[RFI] LED Christmas Light RFI -> LED lead corrosion

[Kim Elmore]

Now THIS is interesting. Up until now, I have used the classic series 
strings and have always suspected that corrosion occurred during 
storage. Yet, I could never find much evidence of corrosion because the 
lamp leads are copper and the socket material is brass.

I recently replaced incandescent lamps in my amplifier meters with white 
LEDs. As I was cleaning up, I noticed to my astonishment that the 
clipped parts of the LED leads are magnetic. I remembered thinking that 
whatever they were, they sure were surprisingly tough to cut... If the 
leads are some sort of ferrous material (likely mild steel) that has 
been copper plated and then tinned to make soldering easier, the 
corrosion you see is easily explained.

It's also interesing that they depend on the (poor) PIV ratings of the 
LED series string for string AC operation. If this is how they manage 
the LED strings, then these will be very RF quiet though they may not 
last long as outdoor lights if this is the common construction practice.

Kim N5OP




On 12/1/2013 6:30 PM, John DeGood wrote:
> My father-in-law WA3GNU has 3 strings of LED holiday lights at his QTH.
> They consist of 60 LEDs per string: each string is divided into 2 halves
> with each half consisting of 30 LEDs in series with 3 x 735 ohm
> resistors (sealed in heatshrink tubing) connected directly across the
> 120 VAC line and protected by a pair of fuses in the plug. Given this
> circuit topology, I would not expect RFI problems and WA3GNU has not
> noticed any.
>
> When we took them down last year they were all working, but all 6 halves
> of the 3 strings were dark when we attempted to return them to a third
> season of service this week. Troubleshooting, I discovered that
> approximately 25% of the LED lamps had suffered corrosion failure in
> which one lead was broken due to corrosion. Many of the remaining LEDs
> with both leads still intact showed obvious evidence of serious
> corrosion in progress.
>
> These LED lamp strings are constructed identically to miniature
> incandescent holiday lamp strings, with the LED leads simply inserted
> through a pair of holes in a small keyed plastic base and then bent back
> against opposite sides of the plastic base to serve as the contacts. The
> plastic base is then inserted into a keyed plastic socket such that the
> LED leads press against a pair of flat contacts which appear to be made
> of brass or a similar metal.
>
> I hypothesize the corrosion resulted from the combined effects of
> moisture, dissimilar metals (LED leads vs. socket contacts), and DC
> current (the series LED string acts as a rectifier). A few of the LED
> lamps showed no evidence of corrosion: I hypothesize they might by
> chance have been positioned such that they did not get wet. Thus, this
> corrosion failure mode might not occur in a dry indoor environment.
>
> John NU3E
>
> On 12/1/2013 3:51 PM, Kim Elmore wrote:
>> Does anyone have experience with RFI from LED Christmas lights? Are some better than others? All bad? No problems?
>>
>> Kim N5OP
>>
>> "People that make music together cannot be enemies, at least as long as the music lasts." -- Paul Hindemith
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