Resistors

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Resistors

• Electronics 1
• CVHS

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Review

• Resistance is the opposition to current flow
  provided by a material
• The symbol for resistance is O
• The schematic symbol for a resistor is
• When measuring resistance YOU MUST ALWAYS REMOVE
  POWER from the circuit

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Resistance

• One ohm 1 amp _at_ 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/v
eqir/VeqIRColored.swf
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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

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Resistor Types

• Fixed (resistance is preset and cannot be
  changed)
• Variable (The internal resistance of the device
  can be adjusted)

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Fixed Resistor Composition
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Fixed resistors

• Carbon Film
• Metal film
• Wire wound
• Wire wound ceramic coated
• Symbol

High Voltage Resistors
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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

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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

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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

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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

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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

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Adjustable

• Used in high power applications
• Used when frequent changes are not necessary
• Power is usually removed from the circuit for
  adjustment
• Symbol

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Thermistor

• Changes internal resistance with temperature
• Used as temperature control and safety device.
• Symbol

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Photocell

• Changes internal resistance with amount of light
• Used in electronic eye
• Symbol

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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 resistors
  physical size (printed on ceramic wire resistors)

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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?