ECE 5320-Mechatronics Assignment 1: literature survey on Sensors and Actuators

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ECE 5320-MechatronicsAssignment 1 literature
survey on Sensors and Actuators

• Topic Thermistors (sensors)
• Prepared by
• Vikas G Pai
• Dept of Electrical and computer engineering
• UTAH STATE UNIVERSITY
• Tel [phone]
• Email vikasgpai_at_cc.usu.edu
• vikas_at_biology.usu.edu

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Overview

• A Thermistor is a type of resistor used to
  measure temperature changes, relying on the
  change in its resistance with changing
  temperature.
• Thermistors can measure temperatures across the
  range of -40 150 0.35 C
• Typical operation resistances are in the kW
  range, although the actual resistance may range
  from few W to several MW.

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Classification

• Thermistors can take various shapes rod, disc,
  washer, bead

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Classification

• Thermistors come in two varieties NTC, negative
  thermal coefficient, and PTC, positive thermal
  coefficient.
• The resistance of NTC thermistors decreases
  proportionally with increases in temperature.
• PTC thermistors have increasing resistance with
  increasing temperature

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Thermistor construction

• NTC Thermistors are most commonly made from the
  oxides of metals such as manganese, cobalt,
  nickel and copper. The metals are oxidized
  through a chemical reaction, ground to a fine
  powder, then compressed and subject to very high
  heat. Some NTC thermistors are crystallized from
  semiconducting material such as silicon and
  germanium.
• PTC Thermistors are generally made by introducing
  small quantities of semiconducting material into
  a polycrystalline ceramic. When temperature
  reaches a critical point, the semiconducting
  material forms a barrier to the flow of
  electricity and resistance climbs very quickly.

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Working principle

• Electrical resistance of a metal depends on the
  temperature.
• The basic principle of thermistors is that
  change in temperature changes its resistance,
  this change can be converted to electrical
  signal.

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PTC working principle

• Resistance of these types of
• thermistors increases with the
• rise in temperature.
• Due to the special Resistance-
• Temperature-characteristic, there
• is no additional temperature
• regulation or safety device
• necessary while reaching high
• heat-power level when using the
• low resistance area

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PTC working principle

• The PTC-heating element regulates the power
  sensitively according to the required
  temperature. The power input depends on the
  requested heat output.
• Courtesy http//www.ptc-ceramics.com/principle.ht
  m

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NTC working principle

• Resistance of NTC thermis-
• tors decreases proportionally with
• increases in temperature.
• Thermistor resistance-temperature
• relationship can be approximated by,

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NTC working principle

• where  T is temperature (in Kelvin), 
• TRef is the reference temperature,
  usually at room temp.
  (25 C 77 F 298.15 K),
•  
• R is the resistance of the
  thermistor (W), 
• RRef is the resistance at TRef, 
• b is a calibration constant depending on
  the thermistor material, usually
  between 3,000 and 5,000 K. 
• Courtesy http//www.dataacquisitionweb.com/sensor
  s/ntc_thermistors

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Sample configuration in application (PTC
Thermistor)

• There are very few commercial applications
  involving PTC thermistors that are based upon the
  resistance-temperature characteristic.
• Most PTC thermistor applications are
• based upon either the steady state self-
• heated condition (voltage-current char-
• acteristic) or upon the dynamic self-heated
• condition (current-time characteristic) or
• a combination of both.

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Sample configuration in application (PTC
Thermistor)

• The dramatic rise in resistance of
• a PTC at and above the transition
• temperature makes it ideal for
• over current protection.
• For all currents below the desired limiting
• current, the power dissipated in the thermistor
• is not sufficient to self-heat the device to its
  
• transition temperature.
• Should an over-current condition occur, the
• thermistor will self-heat beyond the transition
• temperature and its resistance rises
  dramatically.
• This causes the current in the overall circuit
  to be reduced.

