Re: two drums

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.