What are Toroidal Transformers?

Most device transformers are standard laminated core units. These are often referred to as “EI” transformers because they are shaped like the letters E and I. However, there is an alternative to these units and they look like donuts. These are known as toroidal transformers and you may want to take advantage of their benefits. Here is more about them.

It’s All in the Shape

A standard single phase EI transformer has two coils of wire wrapped around laminated cores. They work on the basic transformer principle of electric current passing through coiled wire and transferring itself to another coil of wire in close proximity. Donut transformers work differently.

Why A Donut Shape?

To make one of these transformers, you need a round core made from magnetic material. Wrap wire around the core in two different places. This gives you the primary and secondary windings. EI transformers utilize a square shaped core, made from many sheets laminated together. However, toroidal transformers with their round cores, perform much better and here is why


Just by changing from a square to a round core, you limit the amount of flux loss considerably. Why is this important? Magnetic flux is the main reason for EMI or electromagnetic interference. Many electronic devices today require EMI shielding and if this can be done from the power source it is more efficient. This is especially important for communications, video and audio equipment.

The donut shape helps to keep weight and size down. This is a very important consideration as electronics become more compact and smaller every day. For applications using high frequency, you can buy toroidal transformers with powdered iron cores for maximum EMI protection. Also, the round shape helps the transformer get excellent air circulation and you may notice better overall performance from your electronics.