Page 113 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 4 Electrical Controls 75

Resistivity (p) volume control. When you turn the knob to raise or lower the
Material (ohm/inch) volume on your stereo, you are actually adjusting the resis-
Aluminum .0673 tance of the internal circuit.
Carbon .0762 - 1.5240 Figure 4-116 shows a classic, laboratory type, wire wound,
Copper (Pure) .04267 variable resistor. The primary element is a coil of wire with a
Copper (Common Wire) .0437
Gold .0607 terminal on both ends. A third wiper can slide up and down
Iron .2466 the length of the coil. At any given position, the resistance is
Lead .5588 equal to the length of the wire between one of the terminals
Mercury 2.4890 and the wiper.
Nichrome 2.5400
Nickel .1895 To construct a variable resistor, a coil is wound onto a coil
Platinum .2692 form and connected to a terminal at both ends. A wiper is
Silver .0404 made from a strip of brass and an insulating handle. The insu-
Tungsten .1422
lation must be sanded off the wire at the apex of the coil
Figure 4-115 Resistively of Common where the wiper contacts the wire. Figure 4-117 shows a
Materials bench built, variable, wire wound resistor.
For applications that require setup or periodic adjustments,
there are a wide variety of center-tap resistors available. These
Figure 4-115 provides a list of resistivities that correspond resistors act as a fixed assembly, but allow a degree of adjust-
to different metals commonly used as conductors and/or to ment during setup or de-energized times. Figure 4-118 shows
manufacture wire. a wire wound ceramic resistor with a clamp-on center tap.
For low-power applications, PC board mount 10-turn preci-
sion wire wound resistors are used. These devices are typically
Variable Resistors built into a compact plastic housing and have an adjustment
screw on one end, as shown in Figure 4-119.
For many applications, there is a need to adjust the parameters Rheostats are a type of resistor that is designed to be
of the circuit during operation. The easiest parameter to adjust adjusted while the circuit is energized. They are typically
is resistance. This is accomplished using variable resistors. found in high-current applications such as motor field, reac-
One of the most common uses for variable resistors is a tor, and heater control. Typically, rheostats are wire wound

Schematic Symbol

Wiper Terminal
Handle
Insulators
Wiper
Square Rod Wire Coil

Frame Plates

Terminals

Coil Form
Spacers
Termination Ring
Figure 4-116 Lab Type, Wire Wound, Variable Resistor