Electrical transformers are essentially a coil of electrically conductive wire wound around an iron core, most often a rectangular “EI” core. Some transformers, however, look more like a doughnut, and they are called toroidal transformers.

Either construction makes for an efficient process when you need to raise or lower voltages, and each has its advantages and disadvantages. At Johnson Electric Coil Company, we can help explain the technical differences to help you choose the type of transformer that is right for you!

Creating a Toroidal Transformer

Like a standard EI transformer, a toroidal transformer needs both primary and secondary windings to step voltages up and/or down—in other words, to be a transformer. The fundamental difference between EI and toroidal transformers is not their function but how they are configured. Instead of a rectangular core and coil construction, where sections of the core are exposed, toroidal electrical transformers are ring shaped, and the core is surrounded 360° by the windings. This feature offers certain benefits to a transformer’s overall performance.

Why the Round Shape?

Flux refers to the strength of the magnetic field generated by electric current. Since much of a rectangular, or EI, magnetic core is external to the windings, a fair amount of magnetic flux is dissipated. Conversely, since the windings of a toroidal transformer surround the core, there is a tighter magnetic field. Some applications require special transformer shielding to limit things like humming and buzzing, electrical noise, and disruption due to EMI. But because toroidal electric transformers radiate less magnetic flux and are less prone to electromagnetic interference, they are less likely to need shielding


Space is often limited, and you can typically get the same effect from a smaller ring-shaped design as you can from a larger rectangular construction. In an age of miniaturization, for example, size is a very important consideration. Smaller transformers make many devices possible today.


Weight is also often a vital factor. Lighter devices require less-robust support, are easier and safer to handle and less costly to transport. Because a toroidal transformer is lighter than a comparable EI design, where weight is critical, a toroid can make the difference between go and no-go without sacrificing performance.


One of the biggest enemies of electrical components and circuits is heat build-up, and a toroidal coil typically dissipates heat better than an EI design. When it comes to transformer performance, heat is wasted energy (losses), meaning the cooler-running toroidal transformer is more efficient as well. And when the transformer runs cooler, everything else does too.

Discover the Difference

If you want to learn more about the benefits of toroidal electrical transformers, call Johnson Electric Coil Company toll free today at 800-826-9741.