NTC Thermistors
General Characteristics
2.1.2. Temperature -
Resistance characteristics R (T)
This is the relation between the zero power resistance and
the temperature. It can be determined by experimental mea-
surements and may be described by the ratios R (T) /R
(25°C) where:
1 – INTRODUCTION
NTC thermistors are thermally sensitive resistors made from
a 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.
R (T)
is the resistance at any temperature T
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.
R (25°C) is the resistance at 25°C.
These ratios are displayed on pages 29 to 33.
2.1.3. Temperature coefficient (α)
The temperature coefficient (␣) which is the slope of the
curve at a given point is defined by:
By varying the composition and the size of the thermistors,
a wide range of resistance values (0.1Ω to 1MΩ) and tem-
perature coefficients (-2 to -6% per °C) can be achieved.
100
R
dR
dT
␣ =
•
and expressed in % per °C.
RoHS (Restriction of Hazardous Substances - European
Union directive 2002/95/EC).
2.1.4. Sensitivity index (B)
The equation R = A exp (B/T) may be used as a rough
approximation of the characteristic R (T).
ELV (End of Life-Vehicle - European Union directive
2000/53/EC).
B is called the sensitivity index or constant of the material
used.
All Thermistor Products have been fully RoHS/ELV since
before 2006.
To calculate the B value, it is necessary to know the resis-
tances R1 and R2 of the thermistor at the temperatures
T1 and T2.
Chip Thermistor NB RoHS/ELV Status: external Plating
100% smooth semi-bright Sn as standard SnPb Termination
available on request.
1
1
1
-
The equation: R1 = R2 exp B (T )
T2
leads to:
B (K) =
• ᐍn (RR1 )
1
1
1
1
2 – MAIN CHARACTERISTICS
2
-
(T )
T2
2.1 CHARACTERISTICS WITH NO DISSIPATION
Conventionally, B will be most often calculated for tempe-
ratures T1 = 25°C and T2 = 85°C (298.16 K and 358.16 K).
2.1.1. Nominal Resistance (Rn)
In fact, as the equation R = A exp (B/T) is an approximation,
the value of B depends on the temperatures T1 and T2 by
which it is calculated.
The nominal resistance of an NTC thermistor is generally
given at 25°C. It has to be measured at near zero power
so that the resultant heating only produces a negligible
measurement error.
For example, from the R (T) characteristic of material M
(values given on page 29), it can be calculated:
The following table gives the maximum advised measure-
ment voltage as a function of resistance values and thermal
dissipation factors.
B (25 – 85) = 3950
B (0 – 60) = 3901
B (50 – 110) = 3983
This voltage is such that the heating effect generated by the
measurement current only causes a resistance change of
1% ΔRn/Rn.
When 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.
Maximum measuring voltage
Ranges of
Using the same equation, it is possible to relate the values of
the index B and the coefficient α:
(V)
values
(Ω)
δ = 2 mW/°C δ = 5 mW/°C δ = 10 mW/°C δ = 20 mW/°C
R 10
0.10
1
-B
T2
1
R
dR
dT
=
␣ =
•
• A exp (B/T) •
10 < R 100
0.13
0.38
1.1
0.18
0.53
1.5
0.24
0.24
2.0
A exp (B/T)
100 < R 1,000
1,000 < R 10,000
10,000 < R 100,000
R < 100,000
0.25
0.73
2.1
B
3.2
4.6
thus ␣ = –
expressed in %/°C
T2
6.4
9.7
14.5
2
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