Magnetic Copper: Resistance is High…
Just kidding, we all know copper isn’t magnetic, mild steel on the other hand is most certainly magnetic and has the fun property of a much higher resistance than copper. And with steel being considerably cheaper, it’s no surprise that copper clad steel has been finding it’s way into a lot of electronic products lately. At least I have a new way to hang up my jumpers now, here is one stuck to a magnet on the wall:
So I purchased a pack of alligator jumper wires from a seller on eBay some time ago. They come in handy for a lot of things and I’m sure you’d be hard pressed to find an engineer or hobbyist who doesn’t have at least a few laying around. I tend to use them for quick connect of power, switching loads in and out of a circuit or measuring a voltage with the multimeter when I want my hands free.
The trouble (and discovery) all started when I was attempting to source about half an amp from a series pair of 1.5v batteries using a very capable 10 Amp boost converter. I had two wires in series with the batteries, one to positive and one to ground, for a combined total of 2 ohms. Not terribly much until you look at the math (V = iR) and find your dropping a volt from an already stressed pair of batteries. This additional resistance put the source voltage under the boost converter’s minimum startup voltage. The problem wasn’t immediately visible as I was measuring the voltage directly across the batteries and not at the input of the boost converter.
There were also two jumper wires connecting the load, a 3w 260 lumen white LED mounted on an old VGA chip cooler. Here the added resistance limited the current to about 500mA. The LED was rated for about 1 Amp.
This isn’t the first time I’ve crossed tracks with copper clad steel in non-ideal situations. Not more than a few months back a friends replacement laptop brick died. It turned out the AC plug had a one shot fuse built in. While replacing the plug end, the bare wire was really springy, like nylon fishing line and it wouldn’t hold a twist. It could be bent, but it would never hold it’s shape like normal copper. At the time I didn’t record the resistance but with measurement, we figured there must have been a couple of watts of dissipation under normal operating conditions. I did however get a couple of pics at 200x, here’s a close up of a single strand of the wire:
The next picture is the stripped jumper wire at 20x magnification, it looks a lot like normal copper wire from the outside:
And the same jumper wire at 200x magnification, it’s awfully smooth and round, perhaps that’s a clue to it’s metallic composition:
For comparative purposes here is some oxygen free pure copper speaker wire, I can tell it’s pure copper from the sound alone:
And for further reference, here is some solid core copper phone wire, it’s definitely not oxygen free, just look at those voids. It’s no wonder the phone quality was bad, I can just imagine the sound molecules bouncing off those giant voids!!
So there you have it, be on the look out for this mystery material, under certain circumstances it can be quite a load of fun or a complete headache…