Toroidal transformers are circular in shape and will usually have a circular cross-section, though the cross section may sometimes be square or circular. Commercial toroids have a magnetic core. Examples include the powdered iron toroids and ferrite toroids. Because the core provides a closed magnetic path, fewer turns are required, giving the toroidal coil a higher Q and a greater inductance than a solenoid of the same size.

You will find different toroids for sale, designed for a range of uses. There are high frequency toroids, some are designed as transformers, and there are even very large ones that are designed as high-current sensing coils. The Rogowski coil is one in the class of industrial toroids that is large enough to fit around a bus bar. After safely measuring the small voltage from the Rogowski coil, it's easy to calculate the huge current in the bus bar. When using toroids, consider the following:

1. Find an experienced company to design a toroidal transformer to your specifications or according to your needs.

2. Buy toroids for any of a variety of applications.

3. Ensure your equipment is working as designed by having your toroids calibrated periodically.

Action Steps
The best contacts and resources to help you get it done

Have your toroidal transformers built by professionals

Toroidal coils can be fabricated for any of a number of applications, e.g. transportation, audio or power supply. Find a company that has a variety of winding stations to accommodate different requirements. Also, make sure they have test equipment to verify that the toroidal coils they fabricate operate within specifications.

I recommend: Quality Coils has superior ferrite core suppliers, are well equipped with winding stations and test stations and have extensive experience building toroidal coils for the aerospace and transportation industries. Integrity Technology also will fabricate toroids according to your requirements, ranging form 0.2 inch to 3 inches in diameter.

For large speakers, buy special low-noise commercial toroids

A frequent problem in the audio industry is to have a growling noise emanating from the speakers, but originating at the transformer. Toroids are powerful because they are efficient due to the continuous core, however this efficient nature also causes them to become noisy under unfavorable conditions in the line such as overvoltage. This results in inrush currents and thus noise. But as a result of recent advancements in toroid design, significantly cleaner sound is produced using specially designed low-noise toroids.

I recommend: Plitron has researched the noise inherent in transformers for years and has developed what they call LONO technology. Plitron offers LONO toroidal transformers with inrush currents reduced 40 percent. Miracle also carries superior quality audio toroidal transformers.

Choose the right toroidal coil to get your job done right

Whatever your application is for a toroid, it's important to acquire the right toroid for the job, one that is specifically designed for that purpose.

I recommend: I'm especially impressed with the selection of toroids offered by Toroid Corporation of Maryland. From filter chokes operating at 50 to 60 Hz to current-sensing transformers, to toroidal isolation transformers, they will have the right one for the job. If you're searching for medical isolation toroidal transformers in compliance with applicable standards, look no further than Bicron Electronics.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  When using toroids, utilize a wire size somewhat smaller than the maximum so that the spacing between windings can be increased. Space the windings evenly with the leads separated by approximately 30 to 40 degrees. This will minimize the capacitance between turns as well as between the leads and therefore maximize the self-resonant frequency. Small changes in the reactance can be achieved by compressing or expanding the winding.