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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