NTC Thermistors
General Characteristics
2.1.9. Shaping of the R (T) characteristic
By the use of a resistor network, it is possible to modify the
R (T) characteristic of a thermistor so that it matches the
required form, for example a linear response over a restrict-
ed temperature range.
2.2 CHARACTERISTICS WITH ENERGY
DISSIPATION
When a current is flowing through an NTC thermistor, the
power due to the Joule effect raises the temperature of the
NTC above ambient.
A single fixed resistor Rp placed in parallel with a thermistor
gives a S–shape resistance–temperature curve (see Figure 2)
which is substantially more linear at the temperature range
around the inflexion point (Ro, To).
The thermistor reaches a state of equilibrium when the
power supplied becomes equal to the power dissipated in
the environment.
The thermal behavior of the thermistor is mainly dependent
on the size, shape and mounting conditions.
R
(kΩ)
Several parameters have been defined to characterize these
properties:
RTO
2.2.1. Heat capacity (H)
Rp
The heat capacity is the amount of heat required to change
the temperature of the thermistor by 1°C and is expressed in
J/°C.
Rp
2.2.2. Dissipation factor (␦)
This is the ratio between the variation in dissipated power
and the variation of temperature of the NTC. It is expressed
in mW/°C and may be measured as:
RO
U.I
85 – 25
␦ =
TO
T (°C)
where U.I is the power necessary to raise to 85°C the tem-
perature of a thermistor maintained in still air at 25°C.
Figure 2 – Linearization of a thermistor
2.2.3. Maximum permissible temperature (T max)
This is the maximum ambient temperature at which the ther-
mistor may be operated with zero dissipation. Above this
temperature, the stability of the resistance and the leads
attachment can no longer be guaranteed.
It can be calculated that better linearization is obtained when
the fixed resistor value and the mid-range temperature are
related by the formula:
B – To
B+ 2To
Rp = R
x
To
2.2.4. Maximum permissible power at 25°C (Pmax)
This is the power required by a thermistor maintained in still
air at 25°C to reach the maximum temperature for which it is
specified.
For example, with a thermistor ND03N00103J —
R25°C = 10kΩ, B = 4080 K
good linearization is obtained with a resistor in parallel where
the value is:
For higher ambient temperatures, the maximum permissible
power is generally derated according to the Figure 3 here-
after and TL = Tmax – 10°C.
4080 - 298
4080 + (2 x 298)
Rp = 10,000 Ω x
= 8088 Ω
2.1.10. Demonstration of the R (T) parameters
calculation
P
max
To help our customers when designing thermistors for
temperature measurement or temperature compensation,
software developed by our engineering department is avail-
able upon request.
Tmax
Figure 3 – Derating of maximum power
T°C
25°
TL
4
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