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If the secondary winding of the 230v-115v Step Down Toroidal Transformers have multiple separate ends and you are only tapping a portion, would there be uneven magnetic field cancellation? See for a version with dual primary and dual secondary:

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Schematic of which are:

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Compare it with a single primary and secondary where the entire windings are used at once. 2

Do you agree the version where the entire windings were used at once have even magnetic field cancellation than the first one just using a portion of the secondary winding?

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  • \$\begingroup\$ I've never heard of transformer magnetic field cancellation. I Googled it and still didn't find anything particularly relevant. What are you talking about? \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 31, 2018 at 4:35
  • \$\begingroup\$ A toroidal transformer can cancel the fluxes external magnetic field... thats why some application require them. \$\endgroup\$
    – Samzun
    Commented Oct 31, 2018 at 5:00

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Not really... In both cases you use the whole winding (Parallel the connections (watch the phasing, it matters) for the two winding case).

You sometimes get lucky in a particular orientation, but to a first order the core flux is the same all the way around (it kind of has to be), you are dealing with second order effects here from things like imperfections in the core, but while it is a thing, it is very rare for it to be enough of a factor to matter.

The big win for lower external fields is using a transformer rated for more voltage (both primary and secondary, and correspondingly more VA), the lower voltage will lower the operating flux and move further from saturation. For the same reason a 50Hz part is usually better then a 60Hz one (even on 60Hz). Aggressively cost optimised transformers often run on the ragged edge of saturation, especially if your mains line is a bit high.

I get the feeling you may be trying to solve the wrong problem here, what are you actually trying to do, and why does the (really not very large) leakage from a properly sized power transformer cause you so much of a problem?

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  • \$\begingroup\$ Say you have a 750VA capacity toroidal transformer and you are only using 500VA. Won't this produce the same flux as using a 500VA version at near saturation? Won't both produce the same magnetic field say 16 inches away? Well. I'm just using it along with magnetic sensors so need to have minimal stray fields. \$\endgroup\$
    – Samzun
    Commented Oct 31, 2018 at 10:29
  • \$\begingroup\$ Nope, flux is primarily proportional to applied voltage and inversely proportional to frequency. There is a second order effect where as a transformer becomes more heavily loaded the flux actually falls slightly (Due to the resistive copper loss of the primary being in series with the supply, so lowering the effective input voltage as the load increases), but this is very much a second order effect. What does minimal mean to you? You have to have numbers before you can design. \$\endgroup\$
    – Dan Mills
    Commented Oct 31, 2018 at 11:07
  • \$\begingroup\$ I need just 5 milligauss at 20 inches away. Is this possible? What brand of Toroidal Transformer are known to have lowest stray fields so I can check its design? I think the windings have to be even to have lowest stray fields, so perhaps the better quality brand, the better is the symmetry, right? Or are those cheap china brand and top brands same in the stray fields performance? \$\endgroup\$
    – Samzun
    Commented Oct 31, 2018 at 11:29
  • \$\begingroup\$ @Samzun Talk to a custom transformer winding company and get them to build you something with an oversized core and a flux band, the better sort of vendor will take your target numbers and make sure the thing meets them. You need to deal with a vendor because such things as the use (or not) of grain oriented steel for the core matters, as does the core tape geometry and the annealing, and your vendor will know their materials choice. If you need more then the vendor can do in the transformer (should not be the case) then a heavy mild steel sheet forms a reasonably effective shield. \$\endgroup\$
    – Dan Mills
    Commented Oct 31, 2018 at 12:55
  • \$\begingroup\$ Ok. As summary. For normal EI core... the bigger it is.. the larger is the flux and magnetic field... So you are saying that for Toroidal core, the larger it is, the lesser is the flux and magnetic field? I only need 300VA. Should I buy 500VA just to have lower field.. or did I misunderstand you? \$\endgroup\$
    – Samzun
    Commented Oct 31, 2018 at 13:11

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