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					  ECE5320 - Mechatronics
Assignment 1: Literature Survey on
      Sensors and Actuators

 Topic: Thermistors (Sensors)
                        Prepared by:

                  SIDDHARTH P. RAO

          Dept. of Electrical and Computer Engineering
                       Utah State University
    Tel: 435-753-4306(Home) Email: siddharth@cc.usu.edu
    Tel: 435-797-5237(Work) Email: siddharthr@ext.usu.edu
                        Overview
   “Sensor is a device that when exposed to a physical
    phenomenon (temperature, displacement, force, etc.) produces
    a proportional output signal (electrical, mechanical, magnetic,
    etc.)”.

   The „Thermistor‟ uses resistance to detect temperature.

   Thermistors can measure temperatures across the range of -40
    ~ 150 ±0.35 °C (-40 ~ 302 ±0.63 °F).

   Typical operation resistances are in the kW range, although the
    actual resistance may range from few W to several MW.
          Typical Thermistor Types
   The adjoining figure shows
    typical types of thermistors.

   The shape of the thermistor
    probe can take the form of a
    bead, washer, disk, or rod.

   Basically, thermistors are
    broadly classified as
    Ceramic, PTC (positive
    temperature coefficient) and
    NTC (negative temperature
    coefficient) thermistors.
          Basic Working Principle
   The electrical resistance of metals depends on temperature.

   By measuring the changing resistance, the temperature can be
    determined.

   The change in resistance can easily be converted to an
    electrical signal transmittable.

   A thermistor is made of semiconductor, a mixture of metal
    oxide.
          Basic Working Principle
   Metals usually have a positive resistance coefficient with
    respect to temperature.

   Unlike metals, the semiconductors have a negative resistance
    coefficient.

   This is the main difference between a thermometer and a
    thermistor.

   Thus, it can be said that a PTC Thermistor is similar to an
    Resistance Temperature Detectors (RTD).
          Basic Working Principle
   Thus, thermistors are based on the principle of when the
    temperature of the resistors changes, the electrical resistance of
    the resistors will change correspondingly.

   In Negative Temperature Coefficient (NTC) thermistors, when
    the temperature of the resistors increases, the resistance of the
    resistors will be decreased.

   In Positive Temperature Coefficient (PTC) thermistors, when
    the temperature of the resistors increases, the resistance of the
    resistors will also be increased.
        The PTC Working Principle
   The PTC (Positive Temperature
    Coefficient) is a temperature
    sensitive semiconductor, which is
    made of doped polycrystalline
    ceramic on the basis of barium
    titanate.

   The resistance of these thermistors
    increases sharply when a defined
    temperature is reached.

   This property is the reason for the
    self-regulation characteristic,
    which the PTC heating elements
    make use of.
        The PTC Working Principle
   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.

   The PTC-heating element
    regulates the power sensitively
    according to the required
    temperature. The power input
    depends on the requested heat
    output.
      The NTC Working Principle
   The NTC thermistors which
    are discussed herein are
    composed of metal oxides.

   The most commonly used
    oxides are those of manganese,
    nickel, cobalt, iron, copper and
    titanium.

   As seen from the adjoining
    figures, the resistance of these
    thermistors decreases with the
    increase in temperature.
      The NTC Working Principle
   In the basic process of
    fabrication, a mixture of two
    or more metal oxide powders
    are combined with suitable
    binders, formed to a desired
    geometry, dried, and sintered
    at an elevated temperature.


   By varying the types of oxides
    used, their relative proportions,
    the sintering atmosphere, and
    the sintering temperature, a
    wide range of resistivities and
    temperature coefficient
    characteristics can be obtained.
Sample Configuration in Application
         (PTC Thermistor)
   As to their possibilities of application, PTC thermistors can be
    divided on the basis of their „function‟ and their „application‟.

   Out of the so many possible applications, I would like to like
    to show the use of „PTC thermistors for over-current
    protection‟.

   It‟s one of the simplest configurations and is very easy to
    understand.

   Here, PTC thermistor is used in the form of a fuse which is
    connected in series with the load in the circuit.
Sample Configuration in Application
        (PTC Thermistor)
   Ceramic PTC thermistors
    are used instead of
    conventional fuses to protect
    loads such as motors,
    transformers, etc. or
    electronic circuits against
    over-current.

