Dots on inductors

It indicates phasing alone. Constructional details, where critical, will only show on the winding sheet.

You won't get the winding you want without including winding instructions in the transformer drawing and then batch inspecting each purchased lot.

RL

The dot is for phasing of a TRANSFORMER. In such a case, it indicates the winding START point. In many cases, it is already part of the bobbin. In some cases, it gets painted on as part of the winding process. There is no need to mark the winding start of an inductor.

An inductor is a non phased device.

No, it matters not which 'way' it gets utilized. AT ALL.

I don't think that's entirely true.

Which way you use it doesn't change the inductive behavior of the part, but it does change the parasitic capacitance of the part to neighboring parts on the board, and/or how it radiates. If the outside windings are on the "cold" end of the inductor they'll tend to provide some measure of electrostatic shielding. If they're on the "hot" end they'll tend to provide a bit of electrostatic coupling to the world at large, much the same as if you had a similar-sized cylinder of metal attached to that circuit node.

This may not matter for most practical purposes, but it certainly feels like something that would have an effect on EMI, depending on how the circuit is built.

It can matter, just like the "outside foil" of a film cap (black band) sometimes matters. There's less capacitive coupling to the outside world, less radiated EMI, of the outside of the winding is at AC ground.

Also, sometimes magnetic coupling between inductors matters, like in filters. Polarity could matter there, too.

Someone used to, maybe still does, make choke inductors that have internal biasing magnets, to increase saturation current in one direction.

There are hum-bucking applications, too.

Nope, no standard there. Fastest way to find out is crack one open and look. Watch out for ferrite shrapnel. The gentle method is to measure the RF leaking out with one end grounded and the other energized, then reverse. Then talk to the mfg whether their production procedure (SOP) calls out to always connect in this particular way. Typically they are but it can be tough to obtain something in writing about it.

tes

Wrong. If the outermost layer of a multilayer winding is connected to ground at one end, the inductor as a whole will radiate appreciably less than it would if one end of the outermost layer were connected to a high AC voltage.

It only the stray capacitative coupling that changes, but that can matter quite a lot.

I've just checked with a German manufacturer and they confirm the dot on their inductors is the start of the inner winding.

I appreciate not all manufacturers will necessarily be as consistent.

Great, then at least you found one where they have a system to it. If this is for a product design I'd still get that in writing though.

It is highly unlikely that a dot will occur on a magnetic drawing that has only one winding. Inductors with multiple, tapped and phase-coherent windings do exist and are constructed with all of the precautions required for transformers. The are called inductors simply as a convention to indicate their intended use.

Every line-powered computer you've ever seen probably has one of the more common examples; a common-mode choke to assist with EMC compliance. Such a part becomes unpredictable (if not completely useless) if phasing or winding instructions are ignored.

There are many intentional constructional details in magnetic components that are not immediately obvious, even to the experienced technician, that can determine their effectiveness in many ways.

RL

It often does:

Or

What they said by email was:

The dot signifies the start of the winding on the inductor, thus this would be the inner layer of the windings which would connect to the hot side of the output of the DC/DC converter. The outer winding of the inductor in effect self shields the emi on the inner windings.

Perfect, then you got it in writing. I thought it was just a phone conversation across the Channel.

Interesting; both of them. At a quick glance, they are unique in the range of both manufacturers.

For Coilcraft, the 'dot' isn't actually associared with either solder junction, but appears equidistant between the two......so you'd have to crack one open to figure out what they mean, and hope that the meaning isn't subverted in the part's sales lifetime. It's not guaranteed that the coil winding personnel have even a working familiarity with the language of the spec......

The instruction 'winding direction' is unfortunately ambiguous in the English language, as winding is both a noun and a verb. When indicating prefered wire layout, I always indicate the direction of the coil former's rotation (ie the verb sense of the winding machine's function) and illustrate the movement of the physical part, to avoid this (hopefully....).

RL

It's not that unique, even in the distant lands of China they do it:

But it's always good policy to obtain written confirmation about the dot and winding orientation.

and, of course, there was actually a noticeable effect on the EMI plots, when checked........making the effort worthwhile?

There'a difference between anecdote and application. There's a difference between a 200W toroid and a shielded bobbin core.

The costs of playing it safe, just in case, can whittle a budget past the bare bone.

RL

It already applies at much lower power levels. I finished a flyback design a few weeks ago. If you get the winding orientation wrong on those then all hell breaks loose at the EMC chambers.

The word "shielded" in ferrites is often more a marketing term. A closer looks typically reveals a fairly large potting area. The potting compound is colored almost the same as the core.

Since some ferrites are conductive, it's not always obvious which connection will radiate less.

True, but fairly easy to test when you have a near field probe kit. That kit is probably the most worn tool out here. Other than the bottle opener :-)