Chapter 3 Introduction to Basic Electrical Circuit Materials 69
Film technology is not restricted to single resistors
but may also be designed to produce resistor networks on
a single chip. A resistor network consists of two or more
independent resistors contained in one single surface-
mounted package.
For higher current uses, resistors are wire wound. A
thin wire is wound on a ceramic core. The wire has a
specific fixed-value resistance. The entire component is
insulated by a coat of vitreous (opaque) enamel. These
resistors are shown in Figure 3-33. Wire wound resistors
are commonly manufactured in sizes from 5 to 200 watts.
The wattage chosen depends on the heat dissipation
required during operation. Metal oxide resistors are also
used for high voltage and wattage requirements.
Another type of wire wound resistor is the adjustable
resistor. Unlike the standard wire wound resistor, the
adjustable resistor is not entirely covered by enamel
material. Instead, a portion of one side of the wire is
Figure 3-33. Wire wound resistors.
advantage of thin film is smaller components requiring
less height can be made.
The substrate is made from glass, ceramic, or silicon,
and it is used as an insulator base for the resistor. A layer
of resistor material is deposited on the substrate in a
zigzag pattern precisely engineered to produce the
desired resistance value. The resistor materials are made
from metals or carbon mixed in a precise proprietary for-
mula. The result is a thin film, which is only a few
micrometers thick or thick film, which is 10 to 50
micrometers thick. A protective coating is used to cover
the resistor material deposited on the substrate. The ends
of the film resistor serve as connection terminals and are
made of metal such as nickel or silver, or they can be
made from an alloy. Solder is used to mount and connect
the film resistor to the circuit board. See Figure 3-32. The
process is very similar to the same process used to man-
ufacture integrated circuits (ICs). The process is covered
in more detail in Chapter 19.
The number stenciled directly on surface-mounted
resistors indicates the resistance value. When a three-digit
number is used, the first two digits represent the first two
digits of the resistance value and the third digit represents
the number of zeros to add to the first two digits. For
example, the 202 in Figure 3-32 represents the resistance
value 2000 or 2k, + or – 5% tolerance (meaning it is
allowed to vary by 5%). When four digits are used, a tol-
erance of + or – 1% exists. For four-digit numbers, the
same rule applies that the last digit represents the number
of zeros. For example, 5002 would represent 50,000 or
50 k ohms, + or – 1% tolerance. For resistance values that
require a decimal, the letter “R” is used to indicate
the decimal point. For example, 7R5 would represent
7.5 ohms.
Film
resistors
Figure 3-32. You can see film resistors mounted on a
circuit board.
Completed
surface-mounted
resistor
Substrate Resistive
material
Protective
coating
Terminal
Figure 3-31. This cutaway drawing displays how a film
resistor is constructed.