Catalogue AVX NTC Thermistors
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NTC Thermistors
General Characteristics
/aV№(
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 [in R) + C [in R)3
The parameters A, B and C are determined by solving the set of equations obtained by using the measured resis­tances at three temperatures.
The solution of the above equation gives the resistance at any temperature:
+fH/I) + fV 30/27^+ 4@3)]
Thus, the tolerance on the resistance (AR2/R2) at a temper­ature T2 is the sum of two contributions as illustrated on Figure 1:
- the tolerance AR1/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).
Graph with B Graph with B ± AB
RQ
J}(Ar)25°c } Tf
}
= (AR)T
The precision of this description is typically 0.2°C for the range -50 to +150°C (A, B, C being determined with exper­imental 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 36 to 40 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:
Example: consider the thermistor ND06M00152J —
. R (25°C) = 1500 ohms
. Made from M material
• R (T) characteristic shown on page 23 gives: a = - 4.4%/°C at 25°C
• Tolerance AR/R = ±5% is equivalent to: AT = 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.
25 °C
T Temperature (°C)
Figure 1
Differentiating the equation R = A exp (B/T), the two contri­butions on the tolerance at T can also be written:
AR2
~r2
|1-1 I • AB
T Ï2
+
Ri
The T(f) values given with the resistance - temperature characteristics on pages 36 to 40 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 a decreases with temper­ature (a = -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:
T
R
a
Design 1
Design 2
(°C)
(Q)
(%/°C)
AR/R(%)
AT(°C)
AR/R(%)
AT(°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
Only the Design 2 Is able to meet the requirement AT : from 25°C to 100°C.
1°C
3
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