Electronic Troubleshooting

1
Electronic Troubleshooting

• Chapter 12
• Sensors and Transducers

2
Sensors and Transducers

• Characteristic
• Transducers converts the form of energy
• A microphone coverts sound energy into electrical
  energy
• A speaker converts electrical energy into sound
• Sensors are transducers that used to detect
  and/or measure something
• Used to convert mechanical, thermal, magnetic,
  chemical, or etc variations into electrical
  voltages and currents

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Sensors and Transducers

• Temperature Sensors
• Types of Temperature Sensors
• Thermocouple
• Resistance temperature device(RTD)
• Thermistor
• Monolithic IC Sensors
• NOTE See Chart on next slide or page 344
• Thermocouple
• Characteristics
• Most common sensor
• A pair of dissimilar wires welded together at the
  sensing location
• A temperature difference from the welded end and
  the other end causes a DC voltage at the non
  welded end
• Can be used under extreme conditions
• Ovens, Furnaces, Nuclear tests

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Sensors and Transducers

• Temperature Sensors

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Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Operation
• When wires made of dissimilar metals are
• Welded together at both ends
• With different temperatures at both ends
• Current flows

6
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Operation
• Open the pair of wires in
• between the two ends a
• voltage develops
• Called Seebeck Voltage
• Proportional to
• temperature difference

Chart on page 344
Example 12-1 page 345
to 12-4 on page 365
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Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Operation
• Equation is linear over only a small range of
  temperatures
• Tables of corrected voltages in 10 increments is
  available from the NBS for each type
• Reference Junction
• Voltage developed is dependent upon the
  temperature difference between ends NOT
  Absolute Temperature of the welded end
• Wheres the
• cold junction
• Its at room temp
• Voltage will be wrong
• Need a 00C ref

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Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Reference Junction
• Lab Set up
• Not practical for most situations
• Practical Reference Junction
• solutions
• Electronic Ice points
• Available for All types
• of thermocouples
• Encased electronic device
• that balances an internal bridge
• circuit which generates a voltage to cancel out
  effect that
• the measurement end isnt at OOC

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Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Practical Reference Junction
• solutions
• Isothermal block
• Usually used with
• computerized (also microcontrollers) data
  collection systems
• The isothermal block is a good conductor of heat
  not electrical current
• However its resistance is effected measurably by
  changes in temperature
• Block is always near the point were the voltages
  are measured
• Computerized measuring system calculates cool end
  temperature based on the block resistance and
  corrects the voltage reading

10
Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Typical Problems
• A short of the two wires
• Junction then will be at the point of the short
• Temperatures readings will be incorrect
• No Reference Junction Compensation
• Temperatures readings will be incorrect
• Test Short the inputs to the compensator and
  room temperature should be the new reading
• If extensions of the thermal couple wires are
  used they should be of a larger size and material
• Different materials create Incorrect readings
  since the connection of dissimilar materials
  creates a new junction
• Larger size is needed for IR drops

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Sensors and Transducers

• Temperature Sensors
• Thermocouple
• Typical Problems
• Noise pick-up
• Long leads form an antenna uses shielding.
  e.g., grounded over braiding of copper
• Extreme Temperature Gradient
• Can damage the thermocouple should have
  protection
• Environment can change the metal and its
  thermal characteristics
• Chenicals
• molten metals
• new alloys

Commercial thermocouple assemblies see page 348
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Sensors and Transducers

• Temperature Sensors
• Resistance Temperature Device
• Key principle
• As the temperature of a resistor increases so
  does its resistance
• Measure the change in the resistance of a known
  resistor calculate the temperature change
• Linear relation ship for smaller changes more
  linear than thermocouples NBS has correction
  tables for the typical types of measurement
  resistors
• Typical construction
• Wire wound resistor in on a ceramic core using
  platinum wire
• Stable (linear) over a wide range of temperatures
• Temperature coefficient 0.00385/0C
• Typical Values 10 several kilo-ohms
• Most common value 100?

