Electronic Design

Bob's Mailbox

Hi Bob: I see more and more uses of LEDs in place of incandescent lamps every day. However, many of our engineering brethren are not looking at the big picture with these. It would appear that they are not considering the devastating interference that some of these replacement LED (power-supply) units are creating on the AM radio broadcast bands.

For example: traffic signals. While I was waiting under a red traffic signal, I had a barrage of repetitive static wiping out the AM broadcast station that I was listening to. That is, until the light turned green. This RF buzzing interference had not happened before at this location. Upon closer examination the next day, it appears that (just) the red traffic signal incandescent bulbs were replaced with a load of red LED signal lamps, hence the interference during red signals, and not during the yellow and green incandescent illuminations. (Apparently, some guys have used crude switchers to convert high V to high current, and to hell with the RFI/EMI. Quite unfortunate. /rap)

In summary, more thought should be put into our design projects to ensure that we don't cause problems with other existing technologies, and in this case, we need to be aware of working on LED replacement lamp power-supply solutions that don't cause major interference. (For sure!! /rap)

I doubt if these red LED signals would meet FCC Part 15 regulations, as I would have definitely missed a tornado warning broadcast on that station until I was able to drive away from the offending RF generator. (I'll switch over to AM reception. Here in the Sunnyvale area, there are a lot of LEDs. /rap)

The other item down the pike is the proposed Broadband over Power Line (BPL), which has the potential to derail communications across the board. But that's another topic for another day.

--John Pavlica (via e-mail)

Pease: That's a whole other mess. All my ham friends are furious about this proposal.

Hi Bob: Just read Jason Cook's inquiry on eyelet usage in pc boards (electronic design, Feb. 17, p. 20). It sent me back to the early '70s when I serviced consumer Hi-Fi equipment. A well-known manufacturer (KLH) produced the first reel-to-reel tape recorder with an integrated Dolby system. Technically, the machine was quite an evolution and sounded excellent. However, the vast majority developed intermittent problems virtually immediately. The KLH Model 41 was plagued by the use of "eyelets" through the pc board to connect upper and lower layers. I've spent many an hour chasing down intermittent connections in KLH Model 41s. All this to say—eyelets? Please no. A plated-through connection is the better way to go. Surely, there was nothing saved by the use of eyelets if reliability was any concern at all. (It is imaginable that a really heavy component might make you wish you had an eyelet to support the heavy weight—in shock conditions—which would, of course cause them to go open! /rap)

--Geoff Pomeroy (via e-mail)

Pease: Hi, Geoff, your story is consistent with other people's stories. If I had a board without eyelets, I would just wire a wire through the hole and solder the wire (at least 3/16 in. of it) to the top and bottom foils. Or I would preferably do two of those, in parallel, through adjacent holes. I've done that. I am convinced this is very reliable. If I had a pc board that already had an eyelet, I would solder a wire through the hole (or solder it to the component lead that goes through the hole) and extend it about 1/4 in. away and solder it, top and bottom, to the two foils. The 1/4 in. would act like a strain relief. Is this an adequate fix or "band aid"? It's consistent with what I have seen, but I repaired a lot of my pc boards, and almost none of anybody else's, with eyelets. Philbrick used eyelets, in Malter's SP656, in parallel with shorted (top-bottom) edge connectors, so we would never see any failure. Failures would be unnoticeable.

Comments invited! [email protected] —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

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