22
Thermal time constant
If ambient temperature of a thermistor is changed
to T1 from T2 suddenly, temperature of the thermistor
changes slowly.
The time constant means the time when temperature
of the thermistor reaches 63% of the temperature
difference.
Maximum permissible current
If the maximum permissible current flows to a
thermistor at 25 C, temperature of the thermistor
rises to 200 C, (160 C). When ambient temperature is
above 25 C, the maximum permissible current shall be
over reduced as the maximum permissible current
reduction curve.
Reliability tests
Dry heat test
MARK
Test sample is exposed in air at 200 C for 1,000
hours.
R25/R25
20%
Damp heat test
Test sample is exposed in atmosphere of 95%RH at
40 C for 1,000 hours.
R25/R25
10%
Load test
Test sample is applied the maximum rating current
in air at 25 C for 1,000 hours.
R25/R25
20%
Change of temperature
Test sample is given 10 times of the following
temperature cycle,
40 C for 30 minutes
room temperature for 5
minutes
200 C for 30 minutes
room temperature for 5
minutes.
R25/R25
10%
Residual resistance
If current is flowed through a thermistor, any heat
will be generated in the thermistor by which its
resistance will be decreased, however, a decrease
of a resistance will be stabilized at a saturation
resistance value which is determined by impressed
electric power and a dissipation constant. The
residual resistance value means maximum
saturation resistance value when the maximum
permissible current is flowed through the thermistor.
Temperature coefficient
The temperature coefficient of a thermistor is
expressed by the following equation ;
Dissipation factor
If small voltage is applied to a thermistor, small
current will flow which produce enough heat in the
thermistor. Dissipation factor is electric power
which make 1 C raise by heat in a thermistor.
Resistance-temperature characteristics
The theoretical characteristics of a thermistor is expressed by following equation.
P is applied electric power.
t is rised temperature of the thermistor.
New Power Thermistor MARK
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
Ambient temperature ( C)
Power Thermistor
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
20
10
10 0
30 40 50 60 70 80 90 100 110 120 130 140 150 160
Ambient temperature ( C)
Temperature ( C)
D TYPE
W TYPE
(%/ C)
T2
B
exp B
T1
R1
R1 is the zero-power resistance at absolute temperature T1
R2 is the zero-power resistance at absolute temperature T2
R2
1
T2
1
(mW/ C)
t
P
MARK
B : 3150K
Dry heat test
POWER THERMISTOR
Test sample is exposed in air at 160 C for 1,000
hours.
R25/R25
10%
Damp heat test
Test sample is exposed in atmosphere of 95%RH at
40 C for 1,000 hours.
R25/R25
10%
Load test
Test sample is applied the maximum rating current
in air at 25 C for 1,000 hours.
R25/R25
10%
Change of temperature
Test sample is given 10 times of the following
temperature cycle,
30 C for 30 minutes
room temperature for 5
minutes
160 C for 30 minutes
room temperature for 5
minutes.
R25/R25
10%
B is constant which depends on the material used to
make the thermistor. Unless otherwise specified, all
values of B are determined from measurements made
at 25 C and 85 C.
200
0.01
0.1
1.0
10
100
160
100
50
25
25
0
Temperature ( C)
MARK
B : 2900K
200
0.01
0.1
1.0
10
100
160
100
50
25
25
0
Temperature ( C)
Power Thermistor
B : 3250K
0.02
0.03
0.05
0.1
0.2
0.3
0.5
1.0
2.0
3.0
5.0
10.0
20.0
160
100
50
25
30
0
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