Introduction
Resistors are used in electrical
circuits to:
- Reduce the voltage applied to another
component(s).
- Limit the electrical current flowing
through another component(s).
Construction
A “package” of resistance made
up into a single unit is called a resistor. Resistors having the same resistance value may
be considerably different in size and construction. The flow of current through resistance
causes the conductor to become heated; the higher the resistance and the larger the
current, the greater the amount of heat developed. Resistors intended for carrying large
currents must be physically large so the heat can be radiated quickly to the surrounding
air. If the resistor is unable to dissipate the heat quickly it may reach a temperature
that will cause it to melt or burn.
Types of Resistor
Several types of resistors are available
including:
- “Fixed” value small
components with wire leads are commonly either carbon film or metal film types. These
range in size from 0.125 Watt to 2 Watts.
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- Larger fixed values resistors
manufactured using wire wound resistance; available in the range 2 Watts to 100 Watts.
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- Resistor Network components that
comprise a number of separate film resistors contained within a single integrated package.
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- Adjustable “potentiometer”
and pre-set types, comprising a sliding resistance on an exposed section of resistance
winding or film.
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Specification and
Calculation of Resistance
Three (3) separate criteria determine
the suitability of a resistor for use within any circuit:
The resistivity of fixed value
resistors are measured in “Ohms” (designated “ “).
Resistance is calculated using the
“Ohms Law” formula:
where Voltage
= voltage across the resistor expressed in “Volts”
and Current
= current flowing through the resistor expressed in “Amperes” (or
“Amps”)
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