Re: two drums

On 2025-03-13 17:30, Joe Gwinn wrote:> 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
 >
A quadrupole is basically a current distribution whose total dipole moment is near zero, e.g. two closely-spaced antiparallel dipoles like John's.
Cheers
Phil Hobbs