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Thread: Noisy LED bulb

  1. #1
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    Default Noisy LED bulb

    So a friend of mine who lives off-grid brings in an LED MR-16 type bulb.... His issue was that when the LED bulbs are on, they completely wipe out AM, FM and weather radio reception in his place.....

    He has one type, a warm white that he likes from Home Despot for $30, and a bluer type that he doesn't like, but that is 8 bucks delivered from a chinese source.

    Both are to blame for this noise. Both are CE marked, which may be a simple "self-certified lie", at least as far as the EMI situation.

    Whan I checked the 8 buck one out, it produced full quieting of an AM radio several feet away from the bulb and wires, completely wiping out a local AM station that the radio was actually tuned to..... all you heard was either near silence at the 550 khz end, or strong hash noise at the 1600 kHz end of the dial.

    Investigation with the equipment here produced the information that it gave a strong wide-band noise output up to at least 5 mHz, which was as far as my easily-brought-in radios would go.

    Interference with FM was also noticed, but not as strong. He plugs his FM radio into the 12V DC wiring at his place, which also runs the 12V lights. If he plugs into the inverter output 120v wiring, he has less problem with the noise, presumably due to the isolation between them.

    These little bulbs apparently use a totally unfiltered SMPS to reduce 12V input (AC or DC) to 4V or so to run the LEDs.

    At 12V input, they are as noisy as sin. With 9V DC input, they are pretty quiet, no doubt due to the variation of SMPS duty cycle with input voltage.

    I intend to use the spectrum analyzer at work to check them out in detail, but even without that, I am shocked at the amount of RF noise this little 4W LED "bulb" puts out.

    I had figured that OK, a capacitor across the DC input would probably do the job.....

    Uh-Uh.... not even close. The direct broadcast noise, presumably from an inductor in the SMPS, evaded any line filters I tried, so that even though the DC lines were filtered and quiet, the radio still picked up strong interference. It took a pretty decent Corcom line filter to clean up just the wires, with no effect on directly emitted RF noise.

    I do NOT like this....................

    Here is the perpetrator..... known as an "LX051S"




  2. #2
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    Report it to the FCC. Seriously. That is a hazard to the entire RF spectrum.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  3. #3
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    My mom had recently sent me a link to a scare documentary that talked about how compact fluorescents throw out horrible radiation as well.

  4. #4
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    Quote Originally Posted by beanbag
    My mom had recently sent me a link to a scare documentary that talked about how compact fluorescents throw out horrible radiation as well.
    It's been known for a long time that any kind of fluorescent bulbs generate strong interference to AM / shortwave listening.

    I was really hoping that LED bulbs would not, but it looks like the store-bought kind may not be useful there. Have to build yer own with a stepdown transofrmer, a couple rectifying diodes and a resistor I spose....

  5. #5
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    I have the same bulb in front of me. The output is about 6 volts since the diodes are wired series-parallel. It is filtered with a small electrolytic which can be verified by waving your fingers in front of the bulb when it is on. The inductor is about .1" in diameter and the lead to the diodes about an inch long. The heat sink shell is electrically isolated from the circuitry so it cannot act as an emitter. I find it hard to believe that such a small inductor with such a short length of wire can transmit much on the AM frequencies.

    Using a handheld multiband radio on a local AM station I get mild interference from my track lights within 18 inches or so. The track lights in question run from 120vac so they have indvidual ballasts for each light. The noise is coming from the ballast since the one I took the bulb from is still making noise without the bulb installed.

    Correction: I also have a string of three that are running from 12vac. Those bulbs are making some RFI but it only interferes at a distance of less than 18 inches or so. We have a pretty weak AM signal here as we are in a deep valley and the transmitter is on the other side. Incidentally, quieting is produced by a carrier, not by RFI. It isn't a term used to describe AM receivers since a carrier will beat with another AM signal. Quieting applies to FM discriminators since they will lock on the strongest signal and ignore the rest.
    Last edited by Evan; 03-17-2011 at 07:54 AM.
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  6. #6
    Rosco-P Guest

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    Quote Originally Posted by J Tiers
    He has one type, a warm white that he likes from Home Despot for $30, and a bluer type that he doesn't like, but that is 8 bucks delivered from a chinese source.

    Both are to blame for this noise. Both are CE marked, which may be a simple "self-certified lie", at least as far as the EMI situation.
    What about the findings on the $30 HD bulb? The one I saw seemed to be intended as a direct replacement for PAR bubs in recessed cans. The color temperature was a little on the cool side, but it did have a high CRI.

  7. #7
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    As a point of reference I just measured the 20 watt LED bulb I built using a standard Chinese off the shelf emitter and switch mode power supply. It's installed in a can on a track light. It produces detectable interference only directly in front of the emitter at a maximum distance of about 2 inches.
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  8. #8

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    Quote Originally Posted by Evan
    I have the same bulb in front of me. The output is about 6 volts since the diodes are wired series-parallel. It is filtered with a small electrolytic which can be verified by waving your fingers in front of the bulb when it is on. The inductor is about .1" in diameter and the lead to the diodes about an inch long. The heat sink shell is electrically isolated from the circuitry so it cannot act as an emitter. I find it hard to believe that such a small inductor with such a short length of wire can transmit much on the AM frequencies.

    Using a handheld multiband radio on a local AM station I get mild interference from my track lights within 18 inches or so. The track lights in question run from 120vac so they have indvidual ballasts for each light. The noise is coming from the ballast since the one I took the bulb from is still making noise without the bulb installed.

    Correction: I also have a string of three that are running from 12vac. Those bulbs are making some RFI but it only interferes at a distance of less than 18 inches or so. We have a pretty weak AM signal here as we are in a deep valley and the transmitter is on the other side. Incidentally, quieting is produced by a carrier, not by RFI. It isn't a term used to describe AM receivers since a carrier will beat with another AM signal. Quieting applies to FM discriminators since they will lock on the strongest signal and ignore the rest.
    ????
    How does waving your hand verify it is using an electrolytic? That only shows the bulb is flickering.

    The case being unconnected DOES allow it to radiate, it is capacitively coupled to the noise inside. If you don't want it to radiate it needs a low impedance path to ground.

    What is your theory as to why your bulbs/setup are producing so much less noise than the OP? What you are seeing is what I would expect of a bulb like this.

    Do either of you have pics of the bulbs disassembled?

  9. #9
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    Waving your fingers in front of a bulb is very good way to see how well the DC supply is filtered. Wave your hand in front of your TV to see what I mean. Lots of flicker there. These bulbs don't produce any noticable flicker which indicates decent filtering.

    The case being unconnected DOES allow it to radiate, it is capacitively coupled to the noise inside. If you don't want it to radiate it needs a low impedance path to ground.
    That isn't possible with an MR16 bulb. There is no ground available. Capacitive coupling will be very slight. The wires are twisted together and run through a hole in the center of the heat sink shell to the plate that has the emitters bonded to it. It is very symmetrical which reduces coupling. I will takes some pictures shortly.
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  10. #10
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