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U.S.
Sensor
1832
W. Collins Ave
Orange, CA 92867
Tel: 800-777-6467
Tel: 714-639-1000
Fax: 714-639-1220
Email: sales@ussensor.com |
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TERMINOLOGY
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ZERO-POWER
RESISTANCE
The zero-power resistance is the dc resistance value of a thermistor
measured at a specified temperature with power dissipated by the
thermistor low enough that any further decrease in power will result
in not more than 0.1% (or one tenth of the specified measurement
tolerance, whichever is smaller) change in resistance.
RESISTANCE
RATIO CHARACTERISTIC
The resistance ratio characteristic identifies the ratio of the
zero-power resistance of a thermistor measured at 25°C to that
resistance measured at 125°C.
ZERO-POWER
TEMPERATURE COEFFICIENT OF RESISTANCE
(alpha
 T
)
Zero-power coefficient of resistance is the ratio at the specified
temperature (T) of the rate of change of zero-power resistance with
temperature to the zero-power resistance of the thermistor.
NEGATIVE
TEMPERATURE COEFFICIENT (NTC)
An NTC thermistor is one in which the zero-power resistance decreases
with an increase in body temperature.
POSITIVE
TEMPERATURE COEFFICIENT (PTC)
A PTC thermistor is one in which the zero-power resistance increases
with an increase in body temperature.
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MAXIMUM
OPERATING TEMPERATURE
The maximum operating temperature is the maximum body temperature
at which the thermistor will operate for an extended period of
time with acceptable stability of its characteristics. This temperature
can be the result of internal or external heating, or both, and
should not exceed the maximum value specified.
MAXIMUM
POWER RATING
The maximum power rating of a thermistor is the maximum power
which a thermistor will dissipate for an extended period of time
with acceptable stability of its characteristics.
DISSIPATION
CONSTANT
The dissipation constant is the ratio, (expressed in milliwatts
per degree C) at a specified ambient temperature, of a change in
power dissipation in a thermistor to the resultant body temperature
change.
THERMAL
TIME CONSTANT
The thermal time constant is the time required for a thermistor
to change 63.2% of the total difference between its initial and
final body temperature when subjected to a step function change
in temperature under zero-power conditions.
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RESISTANCE
TEMPERATURE CHARACTERISTIC
The resistance temperature characteristic is the relationship
between the zero-power resistance of a thermistor and its body
temperature. The Steinhart and Hart equation is an empirical
expression that is the best mathematical expression of the resistance
versus temperature characteristics of an NTC thermistor. The
calculation for determining the constants
 is
quite lengthy. To solve for the constants, contact U.S. Sensor's
application engineering department for a copy of a BASIC program
listing.
TEMPERATURE
WATTAGE CHARACTERISTICS
The temperature wattage characteristic of a thermistor is the
relationship at a specified ambient temperature between a thermistor
temperature and the applied steady state wattage.
CURRENT
TIME CHARACTERISTIC
The current time characteristic is the relationship at a specified
ambient temperature between the current through a thermistor and
time, upon application or interruption of voltage to it.
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STABILITY
The
stability of a thermistor is the ability of a thermistor to retain
specified characteristics after being subjected to designated
environmental or electrical test conditions.
BETA
(°K),
(EXPRESSED IN ° KELVIN)
The material constant of a thermistor. Unless otherwise specified,
Beta is derived from thermistor resistance measurements obtained
at 0° and 50°C.
MAXIMUM
STEADY STATE CURRENT (IMAX)
For power thermistors, the maximum continuous steady state current,
either DC or RMS AC, which the device is capable of passing. The
maximum steady state current for US Sensor power thermistors is
determined assuming a maximum operating ambient temperature of
65°C. If a specific application requires ambient temperature
operation above 65°C, custom designed devices are available.
RESISTANCE
AT MAXIMUM CURRENT (RImax)
For power thermistors, the approximate resistance of the device
under maximum steady state current conditions.
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