6
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
B:3350
B:3400
1k
10k
100k
Temperature(1/ Tx10
3)
THERMISTOR
"Thermistor" is the generic name given to
thermally sensitive resistors.
Negative temperature coefficient thermistor
is generally called as thermistor. Thermistor
is a semiconducting ceramic resistor
produced by sintering the materials at high
temperature and made mainly from metal
oxide.
Depending on the manufacturing method
and the structure, there are many shapes
and characteristics for various purposes
such as temperature measurement,
temperature compensation and etc.
The thermistor resistance values, unless
otherwise specified, are classified at a
standard temperature of 25 C.
B constant is calculated from the resistance
values at 25 C and 85 C.
The resistance of a temperature is solely a function
of its absolute temperature. Since electrical power
being dissipated within a temperature might heat
above its ambient temperature and thereby reduce
its resistance, it is necessary to test for resistance
with temperature. The resistance so measured is
called RT, which means the resistance at essentially
zero-power.
The mathematical expression which relates the
resistance and the absolute temperature of
a thermistor is as follows:
Ra Rb exp B
1
T1
1
T2
B
T2
Ra
T1
Where:
is the resistance at absolute temperature
is the resistance at absolute temperature
Rb
T2
B is a constant which depends on the
material of the thermistor
Unless otherwise specified,all values of B are
determined from measurements made at 25 C and
85 C.
The temperature coefficient of resistance
is expressed in the following equation:
100 (%/ C)
Resistance -Temperature Characteristics
Dissipation factor
Thermal time constant
Dissipation factor ( ) is power in milliwatts
required to raise thermistor temperature 1 C.
Measured with thermistor suspended by its leads in
a specified environment.
t:Raise temperature( C)
t
P
(mW/ C)
P :Power(mW)
Thermal time constant ( a) is the time required by a
thermistor to change 63% of the difference between
its initial and final temperature. Measured with
thermistor suspended by its leads in specified
environment.
95
87
63
a
2 a
3 a
5 a
Time (s)
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