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I just experienced for the first time something really weird & very bothersome with a speaker cable that had a white stripe on one of the conductors to indicate polarity (positive or negative whatever you choose it to be).

This was a heavy gauge wire which I used for the 5V power supply to feed the IBL panels on the overhead. Lucky for me, the IBL panels are not polarity sensitive otherwise I might have fried them.

Although the wire with the white stripe was definitely connected to the positive terminal in the power supply, at the other end of the cable where the IBL panels were located, I was getting a negative voltage on that same wire.

After scratching my head for a while, I did a continuity check on the cable & found out that the white stripe had changed sides (wires) from one end of the cable to the other, so the polarity was reversed at the IBL end.

I thanked my lucky stars that I hadn't yet connected the Flight Illusion board for the gauges otherwise that board might have been toast if the 5V had been reversed.

Moral of the story: NEVER EVER assume that a polarity marking on one of the conductors will always remain on that same conductor. Do a continuity check first to make sure that is indeed the case. Admittedly, this was a speaker wire and the wrong polarity would not have blown the speaker but the sound waves would have been 180 degrees out of phase.

Hope this might prevent someone from experiencing fried electronics  :)

Maurice

I've never seen that with normal speaker wire or twisted pair, but I've seen it with all sorts of cables. Especially RJ-45 cables, which can have cross-overs galore.

Are you sure the speaker wire isn't a crossover cable of some sort?

Quote from: rprather on December 24, 2011, 02:59:23 PM
I've never seen that with normal speaker wire or twisted pair, but I've seen it with all sorts of cables. Especially RJ-45 cables, which can have cross-overs galore.

Are you sure the speaker wire isn't a crossover cable of some sort?

Absolutely not. No way that any speaker wire comes with crossover since that would defeat the purpose of the markings which is to get the same polarity on all the speakers so that the sound waves are not 180 degrees out of phase & therefore cancel each other.

This must have been just a fluke in the way the wire was marked during the manufacture.

Maurice