P = I2R = 22 x 10 = 4 x 10 = 40 watts.
Example 2:
A 20-ohm resistor is required to drop the 12-volt battery voltage
of a vehicle to 6 volts for the lighting of a t-volt radio tube.
What is the power at the 20-ohm resistor?
P = E2/R = (6-volt drop) 2/20 = 36/20 = 1.8 watts.
Example 3:
A 1,200-watt electric soldering iron is connected to a 120-volt
power source.
How much current will the iron draw?
I = P/E = 1,200/120 = 10 amperes.
c. Power Losses.
(1) Most of the heat loss (power loss) in communication equipment occurs in
electron tube filaments.
In a man-pack radio set, the battery must
furnish the power required to heat the filaments of the tubes.
This
necessitates the use of heavy and cumbersome battery packs. One of the
reasons that radio sets using transistor circuits are smaller and
lighter is that the transistors, having no filaments as tubes do,
consume far less power.
The battery can therefore be smaller and
lighter in weight.
(2) Power loss due to resistance is an important consideration in
communication work. The resistor used must be capable of radiating the
heat generated without becoming hot enough to burn insulation or to
start fires. For this reason, resistors are rated in watts as well as
in ohms. This wattage rating indicates the amount of current that the
resistor can conduct without becoming damaged by heat. When resistors
are inclosed, the heat they radiate may cause troubles in other parts
of the equipment.
8.
RESISTORS
A resistor is a circuit element
designed to introduce resistance into
the
circuit
for
the
purpose
of
reducing or controlling the flow of
current.
Resistors may be divided
into three general types, according to
their construction.
These are known
as
fixed
resistors,
adjustable
resistors, and variable resistors.
a. Fixed
Resistors.
Fixed
resistors
are
used
to
introduce
constant values of resistance into
circuits.
For low-power requirements
(up to 20 watts), small carbon or
metalized resistors are used; with
heavier
power
requirements,
larger
resistors of wire-wound construction
are employed.
Several types of fixed
Figure 9.
Types of fixed resistors.
resistors are shown in figure 9.
308
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