Coils, inductors, and chokes are the names used to indicate a coil of wire. “Inductor” is preferred because inductors have inductance, a property that is utilized in many electrical circuits.
FIXED INDUCTORS
The simplest coil or inductor has an air core and is made by winding a wire in a series of loops, which may or may not have a form to hold them in place. Coils are seldom color coded for value, so we look at the schematic or a parts list for the inductance value of a coil. Inductance is the electrical property of a coil, just as resistance is the electrical property of a resistor. Many coils are wound on plastic forms that support the loops of wire. The form has no effect on the operation of the coil. The symbols for air-core coils are shown in Figure 1.
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| Figure 1 Symbols for air-core coils. |
Other types are powdered iron core and iron core. Symbols for these types are shown in Figure 2.
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| Figure 2 Powdered-iron-core and iron-core inductors. |
VARIABLE INDUCTORS
Some circuits need inductors that can have their values changed, some by screwdriver adjustment and others by changing the core material. Figure 3 shows the symbols for variable inductors. Note the differences for iron-core variable inductors. Iron-core (made of iron or steel) chokes are indicated by two straight lines over the loops. Dashed lines indicate powdered iron cores.
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| Figure 3 Symbol for variable inductors. |