Who owned Toroidal Isolation Transformers like this:


Do you have a gaussmeter (or any magnetic field meter like the following)?


How low did you measure the magnetic field of it compared to a normal transformer? Is it one half or 1/3 or even lower?

I need very low magnetic field for some applications. In my testing. When I put the above EM meter 16 inches away from a normal transformer. I was getting 100 milligauss. I need it to be 20 milligauss or lower.

This is for one without Mu-Metal shielding.. and what did you measure when you add Mu-metal shielding?

  • \$\begingroup\$ You tell us. The initial research is up to you. Then we can discuss what you find after trying Mu-Metal, which is a bit expensive, and tough to machine. Buy it in tape form and wrap around the outside of the toroid only. If you loop it through the center of the toroid it will short out the magnetic field, and blow a fuse. \$\endgroup\$
    – user105652
    Commented Oct 30, 2018 at 4:01
  • \$\begingroup\$ Mu-metal is very expensive.. so if a normal toroidal transformer would have much diminished magnetic field say one half or more.. that's good enough. And this would be sufficient for me so I'm asking if anyone has measured magnetic field 16 inches away from any Toroidal transformer, what milligauss value did you get? \$\endgroup\$
    – Samzun
    Commented Oct 30, 2018 at 4:05
  • \$\begingroup\$ If you want maximum magnetical shielding, you don't need a torodial core but a cup core. For 50Hz, they are going to be huge and expensive. \$\endgroup\$
    – Janka
    Commented Oct 30, 2018 at 4:23
  • \$\begingroup\$ One of standard methods for reducing parasitic fields from a standard transformer is to put a "flux band", a shield of solid copper outside of the core, as shown in Figure 13.4 of this article, sound.whsites.net/xfmr2.htm \$\endgroup\$ Commented Oct 30, 2018 at 4:34
  • \$\begingroup\$ Isn't the purpose of toroidal core for magnetic shielding? \$\endgroup\$
    – Samzun
    Commented Oct 30, 2018 at 6:27

1 Answer 1


It very much depends on the transformer (and how close to saturation the thing is being run), a good quality toroid run well within its primary voltage rating will have surprisingly low leakage, while a cheap one run on the ragged edge of saturation (Say because it is a minimum cost 60Hz part being run on 50Hz without appropriate derating) will spray (harmonically rich) fields around.

For shielding, flux bands are good, and actually once you are a few inches away, ordinary mild steel is surprisingly effective (And cheap), mu metal is expensive and saturates easily, it is more applicable to small fields then mains transformers of any size.

One other thing that is something of a surprise is that rotating the toroid on its mounting bolt can often find an angle where the field at whatever device matters is largely cancelled.

The valve audio guys tend to be the people who really have to deal with this crap.

I would BTW be highly sceptical of that meter, direction matters and anything which just has a 'microwave' range without a table of correction factors has me dubious.

  • \$\begingroup\$ toroid.com/Products/Standard-Designs here their medical grade toroidal transformers have low leakage current.. does low leakage current mean lesser magnetic field? or just the same? \$\endgroup\$
    – Samzun
    Commented Oct 30, 2018 at 13:35
  • \$\begingroup\$ @Samzun Nope, completely unrelated, they refer to the leakage from primary to secondary due to things like inter winding capacitance which can be an issue for things medical. \$\endgroup\$
    – Dan Mills
    Commented Oct 30, 2018 at 18:33
  • \$\begingroup\$ Is expensive toroidal transformer characterized by a uniformly wound toroidal core to produce low external field? While cheap ones are characterized by uneven winding? Meaning the more expensive is the brand. The more even is the winding or no relation? \$\endgroup\$
    – Samzun
    Commented Oct 30, 2018 at 19:40

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