so to get full wave operation, I could use a bridge rectifier off the line, and run the scrs and light from that. Now that will feed pulsating dc to the halogens-

    I'm sure it will work, but one thing I'm curious about- will running these lights on dc make any difference to them? I read somewhere that this shouldn't be done, but I can't think of any good reason why. Something to do with degradation at one of the electrodes-

    Car headlights don't have a problem- and I'm sure there are lots of examples to show that there isn't any problem with running a halogen on dc. Any comments?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Why don't you just run the SCR's in inverse parallel? That's what a triac is, after all.

    High-voltage lamps (ie, 120V) don't like DC because it causes migration of the filament's metal from one side to that other (I can't remember the direction of flow). This eventually creates a thin spot which fails mechanically - it takes a long time, though. 12V lamps have a much heavier wire in the filament, and much less of it, so they are not bothered so much.

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    • #3
      I made a little gadget for my LED xmas lights. It's a full wave bridge, a filter cap and fuse with a ballast resistor that limits string current to 40 ma or less. It's overdriving the LEDS somewhat but they can take it. It's cold here so they won't overheat. It really pumps up the brightness and part of that is by removing the irritating flicker. They look much better, almost like incandecent brightness but with really pure colors. The colors are much better on DC I'm guessing because the LED isn't constantly going off and on and shifting color in the process.

      Here is the "booster" box.



      Here is a before and after shot. I took the streak photo to show the difference in flicker by just waving the camera during the exposure. On DC the apparent brightness looks to be about twice as bright.

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      • #4
        Ok, I think that explains it about the halogens on dc. If it takes a long time- how long is that- they will probably burn out from the constant cycling on and off before that happens. I don't care, they're christmas lights, they're supposed to burn out

        I could use the scrs back to back. I suppose the triggering would be by putting one resistor in series with the triac in the opto, and in between the gates, with a resistor from each gate to cathode, as is usual.

        Evan, I think you're more irritated by flicker than I am, but I think I'm going to copy your idea about the filtering. My string uses 20vac to run the circuit, since it relies on the ac pulses to run the clock. I might have to use a half wave rectifer and cap so I don't upset that circuit, or run the led strings themselves from a separate dc adapter.

        I have a few of these strings- one I modified to work in the van. Yeah, I had a string of leds merrily flashing away in the van for the duration last year. I had to add a 555 circuit to supply the clock pulse to keep the flasher circuit going, and had to cut the number of leds per string so they would still work on the lower voltage. Actually I cut each string in half and paralleled them, giving each it's own limiting resistor. This story wouldn't be complete without me telling how many hours I spent unravelling those confounded strings. So- this story won't be complete.

        Looking at your booster box- careful where you put your fingers- 179 volts can give quite a shock!
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          I would expect to see some Edison effect with DC driving the lamps at the extreme temperature they operate at. Most store-bought lamp dimmers use triacs and the also have all the zero-crossing electronic controller you need, including RFI filtering. Hang a FW diode bridge on the output, clamp that with a healthy HV electrolytic cap and a current limiting resistor and you have your adjustable full-wave semi-filtered DC good for 1kW.

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          • #6
            Originally posted by Evan
            I made a little gadget for my LED xmas lights. It's a full wave bridge, a filter cap and fuse with a ballast resistor that limits string current to 40 ma or less. It's overdriving the LEDS somewhat but they can take it. It's cold here so they won't overheat. It really pumps up the brightness and part of that is by removing the irritating flicker.
            Great demo of strobing LED's Evan. It's fun to do that with a florescent tube swung in a circle just to convince the non-believers they really do shut off 120 times a second. Who was your invisible assistant?

            Edit: Oops - never mind - just re-read that you shook the camera, not the wire

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            • #7
              Originally posted by dp
              Great demo of strobing LED's Evan.
              -Agreed, that was clever. I have a couple of applications that might be useful at least as a quick-and-dirty diagnosis. Kind of like using my little point-and-shoot digital (so I can watch live via the screen on the back) to see if infrared LEDs are working.

