The properties of a transformer depend on the shape of its core, and on the way in which the wires are wound on it. There are several different geometries used with transformers.
A common core for a power transformer is the E core, so named because it is shaped like the capital letter E. A bar, placed at the open end of the E, completes the core assembly after the coils have been wound on the E-shaped section (above figure A).
The primary and secondary windings can be placed on an E core in either of two ways. The simpler winding method is to put both the primary and the secondary around the middle bar of the E (above figure B). This is called the shell method of transformer winding. It provides maximum coupling between the windings. However, this scheme results in considerable capacitance between the primary and the secondary. Such interwinding capacitance can sometimes be tolerated, but often it cannot. Another disadvantage of the shell geometry is that, when windings are placed one on top of the other, the transformer cannot handle very much voltage. High voltages cause arcing between the windings, which can destroy the insulation on the wires and lead to permanent short circuits. Another winding method is the core method. In this scheme, one winding is placed at the bottom of the E section, and the other winding is placed at the top (above figure C).
The coupling occurs by means of magnetic flux in the core. The interwinding capacitance is lower than it is in a shellwound transformer because the windings are physically farther apart. Also, a core-wound transformer can handle higher voltages than a shell-wound transformer of the same physical size. Sometimes the center part of the E is left out of the core when the core winding scheme is used. Shell-wound and core-wound transformers are almost universally employed at 60 Hz. These configurations are also common at AF.
A solenoidal core transformer.
A pair of cylindrical coils, wound around a rod-shaped piece of powdered iron or ferrite, was once a common configuration for RF transformers. Sometimes this type of transformer is still seen, although it is most often used as a loopstick antenna in portable radio receivers and in radio direction finding equipment. The coil windings can be placed one atop the other, or they can be separated (Fig. 18-5) to reduce the capacitance between the primary and secondary.
In a loopstick antenna, the primary serves to pick up the radio signals. The secondary winding provides an optimum impedance match to the first amplifier stage, or front end, of the radio receiver. The use of transformers for impedance matching is discussed later in this section.
A toroidal-core transformer.
The toroidal core (or toroid ) has become common for winding RF transformers. The core is a donut-shaped ring of powdered iron. The coils are wound around the donut. The complete assembly is called a toroidal transformer. The primary and secondary can be wound one over the other, or they can be wound over different parts of the core (above figure). As with other transformers, when the windings are one on top of the other, there is more interwinding capacitance than when they are separated.
Toroids confine practically all the magnetic flux within the core material. This allows toroidal coils and transformers to be placed near other components without inductive interaction. Also, a toroidal coil or transformer can be mounted directly on a metal chassis, and the operation is not affected (assuming the wire is insulated or enameled).
A toroidal core provides considerably more inductance per turn, for the same kind of ferromagnetic material, than a solenoidal core. It is common to see toroidal coils or transformers that have inductance values as high as 100 mH.
Exploded view of a pot-core transformer.
Even more inductance per turn can be obtained with a pot core. This is a shell of ferromagnetic material that is wrapped around a loop-shaped coil. The core is manufactured in two halves (above figure). You wind the coil inside one of the halves, and then bolt the two together. The final core completely surrounds the loop, and the magnetic flux is confined to the core material.
Like the toroid, the pot core is self-shielding. There is essentially no coupling to external components. A pot core can be used to wind a single, high-inductance coil. Inductance values of more than 1 H are possible with a reasonable number of wire turns. In a pot-core transformer, the primary and secondary must be wound next to each other. This is unavoidable because of the geometry. Therefore, the interwinding capacitance of a pot-core transformer is high. Pot cores are useful at AF and the lowest-frequency parts of the RF spectrum. They are rarely employed at high radio frequencies.
Schematic symbols for autotransformers. At A, air core, step-down. At B, laminated iron core, step-up. At C, ferrite or powdered iron core, step-up.
In some situations, there is no need to provide dc isolation between the primary and secondary windings of a transformer. In a case of this sort, an autotransformer can be used. It has a single, tapped winding. Above figure shows three autotransformer configurations. The unit shown at A has an air core, and is a step-down type. The unit at B has a laminated iron core, and is a step-up type. The unit at C has a powdered iron core, and is a step-up type. You’ll sometimes see autotransformers in radio receivers or transmitters. Autotransformers work well in impedance-matching applications, and also perform well as solenoidal loopstick antennas. Autotransformers are occasionally, but not often, used in AF applications and in 60-Hz utility wiring. In utility circuits, autotransformers can step the voltage down by a large factor, but they aren’t used to step voltages up by more than a few percent.