Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: two drums Date: Fri, 14 Mar 2025 16:46:18 +1100 Organization: A noiseless patient Spider Lines: 44 Message-ID: References: <4bs1tj5ua7r2kbcs8na9ij37pn3rk5dqbn@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Fri, 14 Mar 2025 06:46:34 +0100 (CET) Injection-Info: dont-email.me; posting-host="ff3022b65d8a3991bfddd3dff4e336b2"; logging-data="750290"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18PAqRRstYxXn/2Id5DOwNVYnvgeYywBKY=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:xc2IBVMJ2El3Ok2eBPAXqAQCkYA= Content-Language: en-US X-Antivirus: Norton (VPS 250314-0, 14/3/2025), Outbound message X-Antivirus-Status: Clean In-Reply-To: Bytes: 3142 On 14/03/2025 1:41 am, john larkin wrote: > On Thu, 13 Mar 2025 07:42:28 -0400, legg wrote: > >> On Tue, 11 Mar 2025 19:35:38 -0700, john larkin 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. It doesn't use the whole universe - just the bit in the immediate vicinity of the inductor. The bonus L comes from the field lines that go through the adjacent ferrite. A inductor wound onto a toroid can be completely shielded - but only if the winding is non-progressive. Something wound around a pot core is less perfectly shielded, so the semi-shielded inductor was invented a long time before people realised that toroidal cores were a good thing. -- Bill Sloman, Sydney