13
Sensors and Transducers

• Temperature Sensors
• Resistance Temperature Device
• Measuring Circuit Types
• RTD Bridge circuit
• Constant Current Source
• RTD Bridge circuit
• Platinum resistor is remote from the bridge
  circuit which is isolated from the sensing point
• Bridge is balanced at a known temperature
• Eliminates consideration of the connecting leads
• Voltage developed is proportional to the
  temperature change

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Sensors and Transducers

• Temperature Sensors
• Resistance Temperature Device
• Constant Current through RTD
• Voltage across the RTD rises and the resistance
  increase with the rise in temperature
• The constant current also increases the
  temperature of the resistance and effects the
  temperature reading
• The correction factor for common platinum RTDs
  has been determined
• Example Problems on page s 349 350

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Sensors and Transducers

• Temperature Sensors
• Thermistor
• Resistors with high negative temperature
  coefficients
• Resistance decreases with an increase in
  temperature
• High temperature coefficients means that there is
  a significant change in resistance for a small
  temperature change
• Construction
• Semiconductor material
• In either tube or bead shapes
• Can be used as a plain resistor in circuits such
  as a bridge or voltage divider
• Come in a Wide range of values
• Also come with manufacturer provided resistance
  vs temperature curves

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Sensors and Transducers

• Temperature Sensors
• Thermistor
• Construction
• Also come manufacturer provided resistance vs
  temperature curves
• Sample for thermistor with nominal value of 5k?
  at 00C

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Sensors and Transducers

• Temperature Sensors
• Monolithic IC Sensors
• Current or voltage types are available
• They have linear output voltages or currents with
  temperature changes
• Typical values 1µA/0K 10mV/0K
• 1 0K 1 0C

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Sensors and Transducers

• Light Sensors
• Typical uses of the sensors
• Measure intensity of the light
• Detect the presence or absence of light
• Types of Light Sensors
• Photovoltaic Cells
• Photoconductive Cells
• Photo Diodes
• Phototransistors
• Photovoltaic Cells
• aka, Solar Cells
• Semiconductor material that generates a voltage
  when light shines on it
• 2.5 by 5 cm cell can produce 0.4 V with 180mA of
  current

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Sensors and Transducers

• Light Sensors
• Photovoltaic Cells
• Sometimes used to detect the presence of light
• Photoconductive Cells
• aka, photoresistors
• Characteristics of Photoresisters
• Uses bulk resistivity which decreases with
  increasing illumination, allowing more
  photocurrent to flow.
• Signal current from the detector can be varied
  over a wide range by adjusting the applied
  voltage.
• Thin film devices made by depositing a
• layer of a photoconductive material
• on a ceramic substrate.

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Sensors and Transducers

• Light Sensors
• Photoconductive Cells
• Characteristics of Photoresisters
• Metal contacts with external connection. These
  thin films have a high sheet resistance.
  Therefore, the space between the two contacts is
  made narrow and long for low cell resistance at
  moderate light levels.

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Sensors and Transducers

• Light Sensors
• Photoconductive Cells
• Light Intensity Application
• With little or no light the
• voltage at point X is low
• As the intensity of the
• light on the sensor increases
• the voltage at X will increase
• By adjusting Rf,a usable output range of voltages
  that the is proportional to the light intensity
  can be obtained
• Presence or Absence of
• Light application
• Activates a electromechanical
• counter when the light is blocked

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Sensors and Transducers

• Light Sensors
• Photoconductive Cells With a Microcontroller
• Critical aspect of this application a BASIC
  command for measuring the RC decay time on a
  connected circuit
• RCTIME command is designed to measure RC decay
  time on a circuit like the one below. The lower
  the count recorded the brighter the light
  measured
• RCTIME Pin, State, Duration
• Pin argument is the number of
• the I/O pin that you want to measure
• State argument - 1 if the voltage across the
  capacitor starts above 1.4 V and decays downward.
  0 if the voltage across the capacitor starts
  below 1.4 V and grows upward
• Duration argument has to be a variable that
  stores the time measurement, which is in 2 µs
  units
• Very simple circuit range of measured light is
  limited only by the size of the variable used to
  store the count.

