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Author Topic: Mobile Noise from Leds  (Read 4502 times)
NC4TB
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Posts: 38




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« on: December 18, 2015, 08:04:24 AM »

Anyone else had problems with LEDs causing moderate white noise on ten meters and horrendous banshee like wail on the am radio in my 1989 S10 Blazer? It's worse with the engine off. I've narrowed down the primary source to be a festoon led bulb in the roof. So I placed a .01 mfd disc cap across the led with no change at all. Do I need a larger capacitor or should I extend the wiring and run several turns through a ferrite? That would be a pain to accomplish, as the wiring is under the headliner. As an aside, the LEDs don't seem to produce much noise on 2 meters, and none on my public service Scanner.
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WB4SPT
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« Reply #1 on: December 18, 2015, 08:23:50 AM »

Surprising.  Sounds like that LED assembly is more than just leds and resistors.  I would try a different manufacturer.  The .01uF is actually a good choice for value.  A leaded .01 has a SRF at ~26MHz. 
More heroic are caps to ground at the device, or try a so-called alternator feedthru cap.  Might be a bit big however. 
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NC4TB
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Posts: 38




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« Reply #2 on: December 18, 2015, 11:40:46 AM »

LEDs control their power consumption and thereby avoiding overheating by shutting off the power to the LED at a rapid rate. It acts sort of like a dimmer with a variable SCR .Think of it like a rapid flicker but so fast the human eye can't see it. I actually soldered the disc cap to the ends of the festoon bulb, basically shorting the ac/rf output at the bulb. I suspect the hot wire and possibility the ground wire are acting as radiators even with the cap across them. And being up in the roof of the car, the ground is actually somewhere in the dash. There really isn't anything to ground the "ground" to up there, short of putting a screw through the sheet metal, which isn't going to happen.. If I ever have to replace the headliner I'd try a shielded cable instead of the original wiring. I'll try a different brand, but they are all made in China. The reason I went with interior LEDs was for more light, and the fact that I accidentally ran the battery down several times after leaving the incandescent bulbs on. They really weren't bright enough to notice in the illuminated garage. With LEDs I have much more time to notice they are on before the battery is ruined.
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WX7G
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« Reply #3 on: December 20, 2015, 05:24:36 PM »

A capacitor at the noise source (the switching circuit that drives the LED) might reduce the noise. A capacitor at the LED will not decrease the noise. One troubleshooting test and a possible fix is to remove the LED.
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N1UK
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Posts: 1946




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« Reply #4 on: December 21, 2015, 11:15:21 AM »

Am I missing something but wouldn't it just be easier to replace the LED with an incandescent bulb which was no doubt a standard fitting in 1989 ?


73 Mark N1UK
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NC4TB
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« Reply #5 on: December 25, 2015, 12:06:56 PM »

Mark, I went to LEDs because I seem to have developed a bad habit of accidentally leaving the interior lights on and running the battery down to the point of ruining it. Twice. The LEDs are much brighter and more likely to be noticed. Plus a longer run time to failure of the battery. And, as mentioned, the incandescent bulbs were terribly dim.
On a side note, I put a different brand in the ceiling light with much reduced noise, so all LEDs are not as noisy as others.
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WX7G
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« Reply #6 on: December 25, 2015, 04:22:51 PM »

Interesting. I wonder if the LED lamp has a buck regulator (RF noisy) to eliminate a lossy (heat) series resistor and to allow a small number of LEDs to be used? If so, an incandescent lamp should eliminate all of the noise.
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WA2ISE
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« Reply #7 on: December 27, 2015, 01:31:59 PM »

... If so, an incandescent lamp should eliminate all of the noise.

A resistor would eat less energy than the incandescent bulb would.  3 white LEDs in series and then the resistor would do it. 
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AA4PB
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« Reply #8 on: December 27, 2015, 02:02:05 PM »

I've heard the LED turn signal lamps on other vehicles making noise on HF bands when stopped behind them.
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KC2MMI
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Posts: 715




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« Reply #9 on: December 31, 2015, 05:23:04 PM »

"LEDs control their power consumption and thereby avoiding overheating by shutting off the power to the LED at a rapid rate."
 Sorry, that's just not so. An LED by itself has NO control over power and overheating that way, it is "just" a diode.
Many LED lamp assemblies have some type of digital power control and switching circuitry added to them, but others just use a dropping resistor. (I suspect the digital circuitry is actually cheaper these days.)

 But a single "white" LED is typically a 4.5v device, so a plain white LED bulb will always have some other components associated with it in a car, unless they're ganged in 3's to use the nominal "12" volts. I think they've gotten white LED's down to 3v or less these days, not sure how far that has been pushed.

 "LED" noise is always from other components in the assembly.
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W9IQ
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Posts: 399




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« Reply #10 on: January 01, 2016, 05:49:03 AM »

If the goal is technical accuracy, then I must point out that LEDS are current driven, not voltage driven, devices.  So to say that an LED is a 4.5 volt device is incorrect. At the specified current, the voltage drop across the LED is, for example, 4.5 volts.

The current through, and brightness of, an LED can be controlled through PWM of the current. It is the PWM circuit that is often the source of RFI if RFI mitigation is not properly applied.

- Glenn W9IQ

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