Resistors are made in many forms, but all belong in either of two groups: fixed or variable. The relative sizes of all fixed and variable resistors change with the wattage (power) rating, increasing in size for increased wattage ratings in order to withstand the higher currents and dissipation losses.
- Fixed Resistors
Resistors of this type are readily available in values ranging from 2.7 ⍀ to 22 M⍀. For use with printed circuit boards, fixed resistor networks in a variety of configurations are available in miniature packages.
- Variable Resistors
Variable resistors, as the name implies, have a terminal resistance that can be varied by turning a dial, knob, screw, or whatever seems appropriate for the application. They can have two or three terminals, but most have three terminals. If the two- or three-terminal device is used as a variable resistor, it is usually referred to as a rheostat. If the three- terminal device is used for controlling potential levels, it is then commonly called a potentiometer. Even though a three-terminal device can be used as a rheostat or potentiometer (depending on how it is connected), it is typically called a potentiometer when listed in trade magazines or requested for a particular application .
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