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

To: rfi@contesting.com
Subject: Re: [RFI] LED Christmas Light RFI -> LED lead corrosion
From: Kim Elmore <cw_de_n5op@sbcglobal.net>
Date: Sun, 01 Dec 2013 21:58:39 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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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