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For years and years, for part of our Christmas decorations I have always run a length of pre-lit garland across the front of our stage and I've plugged it into an outlet that is connected to our house lights dimmer. The lights in the string are the old incandescent mini-lights, so they work fine on the dimmer. This year, part of the lights decided to give up the ghost and I am wondering if I substitute a string of the new-fangled LED lights in there, will they work with the dimmer? I don't really care if the "fade out" effect doesn't happen, but is it going to damage the lights or the dimmer? We have a Kelmar dimmer from about the late 80s.
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As long as there is enough overall load on the dimmer circuit, it should be OK; though as you predict, the ramp up and down might not look very elegant. As long as you're not trying to hold it at an intermediate setting for prolonged periods of time (i.e. the outlet is getting the full 120v when on), I can't see how this could damage either the lights or the dimmer. We've had a few problems with customers using our (MiT) dimmers, when either they have replaced incandescent house light bulbs with LEDs and the total load on the circuit (in amps) has not been enough to enable the dimming action to work properly, and/or they have used non-dimmable bulbs and tried to hold them for prolonged periods of time at an intermediate voltage. This tends to fry the power supply boards in non-dimmable LED bulbs. Otherwise, they'll just flicker a bit on their way up and down. -
It most likely won't damage the dimmer, but depending on the led light string's voltage drop circuit for the LEDs. that dropper may not like the dimmer much. Only way to find out is do a test, first find the dropper circuit (often in what looks like a socket but with no led in it), then power the string though the dimmer and make sure that dropper doesn't get more than slightly warm. I think most led strings would be ok, but might behave oddly during the dim up/down phases.
If that outlet is the only thing on the dimmer, to avoid the LED string from flashing during the dim up/down, you may need to add a small amount of incandescent load to it.Comment
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Exactly what type of dimmer is it? We use a variable voltage dimmer for our stage lights, it works nice enough with a switch-mode LED chain supply. No flashing, just a faster ramp up and down. However, with modern electronic dimmers, everything can happen. If you use this outfit only during christmas, you should probably better stick with the old classic incandescent mini lights. You should be able to source replacement mini-bulbs for them on ebay for very little money. Something on the plug or a label should tell you the proper voltage. Typically, you can also count the number of bulbs and divide the mains supply voltage by that number to find the right voltage. Since our chain is only ramped up and down through the dimmer, our bulbs hardly ever fail as they would when being hard-switched.
- CarstenLast edited by Carsten Kurz; 12-26-2020, 03:42 PM.Comment
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I concur with Leo and Tony here. In a similar though unrelated situation, I have a client that insisted that their stair tread lights be dimmed during the feature (not to off just that they sort of track with the overhead so things were nice a bright during show changes but "bright enough" during the show. A typical "chopper" dimmer was used and while the LEDs did, in fact, dim, the results were less than satisfying and stable. There was a VERY fine line between "bright enough" and either essentially off or flickering. There was also a very small range between the limits of "bright enough" and "full" .
So, until you can experiment with the LEDs you are using and the dimmer you want to use, you really won't know.Comment
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A little more info -- we have six or eight spotlights that flood the screen before the show (in place of curtains) so those are on the dimmer in addition to the regular house lights. We have one extra outlet tied into the same circuit as the stage lights, so that's where I plug in the Christmas lights. So, there is already an incandescent load on the circuit. The spotlight bulbs are #4515 which are a 6-volt bulb -- each of the light cans has a step-down transformer in it. They are leftovers from my days as a dance-floor DJ.
The light string that has quit has a weird looking fuse that I've never seen before - it looks like a letter H, plastic, with filaments down both sides. I'm sure it would be cheaper to just replace the lights. I may just find a spare set of LED lights and give it a shot.
The dimmer is one that has two circuits -- a "house lights" circuit and a "stage" circuit, so these lights and outlet are wired to the Stage side. The circuit is never left "half on," it's always full-on or full-off after a dim cycle.
