Dots on inductors

On Wed, 28 Nov 2012 15:29:24 -0800, Joerg <invalid@invalid.invalid>
wrote:

>legg wrote:
>> On Tue, 27 Nov 2012 14:06:35 -0800, Joerg <invalid@invalid.invalid>
>> wrote:
>> 
>>> legg wrote:
>>>> On Mon, 26 Nov 2012 15:44:04 -0800, SoothSayer
>>>> <SaySooth@TheMonastery.org> wrote:
>>>>
>>>>> On Mon, 26 Nov 2012 16:40:18 -0500, legg <legg@nospam.magma.ca> wrote:
>>>>>
>>>>>> On Mon, 26 Nov 2012 16:48:51 +0000, Raveninghorde
>>>>>> <raveninghorde@invalid> wrote:
>>>>>>
>>>>>>> I read many years ago (National app note from memory) that one should
>>>>>>> connect the inner layer of an inductor to the switch in a buck
>>>>>>> regulator and the outer layer to the capacitor as the outer layer will
>>>>>>> then act as a screen.
>>>>>>>
>>>>>>> I don't know if it makes a difference in practice but it's free.
>>>>>>>
>>>>>>> With many modern inductors it is impossible to see how they are wound
>>>>>>> so the only indication of "polarity" is the dot. Is the dot arbitary
>>>>>>> and down to the winder or does it actually relate in anyway to the way
>>>>>>> the inductor is wound?
>>>>>> 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.
>>>> It is highly unlikely that a dot will occur on a magnetic drawing that
>>>> has only one winding. ...
>>>
>>> It often does:
>>>
>>> http://katalog.we-online.de/kataloge/eisos/media/pdf/744778920.pdf
>>>
>>> [...]
>> 
>> 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.
>> 
>
>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 function of the rf-conductive core body has also to be taken into
account in the radiation process. Capacitance to the core from
different portions of a winding (or windings) has to be taken into
account, as well as possible core body and screen current paths.
>
>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.

A shielded bobbin core, on the other hand, specifically indicates the
employment of a sleeve structure with a controlled gap, whether SMD or
through-hole. This controlled gap is where the filled adhesive is
present.

RL