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Sample configuration in application (PTC
Thermistor)

• 3 more applications are listed , in that they all
  rely on the dynamic operation (Current-Time
  Characteristic) of a self-heated PTC thermistor.
  In each case, current is allowed to pass through
  a series circuit for a prescribed amount of time
  before the thermistor self-heats into a high
  resistance condition.
• Time Delay circuit

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Sample configuration in application (PTC
Thermistor)

• Motor starting Degaussing
• Courtesy http//www.thermometrics.com/assets/imag
  es/ptcnotes.pdf

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Sample configuration in application(NTC
Thermistor)

• NTC thermistor is a versatile component that can
  be used in a wide variety of applications where
  the measured is temperature dependent.
• Thermistor applications are grouped according to
  one of the three fundamental electrical
  characteristics
• The current-time characteristics
• The voltage-current characteristic
• The resistance-temperature characteristic

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Sample configuration in application(NTC
Thermistor)

• Application based on Current-Time characteristic
• Time delay, surge suppression, inrush current
  limiting and sequential switching represent some
  of the earliest, high volume uses of thermistors.
  These thermistor applications are all based upon
  the current-time characteristic.
• Application based on Resistance -temperature
  characteristic
• Applications that are based upon the resistance
  temperature characteristics include temperature
  measurement, control, and compensation.

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Sample configuration in application(NTC
Thermistor)

• Linear Voltage Divider
• The simplest thermistor network used in many
  applications is the voltage divider circuit
• The output voltage is taken across the fixed
  resistor.
• This has the advantages of providing an
  increasing output voltage for increasing
  temperatures and allows the loading effect of any
  external measurement circuitry to be included
  into the computations for the resistor, R .
• The loading will not affect the output voltage as
  temperature varies

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Sample configuration in application(NTC
Thermistor)

• Linear Voltage Divider
• The output voltage as a function of temperature
  is as follows
• Courtesy http//www.thermometrics.com/assets/imag
  es/ntcnotes.pdf

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Specification

• Major specifications to be considered while using
  a thermistor.
• Resistance temperature curve it varies from
  thermistor to thermistor and the specifications
  are provided by the manufacturer.
• Nominal resistance value
• Resistance tolerance The specifications for this
  is provided by the manufacturer
• Beta tolerance This depends on the material
  being used for the thermistor

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Applications

• Thermistor is a versatile component and used in
  various applications where temperature is a
  factor to be considered.
• Depending on type of application and specific
  output,either PTC or NTC thermistors are used.
• The application part is broadly divided into PTC
  thermistor application and NTC thermistor
  application.

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Application of PTC Thermistors

• They are used as resettable fuses.
• They are used in time delay circuits.
• PTC Thermistors are used in motor starting
  circuits.
• They are also used in Degaussing circuitry.
• The PTC Thermistor can provide a combination of
  heater and thermostat in one device
• They are used as liquid level and flow
  sensors.

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Application of NTC Thermistor

• General industrial applications
• Industrial process controls
• Plastic laminating equipment
• Fiber processing manufacturing
• Hot mold equipment (thermoplastics)
• Solar energy equipment
• Automotive and Transportation Application
• Emission controls
• Engine temperatures
• Aircraft Temperatures.

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Application of NTC Thermistor

• Medical Applications
• Fever Thermometers
• Fluid temperature
• Dialysis Equipment
• Consumer/Household Applications
• Burglar alarm
• Refrigeration and air conditioning
• Fire detection
• Oven temperature control

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Advantages of Thermistors

• Thermistors have high sensitivity, better then
  that offered by thermocouples, RTDs.
• High accuracy, 0.02 C (0.36F)
• They offer a wide range of high resistance
  values.
• They have a small size.
• Thermistors have a faster response time then that
  of RTDs

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Limitations

• Limited temperature range, typically -100
  150 C (-148 302 F).
• Nonlinear resistance-temperature relationship,
  unlike RTDs which have a very linear
  relationship.
• Errors can result from self excitation currents
  being dissipated by the thermistors.
• They get de-calibrated on exposure to higher
  temperatures

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Selection, cost, buying info

• Based on the application, the type of Thermistor
  is decided.
• Thermistors can cost from 0.5 and above.
• Some online stores for buying thermistors.
• www.ussensor.com , www.preconusa.com ,
  http//www.vishay.com ,

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References

• www.efunda.com (Introduction to Thermistors)
• www.thermometrics.com/assets/images/ntcnotes.pdf
• www .thermometrics.com/assets/images/ptcnotes.pdf
• http//www.dataacquisitionweb.com/sensors/ntc_ther
  mistors
• http//www.ptc-ceramics.com/principle.htm

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Thank You !

• Vikas Pai