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"Noisy" LEDs


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A number of people have been having trouble with noise from the LED strings coupling over to other strings. Most of the reports have only involved 2008 CDI lights, although people have been so sensitive to problems with that brand that I don't know if the problems are confined to those lights or not.

At this point it doesn't appear to me that there is a clear way forward on this matter. So far nobody has reported this problem with the 2009 sample strings. The number of strings out there is so small and the testing is so cursory to date that I don't know if this problem will recur with next years strings or not. The most likely way that I see to prevent the problem would be for CDI to revert to the 2007 strings, as I haven't seen this problem reported with those items.

So, I'm starting a thread to discuss this topic...

Comments (starting with problem reports)?

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Phil,

I have the '08 CDI strings in my display. I am having some crosstalk issues with parts of some sequences, but it is not too serious. The most visible case is when sequentially fading the Blue 50s on my little Mega tree (16 strands of Blue M5 50s), some static wireframe deer with standard minis set to 50% on a different controller will flicker dimmer for several seconds, then back to normal in a repeating pattern as the tree "rotates".

To help my strings (M5 red, green, and blue 50s and M5 cool white 100s) survive the season I put resistors in series with the AC (before the rectifier/doublers). This limits the peak current and I have not had any failures. I am fading/dimming all of them, including the red 50s. I have not modified the actual voltage doublers as some here have done. Where I had several series strings I put a resistor at the end of my extension cord to limit current for all of them in one place. On individual strings I put a resistor between the strings AC plug and first rectifier/doubler module.

I am wondering where the noise is actually causing the problem, are the Triacs triggering, latch outputs getting set, or is it getting into the PIC micro? My case does not seem to noise on the comm link, I believe my static elements get a single command for 50% at the start of the sequence, I do not change it for the duration. (at least I think this is how LOR works). If this is true, comm noise would not fix itself until a new command is sent for a channel.

I do not think there is any AC filtering in the LOR controllers. I built some LOR PC series controllers as basic kits and do not remember any AC filter type parts being installed. Only a few decoupling caps on the logic DC supply. Maybe a noise filter for the card logic power AC input would help? Maybe some filter on the Triac circuits of this is where noise is causing the problem?

A lot of LED strings with a diode-capaciter input doubler would generate significant noise spikes, particularly at 40-60% dimming when the triacs are switched on near or at the AC peaks.

The strings might generate less noise with a larger AC input resistor and smaller DC limiting resistor. This would reduce the peak current charging the caps. Basically limit the current more at the AC side instead of the DC side.

I think the voltage doubled strings are here to stay, these are probably cheaper to build and some people prefer no "blobs" througout the string. I do not care but also understand how some would prefer no "blobs".

Howard

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I am using a significant number of 100ct Warm White strings from Costco and they definitely have the noise problem when fading. I've gotten rid of the worst problems by modifying my sequences to use less fading, but also i've added some in-line EMI filters that have helped with the worst offenders.

The EMI filters I used are similar to the X10 noise filters that are sold.

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I am wondering where the noise is actually causing the problem, are the Triacs triggering, latch outputs getting set, or is it getting into the PIC micro? My case does not seem to noise on the comm link, I believe my static elements get a single command for 50% at the start of the sequence, I do not change it for the duration. (at least I think this is how LOR works). If this is true, comm noise would not fix itself until a new command is sent for a channel.

So far I had not considered that the noise was getting through to the 5V power on the controller, although that still has to be ruled out. The reason that I haven't considered that is that I've been assuming that the controller power supplies have a lot of filtering on the transformer outputs before the regulator, at least that is how most transformer-based power supplies are designed.

My conjecture for the moment (and it is just a conjecture) is that the noise is causing the triacs to false-trigger. If you look at the datasheets and application notes for triacs you will see that triacs can also be triggered (turned on) if there is a fast voltage edge on their output (static dv/dt triggering). This application note http://www.littelfuse.com/data/en/Product_Catalogs/PowerThyristorApplicationNotes.pdf (look up AN1001.pdf from Littlefuse if this link doesn't work).

So I think that the noise from one triac might be coupling over onto the outputs from the other triacs, and is causing those other triacs to randomly trigger. I haven't done any analysis to see if this is BS or not. I do suspect that this problem would get worse if there are long AC power cables involved (especially on the output of the controller), and would also be worse if you are using a shared neutral power cable between the controller and the lights. If this conjecture is correct, I think that a snubber on each controller output (such as R3/C3 in figure 14 of that app note) might be helpful. Not being an analog expert, I'm not sure if the snubber should be placed close to the lights or to the controller.

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