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Keep in mind that all dimmable Christmas LED lights come with a of power supply that was designed to support dimming and are often labeled as such, as it requires some extra logic in this power supply to make the LEDs dim. Since LED lights are semiconductors, they don't dim like normal, pure resistive loads like classic light-bulb filaments. While you can regulate the brightness of a LED to some extent via voltage differences, this is very ineffective and often causes damage to the LED. The correct way to reduce the output of a LED, is to pulse a LED very fast. So, in order to make them dim, the power supply has to "translate" the drop and rise in voltage into "pulse widths".
Most of the cheap Christmas LED lights I've encountered are, unfortunately, of the non-dimmable type and like mentioned before, those who are, often come with an entirely different ramp up/down than the classic "bulb based" ones. The most reliable way to dim LEDs is via professional LED drivers, they come with an extra input for dimming, either analog or digitally. Unfortunately, they don't easily integrate with existing voltage-modulating dimmers.Comment
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We've found this to be the solution when customers try to use our M-series dimmer with LED bulbs. Typically what happens is that they've been using a bunch of PAR E27 incandescent bulbs for their main house house lights, swap them all out for cheap (though claimed to be dimmable) Home Depot LEDs, and find that they flicker during the ramp up/down, and/or that they don't completely go out for a few minutes after the ramp down is complete. Replacing the first LED downstream from the dimmer with one of the old incandescents almost always fixes it.Originally posted by Tony Bandeira JrComment
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I haven't really done any teardown or measurements but I'm pretty sure the Christmas light strings are just a bunch of LEDs in series with a resistor as a current limiter in series. They are done the cheapest way possible. If I stare at them in the right conditions I can easily see the 60 Hz flashing because they are only on for part of half of the sine wave. They aren't designed like an LED bulb or decorative LEDs designed for dimming control.Originally posted by Tony Bandiera Jr View PostComment
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[QUOTE=Lyle Romer;n8011]
... I'm pretty sure the Christmas light strings are just a bunch of LEDs in series with a resistor as a current limiter in series. ...
... If I stare at them in the right conditions I can easily see the 60 Hz flashing because they are only on for part of half of the sine wave.../QUOTE]
I think you're right but I also think that they might be series-parallel, having two strings of lights in series connected in parallel so that the diodes are going in opposite directions. That way, one of the parallel strings is always conducting so that all of the sine wave gets used.
You'll still be able to see the individual lamps flicker but, if you slow it down, you'll notice that half of the LEDs are lit and the other half is dark.
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Some cheap versions work like that, others use a DC power supply. I have seen chains with 4.5VDC and 34VDC, but, technically, they can be made in a big variety of schemes.Comment
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I have many customers running LED on Kelmar dimmers. The Cree works fine without any modifications to them at all. But if you use other brands Kelmar has at least one option available.
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Our Christmas tree lights are powered through a 5 volt, 4 amp DC wall wart. Haven't tried dimming the input power, though. Might do for a giggle when we're taking the tree down on Sunday - I have a plug-in dimmer (intended to enable a table lamp to be retrofitted with dimming capability) in a box of crap in the garage somewhere.Originally posted by Carsten KurzComment
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Mike, Call Tom at Kelmar and he should be able to tell you all compatible lamps that work with their dimmers. (631) 421-1230Originally posted by Mike Blakesley View PostComment
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There are several segments (unless they are very short strings) and each segment looks like just a bunch of series LEDs in series with a resistor. The segments are parallel but I don't think they bother reversing polarity on every other segment.Originally posted by Randy Stankey View Post
It's basically (a very crude text based schematic):
|-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-|
|-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-|
|-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-|
|-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-|
When stretched out it turns into
|-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-||-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-||-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-||-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-D-R-|
There's basically a hot wire and a neutral wire that run end to end (but physically have connections on larger sockets) and then each series segment is connected in parallel via the same larger sized sockets. If you untwist the string you can see the way it is connected.
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