   They not only respond to
    inadmissibly high currents
    but also if a preset
    temperature limit is
    exceeded.
Sample Configuration in Application
        (PTC Thermistor)
   Thermistor fuses limit the
    power dissipation of the
    overall circuit by increasing
    their resistance and thus
    reducing the current to a
    harmless residual value.

   In contrast to conventional
    fuses, they do not have to be
    replaced after elimination of
    the fault but resume their
    protective function
    immediately after a short
    cooling-down time.
Sample Configuration in Application
        (PTC Thermistor)
   The adjoining figure
    illustrates the two operating
    states of a PTC fuse.

   In rated operation of the load,
    the PTC resistance remains
    low (operating point A1).

   Upon overloading or shorting
    the load, however, the power
    consumption in the PTC
    thermistor increases.
Sample Configuration in Application
        (PTC Thermistor)
   It increases so much that it
    heats up and reduces the
    current flow to the load to an
    admissible low level
    (operating point A2).

   Most of the voltage then lies
    across the PTC thermistor.
    The remaining current is
    sufficient to keep the PTC in
    high-resistance mode
    ensuring protection until the
    cause of the over-current has
    been eliminated.
Sample Configuration in Application
        (NTC Thermistor)
   There are a variety of instrumentation / telemetry circuits in
    which an NTC thermistor may be used for temperature
    measurements.

   In most cases, a major criterion is that the circuit provides an
    output that is linear with temperature.

   When the use of a constant-current source is desired, the
    circuit used should be a two-terminal network that exhibits a
    linear resistance-temperature characteristic.

   The output of this network is a linear voltage-temperature
    function.
Sample Configuration in Application
        (NTC Thermistor)
   Under these conditions, a digital voltmeter connected across
    the network can display temperature directly when the proper
    combination of current and resistance level are selected.

   Consequently, the design of NTC thermistor networks for
    most instrumentation / telemetry applications is focused on
    creating linear resistance-temperature or linear conductance-
    temperature circuits.

   The simplest NTC thermistor network used in many
    applications is the “voltage divider circuit”. Here, with the
    increase in temperature, the resistance decreases, thus
    increasing the output voltage across the divider network.
Sample Configuration in Application
        (NTC Thermistor)
   In this 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 , and thus the loading will
    not affect the output voltage
    as temperature varies.
Sample Configuration in Application
        (NTC Thermistor)
   The output voltage as a
    function of temperature can
    be expressed as follows:




   From the plot of the output
    voltage, we can observe that
    a range of temperatures
    exists where the circuit is
    reasonably linear with good
    sensitivity.
Sample Configuration in Application
        (NTC Thermistor)
   Therefore, the objective will
    be to solve for a fixed resistor
    value, R , that provides
    optimum linearity for a given
    resistance-temperature
    characteristic and a given
    temperature range.

   A very useful approach to the
    solution of a linear voltage
    divider circuit is to normalize
    the output voltage with
    respect to the input voltage.
Major Specifications in Thermistors
   To the design engineer attempting to specify, or, to the
    purchasing agent attempting to procure, the task of choosing
    the correct NTC thermistor may sometimes seems to be an
    impossible task.

   While the process can be difficult at times because of
    subtleties in the use of each product type, it is not nearly
    impossible if one has a good understanding of the basics.

   This, knowing and understanding the major specifications of a
    Thermistor is important. Following are the major
    specifications of a Thermistor.
Major Specifications in Thermistors
   Resistance-Temperature Curves : Usually varies and is
    provided by the manufacturer.

   Nominal Resistance Value : Usually varies and is provided
    by the manufacturer.

   Resistance Tolerance : The standard tolerances available for
    each thermistor type are given on the specific product data
    sheet.

   Beta Tolerance : The beta of a thermistor is determined by the
    composition and structure of the various metal oxides being
    used in the device.
       Application of Thermistors
   The thermistor is a versatile component that can be used in a
    wide variety of applications where the measurand is
    temperature dependent.

   Depending on the type of application and the specific out put
    requirements, the PTC or the NTC Thermistor is used.

