70 Fundamentals of Electricity and Electronics
exposed. An adjustable sliding tap is attached to move
across the exposed surface. This allows the resistance
value to be varied. Adjustable resistors may have two or
more taps for providing various resistance values in the
same circuit. An adjustable resistor and symbol are
shown in Figure 3-34.
Potentiometers
Most electronic equipment requires the use of vari-
able resistance parts. A potentiometer is a very common
type of variable resistor found in electronic projects. The
potentiometer has a rotary knob that varies the resistance
value as it is turned. The variation in resistance is pro-
vided by a contact that is attached to a ring of resistive
material inside the device. This device is similar to
the wire wound resistor. Many potentiometers are
constructed with thin wire inside as the source of resis-
tance. Various styles of potentiometers are illustrated in
Figure 3-35. Also shown is the potentiometer’s schematic
symbol.
Figure 3-34. This adjustable resistor provides a sliding tap
for voltage divider uses. On the right is the schematic
symbol for an adjustable resistor.
Sliding
contact
or wiper
Figure 3-35. Potentiometers are used in electronic circuitry
for fixed and variable resistance. Notice that the schematic
symbol for the potentiometer is the same as for the
adjustable resistor. Figure 3-36. Color coded bands encircle the resistor.
Thermistors
A special type of resistor is called a thermistor. In
comparison to other types of resistors, the thermistor is
unusual due to its ability to change resistance value rap-
idly as its temperature changes. It is commonly used to
prevent high inrush currents in electrical circuits.
An example of a thermistor use can be seen in a blow
dryer. A common blow dryer has heating elements com-
posed of tungsten wire. The tungsten wire has a very low
resistance value when cold, and a high resistance value
when red hot. The thermistor is placed in series with the
heating elements to prevent a high current value when the
dryer is first turned on. As the blow dryer heats up, the
resistance value goes down. The result is a fairly consis-
tent current value as the dryer’s heating element changes
from low resistance (cold) to high resistance (hot). Early
blow dryer models caused a dimming and flickering
of lights and other electronic equipment in the home
because of inconsistent current draw. The thermistor
eliminates this problem.
Resistor Color Code
Larger resistors are usually marked with their
numerical resistance value printed directly on the side of
the resistor. However, this type of labeling is not always
practical, especially on small resistors. The resistor color
code system was developed for this purpose. The color
code marking system has been adopted by the Electron-
ics Industries Association (EIA) and the United States
Armed Forces. This system of color coding is recognized
throughout the world. Refer to Figure 3-36. Note how the
color codes are printed, or banded, around the entire body
of the resistor. This method of coding permits the value of
the resistor to be read regardless of the mounting posi-
tion. To see how to read the color coded bands refer to
Figure 3-37.
Resistors commonly have three or four (and some-
times five) bands. Each band has a unique meaning. The
first band represents the value of the first digit of the