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 theR (T) characteristic of a thermistor so that it matches the
required form, for example a linear response over a restrict-
ed temperature range.A single fixed resistor Rp placed in parallel with a thermistorgives a S–shape resistance–temperature curve (see Figure 2)
which is substantially more linear at the temperature range
around the inflexion point (Ro, To).
2.2 CHARACTERISTICS WITH ENERGYDISSIPATION
When a current is flowing through an NTC thermistor, thepower due to the Joule effect raises the temperature of the
NTC above ambient.The thermistor reaches a state of equilibrium when thepower supplied becomes equal to the power dissipated in
the environment.The thermal behavior of the thermistor is mainly dependenton the size, shape and mounting conditions.Several parameters have been defined to characterize theseproperties:
R(k Ω )
T ( TO R p 2.2.1. Heat capacity (H)
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.
R p 2.2.2. Dissipation factor ( )
R O This is the ratio between the variation in dissipated powerand the variation of temperature of the NTC. It is expressed
in mW/°C and may be measured as: = U.I85 – 25where U.I is the power necessary to raise to 85°C the tem-perature of a thermistor maintained in
industrial still air at 25°C.
T O ° C)
R Figure 2 – Linearization of a thermistor 2.2.3. Maximum permissible temperature (T max)
It can be calculated that better linearization is obtained when
the fixed resistor value and the mid-range temperature are
related by the formula: 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. Rp = RTox B – ToB+ 2ToFor example, with a thermistor ND03N00103J —R
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 higher ambient temperatures, the maximum permissiblepower is generally derated according to the Figure 3 here-
after and TL = Tmax – 10°C.
25°C = 10k Ω , B = 4080 Kgood linearization is obtained with a resistor in parallel wherethe value is: Rp = 10,000 Ω x4080 - 298= 8088 4080 + (2 x 298) 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.
25 ° T
L T
max T ° C
Figure 3 – Derating of maximum power 4