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N_06U00334 Folha de dados(PDF) 4 Page - AVX Corporation |
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N_06U00334 Folha de dados(HTML) 4 Page - AVX Corporation |
4 / 38 page 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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Descrição semelhante - N_06U00334 |
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