Pot cores
There is another way to confine the magnetic flux in a coil so that unwanted mutual inductance
does not occur. This is to extend a solenoidal core completely around the outside
of the coil, making the core into a shell (Fig. 10-9). This is known as a pot core.
Whereas in most inductors the coil is wound around the form, in a pot core the form is
wrapped around the coil.
The core comes in two halves, inside one of which the coil is wound. Then the parts
are assembled and held together by a bolt and nut. The entire assembly looks like a
miniature oil tank. The wires come out of the core through small holes.
Pot cores have the same advantages as toroids. The core tends to prevent the magnetic
flux from extending outside the physical assembly. Inductance is greatly increased
compared to solenoidal windings having a comparable number of turns. In fact,
pot cores are even better than toroids if the main objective is to get an extremely large
inductance within a small volume of space.
The main disadvantage of a pot core is that tuning, or adjustment of the inductance,
is all but impossible. The only way to do it is by switching in different numbers of
turns, using taps at various points on the coil.
Filter chokes
The largest values of inductance that can be obtained in practice are on the order of
several henrys. The primary use of a coil this large is to smooth out the pulsations in direct
current that result when ac is rectified in a power supply. This type of coil is known
as a filter choke. You’ll learn more about power supplies later in this book.
Inductors at audio frequency
Inductors at audio frequencies range in value from a few millihenrys up to about 1 H.
They are almost always toroidally wound, or are wound in a pot core, or comprise part
of an audio transformer.
Inductors can be used in conjunction with moderately large values of capacitance
in order to obtain audio tuned circuits. However, in recent years, audio tuning has
been taken over by active components, particularly integrated circuits.
