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N_09U00154 Datasheet(PDF) 4 Page - AVX Corporation

Part No. N_09U00154
Description  NTC Thermistors
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Maker  AVX [AVX Corporation]
Homepage  http://www.avx.com
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3
NTC Thermistors
General Characteristics
2.1.5. Further approximation of R (T) curve
The description of the characteristic R (T) can be improved
by using a greater number of experimental points, and by
using the equation:
1 = A + B ( n R) + C ( n R)3
T
The parameters A, B and C are determined by solving the
set of equations obtained by using the measured resis-
tances at three temperatures.
The solution of the above equation gives the resistance at
any temperature:
The precision of this description is typically 0.2°C for the
range –50 to +150°C (A, B, C being determined with exper-
imental values at –20, +50 and 120°C) or even better if this
temperature range is reduced. The ratios R(T)/R(25°C) for
each of the different materials shown on pages 29 to 33
have been calculated using the above method.
2.1.6. Resistance tolerance and temperature
precision
An important characteristic of a thermistor is the tolerance
on the resistance value at a given temperature.
This uncertainty on the resistance (DR/R) may be related to
the corresponding uncertainty on the temperature (DT),
using the relationship:
T = 100 •
R
1
R
Example: consider the thermistor ND06M00152J —
• R (25°C) = 1500 ohms
• Made from M material
• R (T) characteristic shown on page 23 gives:
= - 4.4%/°C at 25°C
• Tolerance
R/R = ±5% is equivalent to:
T = 5%/4.4%/°C = ±1.14°C
2.1.7. Resistance tolerance at any temperature
Any material used for NTC manufacturing always displays a
dispersion for the R (T) characteristic.
This dispersion depends on the type of material used
and has been especially reduced for our accuracy series
thermistors.
Thus, the tolerance on the resistance ( R
2/R2) at a temper-
ature T
2 is the sum of two contributions as illustrated on
Figure 1:
– the tolerance
R
1/R1 at a temperature T1 used as a
reference.
– an additional contribution due to the dispersion on
the characteristic R (T) which may be called
“Manufacturing tolerance” (Tf).
Figure 1
Differentiating the equation R = A exp (B/T), the two contri-
butions on the tolerance at T can also be written:
R2
=
R1 +
⎪⎪ • B
R2
R1
The T(f) values given with the resistance – temperature
characteristics on pages 29 to 33 are based on a computer
simulation using this equation and experimental values.
2.1.8. Designing the resistance tolerances
Using the fact that the coefficient
decreases with temper-
ature (
α = –B/T2), it is generally useful to define the closest
tolerance of the thermistor at the maximum value of the
temperature range where an accuracy in °C is required.
For example, let us compare the two designs 1 and 2
hereafter:
Only the Design 2 is able to meet the requirement
ΔT
1°C
from 25°C to 100°C.
R
Ω
R
25
25
°C
T
Temperature (
°C)
Graph with B
Graph with B
±
ΔB
}
(
ΔR)
25
°C
}
} (ΔR)25°C
+
TF
} =(ΔR)
T
1 - 1
T
1
T
2
TR
α
Design 1
Design 2
(°C)
(Ω)
(%/°C)
R/R(%)
T(°C)
R/R(%)
T(°C)
0
3275
-5.2
3.5
0.7
5.0
1.0
25
1000
-4.4
3.0
0.7
4.5
1.1
55
300
-3.7
3.5
1.0
4.0
1.1
85
109
-3.1
4.1
1.3
3.4
1.1
100
69.4
-2.9
4.5
1.6
3.0
1.0
A- 1/T
C
()
A- 1/T
C
()
n R (T) =
]
[ -27
2
1
3
B
C
3
+
3
2
327
2
+ 4
3
()
- 3
+27
2
A- 1/T
C
()
()
()
+
3
2
327
2
+ 4
3
A- 1/T
C
()
B
C
()
()




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