How to Select Toroidal Transformers

Toroidal transformers are electrical components constructed using a wire-wound, ring-shaped metal core. Ring-shaped circular transformers are known as toroidal because their basic construction involves a toroid, the solid form of a geometric torus. Toroids can informally be described as "donut-shaped." A toroidal transformer's core is first wound with wire to form the primary (input) coil, then overlaid with insulation. The secondary (output) wire is then wound over the insulation. In the image below, (8) represents the primary winding, (4) represents the secondary winding, and (5), (6), and (7) are insulation layers between coils.

^{A selection of toroidal transformers. Image credit: Allied Electronics}

Toroidal transformers feature several advantageous differences when compared to traditionally-built devices:

• Overlapped coils, as opposed to two separate coils, allow for much smaller devices.
• Magnetic flux is generally limited to the toroidal core, meaning that toroidal transformers essentially shield themselves from producing electromagnetic interference (EMI).
• By requiring a smaller number of turns per coil, toroidal transformers feature higher inductance relative to a similarly-sized traditional transformer.

Toroidal transformers also include inherent disadvantages. Because each coil winding must pass through the transformer's center hole automated winding becomes difficult and may necessitate a dedicated winding machine specific to toroidal devices. The unique coil winding also renders toroidal transformers more expensive to manufacture.

The advantages of toroidal transformers are best realized in small, specialized devices and as the size of the transformer increases, the advantages tend to diminish. For this reason, toroidal transformers are best applied to signal applications, such as audio and radio frequency (RF), in sensitive circuits because of the product's miniature

construction and noise-limiting attributes. Toroidal transformers are generally not suited to all but the lowest voltage power applications due to their small size.

Configuration

A transformer's configuration refers to the number of input voltages it can accept, and by extension the number of leads it must include. As the number of leads and required copper wiring increase, the manufacturing and product cost also increase.

^{A chassis-mounted transformer, connected to a PCB using leads Image credit: Home Theater HiFi}

Toroidal transformers are used in medical electronics, test and measurement systems, industrial control panels, data communications, motor controls, light fixtures, audio systems, robotics, UPS, and avionics. For more information about basic transformer operation, power ratings, and standards, please visit the Transformers Selection Guide.