Elenco Electronic Component Kit - Manual (Page 4)

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RESISTORS
CARBON FILM RESISTORS
Carbon film resistors are made by depositing a very
thin layer of carbon on a ceramic rod. The resistor
is then protected by a flameproof jacket since this
type of resistor will burn if overloaded sufficiently.
Carbon film resistors produce less electrical noise
than carbon composition and their values are
constant at high frequencies. You can substitute a
carbon film resistor for most carbon composition
resistors if the power ratings are carefully observed.
The construction of carbon film resistors require
temperatures in excess of 1,000
O C.
Metal oxide resistors are also constructed in a
similar manner as the carbon film resistor with the
exception that the film is made of tin chloride at
temperatures as high as 5,000
O C. Metal oxide
resistors are covered with epoxy or some similar
plastic coating. These resistors are more costly than
other types and therefore are only used when circuit
constraints make them necessary.
Metal film resistors are also made by depositing a
film of metal (usually nickel alloy) onto a ceramic
rod. These resistors are very stable with
temperature and frequency, but cost more than the
carbon film or carbon composition types. In some
instances, these resistors are cased in a ceramic
tube instead of the usual plastic or epoxy coating.
When a resistor is constructed so its value can be
adjusted, it is called a variable resistor. Figure 6
shows the basic elements present in all variable
resistors. First a resistive material is deposited on a
non-conducting base. Next, stationary contacts are
connected to each end of the resistive material.
Finally, a moving contact or wiper is constructed to
move along the resistive material and tap off the
desired resistance. There are many methods for
constructing variable resistors, but they all contain
these three basic principles.
Figure 5
METAL OXIDE RESISTORS
METAL FILM RESISTORS
THE VARIABLE RESISTOR
Figure 6
Carbon Film
Ceramic Rod Flameproof Jacket
Leads
Movable
Arm
Wiper
Contact
Stationary
Contact
Thin Layer
of Resistive
Material
Non-conductive
Base Material