   Thus, the application have to be broadly divided as PTC
    Thermistor applications and NTC Thermistor applications
    respectively.

   Following are the various applications.
    Application of PTC Thermistors
   PTC thermistors are used for over-current protection.

   PTC thermistors are used for telecommunication applications.

   PTC thermistors are used for picture tube degaussing.

   PTC thermistors are used for time delay and switching
    applications.

   PTC thermistors are used for motor starting.

   PTC thermistors are used as heating elements.
    Application of PTC Thermistors
   Apart from these, Power PTC thermistors are used as a „Fuse‟
    for Short-circuit and over-current protection.

   They are used as a „switch‟ for Motor start Degaussing.

   They are used as a „temperature sensor‟ in measurement and
    control & over temperature protection circuits.

   They are used to limit temperature for motor protection and
    over temperature protection circuits.

   They are also used as „level sensors‟ and „limit indicators‟.
Application of NTC Thermistors
   NTC thermistors are used in General Industrial Applications
    such as Industrial process controls, Photographic processing,
    Copy machines, Soldering irons (controlled), Solar energy
    equipment, etc.

   They are used in Consumer / Household Appliances like
    Thermostats, Burglar alarm detectors, Refrigeration and air
    conditioning, Fire detection, etc.

   They are used in Medical Applications like Fever
    thermometers, Dialysis equipment, Rectal temperature
    monitoring, Respiration rate measurement, Blood analysis
    equipment, Respirators, etc.
Application of NTC Thermistors
   They are used in Instrumentation Applications like Motor
    winding compensation, Infrared sensing compensation,
    Instrument winding compensation, etc.

   They are used in Automotive and Transportation Applications
    for Emission controls, Differential temperature controls,
    Engine temperatures, Aircraft temperatures, Rotor/bearing
    temperatures, etc.

   They are used in Food Handling Applications like Fast food
    processing, Perishable shipping, Oven temperature control,
    Coffee makers, Freezing point studies.
Application of NTC Thermistors
   They are used in High Reliability Applications for monitoring
    Missiles & spacecraft temperatures, Aircraft temperature,
    Submarines & underwater monitoring and as a Fire control
    equipment.

   They are used in Communications Applications for Transistor
    temperature compensation, Gain stabilization, Piezoelectric
    temperature compensation.

   Apart from all these, they are also used in RF / Microwave
    power measurement, Voltage regulation circuits, Time delay
    devices, Sequential switching, Surge suppression, Inrush
    current limiting, etc.
       Advantages of Thermistors
   High accuracy, ~±0.02 °C (±0.36°F), better than RTDs, much
    better than thermocouples.

   High sensitivity, ~10 times better than RTDs, much better than
    thermocouples. As a result, lead wire and self-heating errors
    are negligible.

   Small in size compared to thermocouples.

   Response time shorter than RTDs, about the same as
    thermocouples.

   Reasonable long term stability and repeatability.
       Limitations of Thermistors
   Limited temperature range, typically -100 ~ 150 °C (-148 ~
    302 °F).

   Nonlinear resistance-temperature relationship, unlike RTDs
    which have a very linear relationship.

   They can be affected by self-heating errors that result from
    excitation current being dissipated in the thermistor.

   Thermistors are also relatively fragile, so they must be handled
    and mounted carefully to avoid damage.

   Exposure to higher temperatures can de-calibrate a thermistor
    permanently, producing measurement inaccuracies.
     Selection, Cost & Buying Info
   Selection of thermistors completely depends on the type of
    applications in which it is being used. It can be a PTC, an NTC
    or a Ceramic thermistor with respective temperature range, etc.

   Based on the type of application, thermistors range from as
    low as $0.5 to as high as over $500 per piece.

   There are many online stores from where thermistors can be
    purchased depending on the type of application. Some of the
    good e-stores are as follows:

   www.omega.com , www.ussensor.com , www.sensorsci.com ,
    www.jameco.com , www.component.com , etc.
                      References
   eFunda: Introduction to Thermistors

   Thermistors : Vishay

   Module 1.4: Sensors and Transducers

   Sensors : September 2000 – Temperature Measurement

   EPCOS AG : PTC Thermistors – Application Notes

   Thermo metrics : NTC Thermistors – Notes
Thank You…!

				
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