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NNTP-Posting-Date: Thu, 13 Mar 2025 21:30:54 +0000
From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: two drums
Date: Thu, 13 Mar 2025 17:30:51 -0400
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On Thu, 13 Mar 2025 17:12:08 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 2025-03-13 12:22, john larkin wrote:
>> On Thu, 13 Mar 2025 11:08:47 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> 
>>> On 2025-03-13 10:41, john larkin wrote:
>>>> On Thu, 13 Mar 2025 07:42:28 -0400, legg <legg@nospam.magma.ca> wrote:
>>>>
>>>>> On Tue, 11 Mar 2025 19:35:38 -0700, john larkin <jlArbor.com> wrote:
>>>>>
>>>>>>
>>>>>> https://www.dropbox.com/scl/fo/qpfhkvbfig7elysx78lq3/ALGqgMaRq1tx8aIiN3p1TfM?rlkey=36bcqfdb9di22ko48j89vocut&dl=0
>>>>>>
>>>>>> If I put two of these drum core inductors close together, and get the
>>>>>> phasing right, I get an extra 5 mH for free, without any more ESR. And
>>>>>> external mag fields drop too, I think.
>>>>>
>>>>> Check ripple and transient load response (load one - measure the
>>>>> other).
>>>>>
>>>>> If chanels not synchronized at the switching frequency, odd results
>>>>> might be seen at mixing frequency.
>>>>>
>>>>> RL
>>>>
>>>> The two inductors will be in the two legs of, basically, a floating
>>>> power supply, to decouple it from whatever customer impedance. So I'll
>>>> be running the exact same current through both inductors. If I get the
>>>> polarities right, I get the bonus inductance and the far-field
>>>> magnetic cancellation.
>>>>
>>>> I don't want signal coupling to other channels on the same board, so
>>>> the field cancellation benefit is squared. That overcomes some
>>>> peoples' objections to unshielded drum cores.
>>>>
>>>> The physics is kinda weird. The 5 mH per inductor uses the universe
>>>> for its energy storage, but the bonus L is energy stored in ferrite, I
>>>> guess. I just invented the semi-shielded inductor.
>>>
>>> The far field won't cancel, unfortunately.  Coupled inductors in series
>>> have a total inductance
>>>
>>> Lseries = L1 + L2 +- 2M.
>>>
>>> Since your inductance is going up, the current is going the same
>>> direction in both cores, so the in the low frequency limit, the B field
>>> contributions add everywhere.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>> 
>> If one core has a north pole up while the other is south, the flux
>> makes a tight local loop. That makes L go up. It's approaching an
>> all-ferrite gapless inductor.
>> 
>> I tested the theory with a probe coil. Along the centerline of the two
>> vertical inductors, induced field is zero. The two 5 mH inductors in
>> series add up to 16 mH in this configuration.
>> 
>> If the two series cores have current in the same direction, I get a
>> bunch of field radiated and only about 8 mH net. It's kinda like
>> winding all that wire onto one big drum core.
>
>Ah, okay, I forgot that your inductors are actually vertical not 
>horizontal.  My bad.

Isn't this the difference between a dipole field and a quadrupole
field, regardless of vertical or horizontal?

Joe