68 Fundamentals of Electricity and Electronics
The chemical makeup that causes resistance is accu-
rately controlled in the resistor manufacturing process.
Resistor values can be purchased in a range of values
from less than 1 ohm to over 22 megaohms. The physical
size and material used for resistance is rated in watts. A
resistor’s wattage rating refers to the resistor’s ability to
safely dissipate heat. Heat is generated by electrons flow-
ing through the resistor. Common wattage sizes range
from 1/4 watt to 25 watts. Resistors are grouped by ohms
and watt sizes. See Figure 3-29. When purchasing a
resistor, the desired resistance and wattage rating must
be specified. For example: 1000 ohms and the watt size,
1/4 watt, 1/2 watt, or 2 watts, etc. In each watt size, the
resistance value would be the same. Electrical power and
wattage will be explained in detail in Chapter 4. See
Figure 3-30 to examine the construction of a molded
composition resistor.
Another type of small wattage, fixed value resistor
is the thin film resistor. The thin film resistor is similar
to the molded composition resistor in appearance and
function. However, the thin film resistor is made by
Suitable for soldering and
welding even after long
periods in stock.
SOLDER-COATED LEADS
SOLID RESISTANCE
ELEMENT
Resistance material has large
cross section resulting in low
current density and high
overload capacity. Uniformity
of material eliminates
"hot spots."
SOLIDLY EMBEDDED
LEADS
Load wires are formed to
provide large contact area
and high pull strength.
PERMANENT COLOR
CODING
Bright, baked on colors are
highly resistant to solvents,
abrasion and chipping.
Colors remain clearly readable
after long service.
RUGGED
CONSTRUCTION
Resistors are hot-molded.
Resistance material,
insulation material, and lead
wires are molded at one
time into a solid integral
structure.
Figure 3-30. Cutaway of a carbon composition resistor.
(Allen-Bradley)
Figure 3-29. The physical size of a resistor can vary
according to wattage rating. The higher the wattage, the
larger the resistor. The largest shown is 20 watts, and the
smallest is 1/8 watt.
depositing a resistance material on a glass or ceramic
tube. A photographic process is used to deposit this film.
Leads with caps are fitted over each end of the tube to
make the body of the resistor. Thin film resistors are usu-
ally color coded.
The term “film resistor” is generally used to classify
very compact resistors used in micro-electronics or on
very small-scale electronic circuit boards. Film resistors
can also be referred to as surface-mounted resistors
(SMRs). The demand for smaller and smaller electronic
devices, such as cell phones, created the need for small
discrete components such as resistors, to be manufactured
in a more compact method.
Thick film and thin film are two general classifica-
tions based on how the film resistor is manufactured.
Thin film deposits resistive material on an insulated sub-
strate. Then the undesired portion is etched away leaving
the desired pattern of resistive material. See Figure 3-31.
Thick film deposits a special resistive paste directly on
the insulated substrate by using a stencil or silk screen
process. As a result, thick film is typically a thicker
deposit of resistive material as compared to thin film.
The advantage of thick film is the resistor can support
higher currents and wattage than the thin film. The
Figure 3-28. Group of carbon composition resistors and
the fixed resistor symbol.