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Sensors and Transducers

• Light Sensors
• Photodiodes
• A diode that is forward biased by light
• Very fast reactions to changing light levels
• Same physical size as LEDs
• Have small windows through which light is
• sensed
• Testing is simple
• When the window is blocked
• High resistance is read
• Shine a bright light (several footcandles) on it
  while still connected to an ohmmeter
• The resistance will drop significantly
• Phototransistors
• Usually used instead of photodiodes when low
  light levels are measured

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Sensors and Transducers

• Light Sensors
• Phototransistors
• Usually used instead of photo resistors when low
  light levels are broken at high rates
• Typical ratings
• Like low power transistors
• 30-50V maximum collector to emitter voltages
• Max collector currents of 25mA
• Typical application
• See slide on the next page or the bottom of page
  355
• Monitors droplets falling through an IV
  administration set
• Drip rate is set by nurses with a small valve not
  shown
• IR LED is the Light beam sourse
• The drops block enough light to turn off the
  phototransistor
• Positive spikes on its collector feed an
  inverter that squares off and amplifies the
  spikes
• Sent to a counter, alarm, or monitoring equip

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Sensors and Transducers

• Light Sensors
• Replacement Considerations
• Best option is an exact replacement
• If not possible match the following
  characteristics
• Voltage, current, power ratings physical size
• Light sensitivity
• Can be specified nm (human sight 400 -700) nm
  (700nm red light)
• Called spectral response
• Can also be specified in angstroms Å. 10 Å 1nm
• Light Insensitivity
• For photoresistors X-k? at Y-footcandles
• 1 Foot candle light falling on 1 square foot
  one foot from a standardcandle
• For phototransistors Collector current at a
  specified light level

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Sensors and Transducers

• Light Sensors
• Other Problems with light sensing systems
• Burned out, weak, or obstructed light sources
• Can be a simple problem of dirty light filters or
  lens
• Light shields may have been misaligned by a bump
• Mechanical Sensors
• Characteristics
• Used to measure
• Force
• motion
• position
• The chapter covers Strain gages
• They measure Forces
• Weight is a common force

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Sensors and Transducers

• Strain gages
• Characteristics
• Sensors used to measure change in the dimensions
  of solid objects caused by forces
• Information is critical to designs of mechanical
  systems
• Used in load cells which are used to measure
  weights of objects
• Measurements can range from a few pounds to the
  weight of a fully loaded tractor trailer rig
• Strain and Stress
• Strain ?L/L0 , where ?L change in length
  due to a force
• and L0 the
  original length before the force was applied
• Can be caused by tension or compression forces

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Sensors and Transducers

• Strain gages
• Strain and Stress
• Strain ?L/L0 , where ?L change in length
  due to a force
• and the
  original length before the force was
• applied
• Can be caused by tension or compression forces
• Stress is a measure of the force applied that has
  been normalized to a unit area
• Stress F/A , where F the total force
  applied and A cross-
• sectional area
• The ratio of Stress/Strain is a constant value
  for each material
• Called Youngs Modulus and has been tabulated for
  many material
• Most metals wont stretch beyond 0.5 without
  deforming

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Sensors and Transducers

• Strain gages
• Strain and Stress
• Resistor conductance can be determined from
  R?L/A
• Where R resistance in ohms, ? (rho) is the
  resistivity of the material, L length of the
  material, A is the cross-sectional area of the
  material
• If the gage material under stress increases it
  length by0.4 - its resistance will increase by
  0.4
• Some commercial gages have been designed to yield
  multiples of the change in length in change of
  resistance A Gage Factor
• Construction
• Metal or semiconductor foil woven back and forth
  to increase the length
• Range of common values 30 -3000 ?
• Most common sizes 120 ? and 350 ?

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Sensors and Transducers

• Strain gages
• Strain and Stress
• Calculations
• Where R resistance of the gage under stress, R0
  Original resistance of the gage, ?L change in
  length of the gage, L original length of the
  gage, GF gage factor
• Example Problems 12-4 and 12-5 on page 359
• Typical Bridge configurations

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Sensors and Transducers

• Strain gages
• Typical Bridge configurations
• The 1/4 bridge has a gain factor of 1
• Change of resistance causes the bridge to
  unbalance
• The ½ bridge has two strain gages
• One in tension mode and one in compression mode,
  like in the metal beam drawing bottom right of
  previous slide
• rg1 is stretched and rg2 is compressed
• Changes double the resistance change
• GF 2
• The full bridge has four gages and a GF of four
• Problems with Strain Gages
• Temperature changes
• If outside the circuitry must have temperature
  compensation
• e.g., Las Vegas temperatures range from the 20s
  115

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Sensors and Transducers

• Strain gages
• Typical