NTC Thermistors

General Characteristics

1 – INTRODUCTION

Ω to 1M Ω ) and tem-perature coefficients (-2 to -6% per °C) can be achieved.RoHS (Restriction of Hazardous Substances - EuropeanUnion directive 2002/95/EC).ELV (End of Life-Vehicle - European Union directive2000/53/EC).All Thermistor Products have been fully RoHS/ELV sincebefore 2006.Chip Thermistor NB RoHS/ELV Status: external Plating100% smooth semi-bright Sn as standard SnPb Termination available on request.

2.1.2. Temperature - Resistance characteristics R (T)

NTC thermistors are thermally sensitive resistors made froma mixture of Mn, Ni, Co, Cu, Fe oxides. Sintered ceramic bodies of various sizes can be obtained. Strict conditions of mixing, pressing, sintering and metallization ensure an excellent batch-to-batch product characteristics.This semi-conducting material reacts as an NTC resistor,whose resistance decreases with increasing temperature. This Negative Temperature Coefficient effect can result from an external change of the ambient temperature or an inter- nal heating due to the Joule effect of a current flowing through the thermistor.By varying the composition and the size of the thermistors,a wide range of resistance values (0.1 This is the relation between the zero power resistance andthe temperature. It can be determined by experimental mea- surements and may be described by the ratios R (T) /R (25°C) where:R (T)is the resistance at any temperature TR (25°C)is the resistance at 25°C.These ratios are displayed on pages 36 to 40.

2.1.3. Temperature coefficient ( α )

The temperature coefficient ( ) which is the slope of thecurve at a given point is defined by: =•and expressed in % per °C.RdT 100dR

2.1.4. Sensitivity index (B)

The equation R = A exp (B/T) may be used as a roughapproximation of the characteristic R (T).B is called the sensitivity index or constant of the materialused.To calculate the B value, it is necessary to know the resis-tances R
1 and R
2 of the thermistor at the temperatures T
1 and T
2 .The equation:R 1 -1T
1 = R
2 exp B

()

1 T
2 R
1

2 – MAIN CHARACTERISTICS

leads to:B (K) =1•

()

1 -1T n

()

R
2 1 T
2

2.1 CHARACTERISTICS WITH NO DISSIPATION2.1.1. Nominal Resistance (Rn)

Conventionally, B will be most often calculated for tempe-ratures T
1 = 25°C and T
2 = 85°C (298.16 K and 358.16 K).In fact, as the equation R = A exp (B/T) is an approximation,the value of B depends on the temperatures T The nominal resistance of an NTC thermistor is generallygiven at 25°C. It has to be measured at near zero power so that the resultant heating only produces a negligible measurement error.The following table gives the maximum advised measure-ment voltage as a function of resistance values and thermal dissipation factors.This voltage is such that the heating effect generated by themeasurement current only causes a resistance change of 1% ∆ Rn/Rn.
1 and T
2 bywhich it is calculated.For example, from the R (T) characteristic of material M (values given on page 36), it can be calculated:B (25 – 85)= 3950B (0 – 60)= 3901 B (50 – 110)= 3983When using the equation R = A exp (B/T) for this material,the error can vary by as much as 9% at 25°C, 0.6% at 55°C and 1.6% at 125°C.Using the same equation, it is possible to relate the values ofthe index B and the coefficient α :
Ranges ofMaximum measuring voltage values(V) ( Ω ) δ = 2 mW/°C δ = 5 mW/°C δ = 10 mW/°C δ = 20 mW/°CR 100.1010 < R 1000.130.180.24100 < R 1,0000.250.380.530.241,000 < R 10,0000.731.11.52.010,000 =•• A exp (B/T) • RdTthus 1dR =1A exp (B/T) -BT
2 = –expressed in %/°C B T
2

2