NTC
Thermistor Theory
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Application
Notes
The
operation of the
industrialized
world depends to
a great extent
on the ability
to measure and
control a variety
of physical parameters.
Temperature is
one of the most
important of those
parameters. In
the present era
of inexpensive,
compact microcontrollers,
display modules
and versatile electronic
instrumentation,
the scope of potential
applications has
grown enormously.
Inexpensive NTC
thermistor elements
are being utilized
extensively as
sensors, probes
and components
in complex circuits
in a variety of
applications.
NTC
Thermistor devices
are extremely versatile
components in electronic
circuits. They
offer distinct
advantages in terms
of matching impedance
levels to available
instrumentation
or compensation
circuit needs.
The thermistor
material composition,
for example, can
be adjusted and
customized to achieve
a desired resistivity-temperature
response, within
certain constraints,
for a sensing device.
Precision
NTC thermistors
offer designers
the greatest sensitivity
to temperature
of any electronic
temperature sensing
component. They
exhibit a negative
temperature coefficient
of resistance in
the region of -3%/°C
to -5%/°C at
25°C. This
is roughly an order
of magnitude higher
than the sensitivity
of positive temperature
coefficient (PTC)
metal resistors
or thermocouple
sensor elements.
This provides some
distinct advantages
in system designs
where sensitivity,
circuit simplicity
and overall system
cost are important.
Drawbacks
of NTC Thermistor
devices include
a non-linear resistance
versus temperature
characteristic
and the fact that
small bead and
chip element devices
have limited power
handling capability.
These disadvantages,
however, are often
overcome with innovative
circuit designs.
Presently, NTC
thermistors are
the preferred sensing
element for many
applications where
precise measurement
and control are
required. Inexpensive
microprocessor
and display components
are now being coupled
with NTC thermistors
and hybrid circuits.
Such designs dominate
industrial applications
and can offer high
performance temperature
measurement and
control capabilities
for very reasonable
overall system
cost.
NTC
thermistor reliability,
performance and
life expectancy
has improved significantly
since the introduction
of such devices
in the 1930’s.
At present, long-term
stability and reliability
of NTC Thermistors
have been demonstrated
in many critical
medical, scientific
instrumentation,
military/aerospace
and industrial
applications. BetaTHERM implements
in-process monitoring
and control methods
that assure NTC
device stability
and performance
throughout the
manufacturing process.
Thermistor
applications make
use of the basic
thermistor features,
such as Resistance
versus Temperature
characteristics,
zero-power characteristics,
self heating effects
and thermal characteristics
like heat capacity
and dissipation
constant. A knowledge
of these factors
is important in
understanding the
principles of thermistor
applications.
The Applications section
provides an overview
of the use of thermistor
properties in practical
situations.
While
an in-depth discussion
of application
principles at text-book
level is beyond
the present scope,
the application
notes provide a
general overview
of methods of using
thermistors. Key
words and headings are
used that may serve
as pointers towards
more detailed sources
of information.
The
application notes
cover temperature
measurement, control
and circuit compensation
applications for
NTC thermistors
based on chip elements.
In addition, several
applications involving
thermistor device
characteristics
such as voltage-current
and current-time
characteristics,
which are also
relevant for rod
and disc thermistors,
are discussed.
The
thermistor application
principles covered
in the next sections
can be classified
in three categories.
These categories
are applications
based on Zero
Power Sensing Mode,
applications based
on Self-Heat
Sensing Mode,
and applications
based on the Time
Dependency of
thermistor characteristics.
In discussing these topics,
the notes on thermistor
characteristics from the
earlier section of the
catalog are relevant in
understanding the principles
involved.
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