Download Resistors

Resistors
Electronics 1
CVHS
Review
• Resistance is the opposition to current flow
provided by a material
• The symbol for resistance is Ω
• The schematic symbol for a resistor is
• When measuring resistance YOU MUST
ALWAYS REMOVE POWER from the
circuit
Resistance
• One ohm = 1 amp @ 1 volt
• A larger resistance value will allow less
current to flow
• A smaller resistance value will allow more
current to flow
http://www.colorado.edu/physics/phet/simulations/veqir/VeqIRColored.swf
Resistor Uses
• Used to limit current flow in a circuit
• Many circuits and components require
different voltage and current levels
• Resistors reduce these levels down to the
required level for each
component or area of
a circuit
Resistor Types
• Fixed (resistance is preset and cannot be
changed)
• Variable (The internal resistance of the
device can be adjusted)
Fixed Resistor Composition
Fixed resistors
• Carbon Film
• Metal film
• Wire wound
• Wire wound ceramic coated
• Symbol:
High Voltage Resistors
Tapped Resistors
• These are wire wound fixed resistors
• One or more “taps” are located between
the ends of the resistor to provide different
resistances
• Symbol:
Variable Resistors
• Used as circuit controls
• Most use a wire wound resistor and a pole
• Carbon coatings, and specialty materials
may also be used for certain applications
Potentiometers
• Has three terminals
• Shaft rotation changes
the resistance between
the center terminal and
two end terminals
• Most “Pots” are linear
• One degree rotation of the shaft = the same
change in resistance regardless of the shaft
location
Potentiometers
• Non-linear Pots (audio taper)
• One degree of shaft rotation does not = the
same change in resistance at different shaft
rotations.
• Used in volume and tone controls
• Symbol:
Rheostats
• Like Pots but only have 2 terminals
• Used to adjust current in high power
circuits
• In low current situations, a pot may be
used as a rheostat
• Symbol:
Adjustable
• Used in high power applications
• Used when frequent changes are not
necessary
• Power is usually removed from the circuit
for adjustment
• Symbol:
Thermistor
• Changes internal resistance with
temperature
• Used as temperature control and safety
device.
• Symbol:
Photocell
• Changes internal resistance with amount of
light
• Used in “electronic eye”
• Symbol:
Resistor Power Ratings
• Resistors convert electrical energy into thermal energy
(heat)
• As current is passed through a resistor
it gets hot
• The power rating of a resistor indicates
how much power the resistor can
dissipate without being destroyed
• There is no relationship between the resistance value and
the power rating of a resistor
• The proper power rating for a resistor is determined by
the circuit designer
• Power rating is displayed by a resistor’s physical size
(printed on ceramic wire resistors)
Review
• What are the two major classifications of
resistors?
• What does one ohm equal?
• As resistance increases, current…
• What is the relationship between r-value
and power rating? … between power rating
and size? … between r-value and size?
• What is the difference between a rheostat
and a pot?