              It's fun to do that with a florescent tube swung in a circle just to convince the non-believers they really do shut off 120 times a second.
              -I once shot a hockey game, at an indoor rink. Brightly lit with something blue-white. Sodium halide? Anyway, I took plenty of shots, but fully a third of them were badly underexposed- like by two full stops.

              I had my ISO up, in order to keep the shutter speed up due to the fast action. But the fast shutter- 1/1000 on average- was faster than the 60hz flicker of the lights, so I'd frequently catch a shot at the bottom of the light/dark cycle, resulting in an major underexposure.

              That was a real bear to try and overcome, and I spent most of the game playing with settings- it didn't help that the kelvin temp of the light changed as it went through the cycle, so one shot would be a cool blue and properly exposed, the very next, taken an eighth-second later, would be two stops underexposed, and heavily red/orange tinted.

              Doc.
              Doc's Machine. (Probably not what you expect.)

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              • #8
                I used to play with fluorescent tubes for strobe lighting. I would make up a pulse generator and sometimes would only make one connection to the tube. The phosphor has a residual brightness after the exciting energy has stopped, but it has a decently fast response between that and full brightness. A friend took a nighttime photo of me walking across the grass with only the 8 ft tube to give light. There were three of me on the print, quite distinct.

                Imagine how bad the flicker could be if the driving energy was not a sine wave- at least with a sine wave the brightness gets to ramp up and down to some extent, minimizing the impact of the flicker.

                Anyway, thanks Doc- that's something to think about when trying to take pics under those types of lighting.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  A friend took a nighttime photo of me walking across the grass with only the 8 ft tube to give light. There were three of me on the print, quite distinct.
                  Only three?

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                  • #10
                    Originally posted by Evan

                    Here is a before and after shot. I took the streak photo to show the difference in flicker by just waving the camera during the exposure. On DC the apparent brightness looks to be about twice as bright.




                    Very cool AC/DC demo pic Ev, the things you can do with a camera...

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                    • #11
                      Looks like it is going to be another Clark Griswald Christmas!!!

                      --Doozer
                      DZER

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                      • #12
                        careful where you put your fingers- 179 volts can give quite a shock
                        That's why it also includes a bleeder resistor on the filter cap. Such resistors should always be applied directly across the capacitor terminals and preferrably be several in parallel for redundancy.
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                        • #13
                          Originally posted by Evan
                          Only three?
                          -I took one like that back in highschool with a K-1000 and some Tri-X film.

                          Dark winter night, no moon, tripod and a "lockable" cable release. I moved around and stood still while a friend of mine swept a flashlight over me. Tried different poses, and we had a thin dusting of snow so I could keep track of where I'd been standing.

                          Wound up with twenty or thirty ghosts of me, some of which I managed to get looking like one was talking while the other was listening, etc. Tried for the "party" look, but was closer to "people milling around a courtyard".

                          Took second place for B&W my sophmore year with it. Wish I still had the photo- I gave the matted copy to the photography teacher, figuring I'd always be able to print more with the negative. I think most of those negatives, though, got tossed years later since I didn't have or have access to, an enlarger or darkroom.

                          I do still have a copy of the one where I waved a plastic-tube "lightsaber" type flashlight thing with a similar timed exposure, but that ones not as cool.

                          Another thing to try; depending on the version of your Canon software, you can cable the camera directly to a PC, and set it up to automatically take an exposure at a given interval. One shot a second for ten shots, or one shot every fifteen minutes until it runs out of memory, or one shot per day precisely at 10:33:55am, or whatever.

                          Fun to play with, especially doing brief time-lapse setups. Crank the camera down to "small JPG" and it'll take thousands of shots before filling the card, hook it to a wall-wart adapter, and have it take a shot every 30 seconds or one minute while you work on a project in the shop or something. (It stays on while waiting between shots- it doesn't hibernate. So you need good batteries or wall power if doing a time-lapse of more than an hour or two.)

                          Doc.
                          Doc's Machine. (Probably not what you expect.)

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                          • #14
                            There is also "ghost writing".



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