Toroidal Transformers FAQ

What applications use a toroidal transformer?

Any electronic transformer application that can accommodate the shape of a toroidal transformer can use one. Although usable, toroidal transformers are not always practical for some applications.

What materials are available for toroidal transformer cores?

Silicon steel, nickel iron, moly-permalloy powder, iron powdered, amorphous, ferrites, and others. Silicon steel and nickel iron are available as tape wound cores or laminated pieces.

Does Butler Winding have non-magnetic toroids to make air core toroidal transformers?

Yes, Butler Winding can manufacture non-magnetic toroids.

What is the shape of a toroidal transformer and what are benefits of this shape?

A 360 degree wound toroidal transformer has a high degree of symmetry because of its circular shape. This leads to near complete magnetic field cancellation outside of its coil, hence the toroidal transformer has less leakage inductance and less EMI when compared against other transformers of equal power rating.

What is the difference between a toroidal transformer with a round cross-section and a rectangular cross section?

Toroidal transformers with a round core cross section are better performers than toroidal transformers with a rectangular cross section. The cancellation is more complete for the round cross section. The round cross section also gives a shorter turn length per unit of cross sectional area, hence lower winding resistances. The toroidal transformer also has better winding to winding magnetic coupling because of its toroidal shape. The coupling is dependent on the winding being wound a full 360 degrees around the core and wound directly over the prior winding, hence sector wound windings do not couple as well and have higher leakage inductance. As winding turns are positioned further away from the core less complete coupling will occur; hence toroidal transformers with multi-layered windings will exhibit more leakage inductance.

What is a gapped torodial transformer?

Gapped toroidal transformers usually require that the gap be filled with some type of insulating material to facilitate the winding process. This is an extra expense. Split core current transformers can be assembled directly on a conductor while toroids must be passed over a disconnected end of the conductor. A toroid can be split in two, but a suitable clamping mechanism (difficult and costly) is required. Some printed circuit boards are space critical. Mounting a toroidal transformer flat on the board may take up too much precious board area. Some applications also have restricted height so the toroid cannot be mounted vertically.

Why is a toroidal transformer generally more expensive than bobbin or tube wound transformers?

Sufficient winding wire must first be wound (loaded) onto the winding shuttle, then wound onto the toroidal transformer’s core. After that, the best situation, from a cost perspective, is no insulation required over the winding and the next winding uses the same wire size. If the wire is different, then the leftover wire must be removed and the wire for the next winding must be loaded. However, if the winding must be insulated, then it must either be insulated (taped) by hand or the toroidal transformer must be removed and taken to a separate taping machine, then placed back on the toroid winding machine after taping. The shuttle must then be loaded with the wire size and type for the toroidal transformer€™s next winding. A toroidal transformer with a single winding (auto-transformer, current transformer) wound on a coated core will probably be cost competitive with an equivalent bobbin or tube wound transformer since the toroidal transformer will not require a bobbin or tube. The cost differential will then depend on the method and cost of mounting the transformers.

If you need assistance with your design, please contact Butler Winding and ask for Engineering.