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I've seen plenty of transformers physically constructed with both (or all, if more than two) windings wound concentrically around the center of an E-core--in fact, that seems to be the most common type of transformer out there. And yet, the only thing separating the primary from the secondary in this case is a thin layer of plastic--I've even seen some in which the inter-winding plastic was actually torn through in places (presumably because it was wound too tightly). And these transformers are ubiquitous in chargers and cheap power supplies, where the primary would be 120/240V mains!

So my question is, how on earth can you have good isolation between the windings in this situation? Magnet wire is certainly not sufficiently well-insulated on its own, and I can't see how that super-thin plastic between the windings could possibly make it isolated enough to pass safety checks.

Or do they not pass safety checks, and that's why they're so cheap?

  • 3
    \$\begingroup\$ Look up breakdown voltages of stuff like mylar, it can be pretty thin. \$\endgroup\$
    – PlasmaHH
    May 4, 2017 at 21:52
  • \$\begingroup\$ With over 100 years of engineering optimizing, don't be so amazed that these things work reliably. Mind you, the 3rd-world countries that make these items might tend to cut too many corners. \$\endgroup\$
    – glen_geek
    May 4, 2017 at 21:59
  • \$\begingroup\$ @PlasmaHH I just did. If I'm reading this right, it says 11kV per mil DC or 7kV per mil AC before breakdown? That seems absurdly high, and I feel like I might be misinterpreting this. \$\endgroup\$
    – Hearth
    May 4, 2017 at 22:01
  • \$\begingroup\$ That sounds correct. Technically you only need one thinn layer but if the transformer goes into a product with double insulation (very common), you are required to have three such layers. Still very thinn. \$\endgroup\$
    – winny
    May 5, 2017 at 5:09

1 Answer 1


There are two keys to making such a concentrically wound design work safely at mains voltage:

  1. For most insulating materials used for the interwinding film, mylar included, DTI (distance through insulation) does not determine how much insulation is needed, and a single layer of mylar tape suffices as a result. Creepage (i.e. distance along the insulation surface) is the key factor here -- this means that the windings simply can't encroach closer to 4mm from the edge of the insulating layer.

  2. Proper SMPS/mains transformer design calls for a special type of magnet wire, known as "triple insulated", for the windings on one side of the transformer (usually the secondary side as that uses fewer turns to begin with), unless some other means of double insulation is used. This wire has multiple layers of film-based insulation on the wire itself vs. the single layer enamel coating on ordinary magnet wire, making it more resistant to insulation damage and able to withstand higher surge voltages. If it is not used, then the requisite double insulation is provided by other means, such as additional wraps of insulating tapes and spacers at the edges to ensure creepage limits aren't infringed upon.

As a result, properly engineered and constructed concentrically wound mains transformers are easily capable of meeting the same safety performance (hi-pot, etal) standards as their split-bobbin cousins, and provide the high coupling needed for an off-line SMPS to work anywhere near efficiently -- split-bobbin design isn't an option for high-frequency switching transformers.

  • \$\begingroup\$ Triple insulated wire is not necessary (obligatory) to achieve safe transformer design. \$\endgroup\$
    – Cubrilo
    Jun 27, 2020 at 12:04

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