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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
typical types of thermistors.

   The shape of the thermistor
probe can take the form of a

   Basically, thermistors are
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
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
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
conventional fuses to protect
transformers, etc. or
electronic circuits against
over-current.

   They not only respond to
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)
illustrates the two operating
states of a PTC fuse.

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

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
(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
measurement circuitry to be
included into the
computations for the resistor,
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
the correct NTC thermistor may sometimes seems to be an

   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.
   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 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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