We have a number of manufacturing engineers running around here at Screaming Circuits. They’re very good at what they do, as are our operators and technicians. They are not, however, electrical engineers. Our parent company has a big group of electrical engineers, but they’re at a different location.

What that means is, though we endeavor to be experts at building things, we often don’t know what the circuits and components do in your specific application. People tend to send us their difficult projects so we’ve probably seen just about everything possible go through our plant. But, every now and then we see something unfamiliar. It doesn’t happen very often, but it does happen.

Sometimes it’s an exotic new package (like the 0.3mm pitch wafer scale BGAs now
showing up). Other times, it’s something a bit older, but just not clear. Rather than put a job at risk, if we aren’t sure, we’ll always hunt down the designer and ask.

Okay. That was a long-winded intro.

We recently ran across just such an unknown; a “polarized” inductor, without an accompanying “polarity” mark on the PC board. Not only that, but the markings on the inductor were a bit ambiguous. One half is black and the other half is green. The datasheet is in black and white, so there’s more room for interpretation than we’re comfortable with.

At first glance, you might wonder why polarity / direction matters in an inductor. I did. It’s just wire. Right?

Almost: it’s not just wire, it’s coiled wire. In most cases, the direction doesn’t matter, but in cases with multiple inductors, or with super high speeds, it can matter due to the fact that the coil winding direction has an influence on the flux and the actual induction.

I won’t go into all of the theory, but think of walking. In most cases, it doesn’t matter whether you start with your left foot or your right. However, if you’re marching in a coordinated group, you want everyone to start with the same foot.

Look at the two sets of air-core inductors above. When set like this, directionality starts to make a bit of sense. Imagine the electrons being pushed around in theses things and try to picture the resulting lines of flux.

The moral of the story: eliminate ambiguity. If the part is polarized, either mark the board, or make it the direction clear to your manufacturer in build documentation. Do this even if the polarity doesn’t matter to you, ’cause we don’t know that.

After photographing these, I ended up recalling this bit of knowledge. It’s just so rarely needed that it had vanished in to the fog. I put a few more photos after my signature.

Duane Benson
Which way did he go?
Which way did he go?

http://blog.screamingcircuits.com/