How to Select Toroidal Transformers
A selection of toroidal transformers.
Image credit: Allied Electronics | Newark / element14 | Digi-Key
Toroidal transformers are electrical components constructed using a wire-wound, ring-shaped metal core.
Toroidal Transformer Basics
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 bare spun toroid. Image credit: capturedlighting.org
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 deconstructed toroidal transformer. Image credit: A+azon
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.
A typical toroidal transformer winding machine. Video credit: altatransformator
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.
Typical toroidal transformer configurations include:
Single configuration transformers include only one primary coil winding and can therefore accept one input voltage.
Dual configuration devices feature two windings in the primary coil and can accept two different voltages. In order to switch input voltages, a two-lead voltage selector must be used in conjunction with the transformer.
5-lead transformers are wired to accept five different input voltages. Like other multiple configuration products, they must be used with a switching device.
A toroidal transformer may be mounted using one of several different methods.
Chassis mount devices are screwed onto an internal framework using built-in tabs.
Chip transformers are integrated circuits (IC) manufactured using thin film technology.
Dish / disk mount products are secured to a flat surface using a simple rubber washer and a metal disk. This method typically uses a through-hole to screw the device onto the surface.
H-frame mounting uses an H-shaped frame to secure the transformer to a flat surface, and is used in high vibration or high shock applications.
Some transformers are compatible with modular jacks, such as RJ-45.
Printed circuit board, or PCB, devices attach directly to a circuit board using through-hole or surface mount technology, and often require solder.
A chassis-mounted transformer, connected to a PCB using leads | A surface mount (SMD) chip transformer
Image credit: Home Theater HiFi | Belden
For more information about basic transformer operation, power ratings, and standards, please visit the Transformers Selection Guide.
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