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Types
and Trends of Resistance Temperature Sensors |
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There are two types of temperature sensors,
one is non-contact type that measures temperature
by brightness, color and infrared strength,
and the other, contact type which utilizes change
of thermoelectric power, electric resistance
and magnetism. Resistance temperature sensors
have been mostly used among them, utilizing
electric resistance change.
There are thermistors, platinum thermal sensors,
etc. of resistance temperature sensors, and
the former has been widely used. Recently design
change from thermistors to platinum thermal
sensors becomes popular, due to the long stability
and demand of compatibility.
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Types and
Trends of Resistance Temperature Sensors |
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Temperature sensors, whose resistance value
changes against the temperature, are mainly
categorized as thermistors, linear resistors,
or platinum resistance thermometer sensors.
Their temperature characteristics are shown
in Figure 1. |
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Thermistors are broken down
into either positive temperature coefficient
(PTC) or negative temperature coefficient (NTC).
For PTC type, the resistance value rapidly increases
at a constant temperature. On the basis of these
characteristics, PTC type is used for overcurrent
protection during thermo-runaway of semiconductors.
NTC type, on the other hand, has resistance
value that decreases in exponentially when temperature
increases. In general, NTC thermistors are often
used for the detection of temperature because
resistance value is high at room temperature
and changes remarkably against temperature change.
A thermistor is usually referred to as NTC type.
The NTC type is hereinafter referred to as thermistor.
Linear resistors have resistance value that
linearly increases when temperature rises. They
have been often used for temperature compensation
of the winding of motors. In recent years, they
are also used for temperature compensation of
high-frequency circuits or displays.
Meanwhile, platinum resistance thermometer sensors
also have resistance value that linearly increases
when temperature goes up. However, they have
higher linearity of temperature and more excellent
in long-term stability than linear resistors.
They are available for wide temperature range
and have high precision TCR tolerance as well
as resistance tolerance. Pt100 series, which
are conformed to the Japan Industrial Standard
(JIS) and International Electrotechnical Commission(IEC),
have compatibility.
Among these resistance temperature sensors,
the demand for SMD type thermistors
has been rapidly increasing since 2000 as the
manufacturing quantity of mobile phones increases.
Therefore, the quantity of thermistors used
is remarkable.
Thermistors offer advantages of large resistance
change, low price, and detection circuit that
is easy to configure. However, general thermistors
are limited by lack of compatibility for precision
applications. It is easy to configure circuits
that turn ON/OFF itself at a certain point of
temperature change. However, it is difficult
to perform high-precision detection in a circuit
for detecting the absolute value of temperature
and feeding it back to the circuit. They are
suitable for the set with short-term guarantee,
but not suitable for the set that requires long-term
guarantee or whose temperature sensor shall
be replaced as occasion needs. Some design engineers
insist on ease of use because of thermistor's
high signal output, but others assert that the
difficulty of the use because the added value
of device operation against temperature cannot
be created to the products.
There were reports about a post-thermistor during
the past two or three years. In reality, however,
linear resistors have been applied instead of
thermistors so as to change the number of fan
rotations according to the temperature of cooling
fan. Or platinum resistance thermometer sensors
have been applied for the cold junction compensation
of thermo-couples because there is no long-term
stability. To enhance world wide compatibility,
however, thermistors have been replaced with
platinum resistance thermometer sensors, Pt
100 types, for the temperature compensation
of measurement equipment.
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Application Notes
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