BASIC ELECTRONICS - OD1633 - LESSON 1/TASK 1
wire is the measure (or ability) of the wire to resist current. Usually the
percentage of nickel in the wire determines the resistivity.
One
disadvantage of the wirewound resistor is that it takes a large amount of
wire to manufacture a resistor of high ohmic value, thereby increasing the
cost. A variation of the wirewound resistor provides an exposed surface to
the resistance wire on one side.
An adjustable tap is attached to this
side. Such resistors, sometimes with two or more adjustable taps, are used
as voltage dividers in power supplies and other applications where a
specific voltage is desired to be "tapped" off.
b. Fixed and Variable Resistors.
There are two kinds of resistors,
fixed and variable. The fixed resistor will have one value and will never
change (other than through temperature, age, etc.). The resistors (shown in
A and B, figure 8 on the previous page) are classed as fixed resistors. The
tapped resistor illustrated in B has several fixed taps and makes more than
one resistance value available. The sliding contact variable resistor shown
in C has an adjustable collar that can be moved to tap off any resistance
within the ohmic value range of the resistor.
There are two types of variable resistors, one called a potentiometer and
the other a rheostat (see views D and E, figure 8).
An example of a
potentiometer is the volume control on your radio, and an example of the
rheostat is the dimmer control for the dash lights in an automobile. There
is a slight difference between them.
Rheostats usually have two
connections, one fixed and the other movable.
Any variable resistor can
properly be called a rheostat.
The potentiometer always has three
connections, two fixed and one movable.
Generally, the rheostat has a
limited range of values and a high current-handling capability.
The
potentiometer has a wide range of values, but it usually has a limited
current-handling capability. Potentiometers are always connected as voltage
dividers.
c. Wattage Rating. When a current is passed through a resistor, heat
is developed within the resistor.
The resistor must be capable of
dissipating this heat into the surrounding air; otherwise, the temperature
of the resistor rises causing a change in resistance, or possibly causing
the resistor to burn out.
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