Toroid Transformers Explained

HF radios often use toroidal transformers and winding them is a rite of passage for many RF hackers. [David Casler, KE0OG] received a question about how they work and answered it in a recent video that you can see below.

Understanding how a conventional transformer works is reasonably simple, but toroids often seem mysterious because the thing that makes them beneficial is also what makes them confusing. The magnetic field for such a transformer is almost totally inside the “doughnut,” which means there is little interaction with the rest of the circuit, and the transformer can be very efficient.

The toroid itself is made of special material. They are usually formed from powdered iron oxide mixed with other metals such as cobalt, copper, nickel, manganese, and zinc bound with some sort of non-conducting binder like an epoxy. Ferrite cores have relatively low permeability, low saturation flux density, and low Curie temperature. The powder also reduces the generation of eddy currents, a source of loss in transformers. Their biggest advantage is their high electrical resistivity, which helps reduce the generation of eddy currents.

If you haven’t worked through how these common little transformers work, [David]’s talk should help you get a grip on them. These aren’t just for RF. You sometimes see them in power supplies that need to be efficient, too. If you are too lazy to wind your own, there’s always help.

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Powering A Lot Of Nixie Tubes

[Limpkin] has an idea for a project that uses a lot of IN-9 Nixie tubes. Where a Nixie tube clock would only use four or six tubes, [Limpkin] is looking at fifty IN-9 bar graph Nixie tubes. These tubes only light up above 100 Volts and draw about half an amp. That’s 64 Watts, according to the math on the project page, so how does [Limpkin] plan on powering these tubes? With a big high voltage power supply.

The power supply [Limpkin] designed is more or less what you would expect to find in any power supply. There’s a transformer, a bunch of caps, and a rectifier. Going with a standard laminated core transformer would mean this power supply would be huge and heavy, but once again eBay comes to the rescue with a small, 150 Watt toroidal transformer. The largest output on the transformer was two 24 V outputs. Combining those outputs gets [Limpkin] to 48V AC, or 68V peak to peak. A full wave voltage doubler with two caps and two diodes gives [Limpkin] the 136V DC that will power the tubes.

Combine the high voltage circuit with a 9V AC tap, a small bridge rectifier, and a few more caps, and [Limpkin] had a supply that would power the tubes and the rest of the electronics in his multiple Nixie tube project. A few passes with a CNC mill gave the power supply a nice case topped off with a foreboding toroidal transformer ready to power a beautiful neon project.