NTC Thermistor：
Introduction
NTC thermistors are resistors with large negative temperature coefficient. Change in resistance of the NTC thermistors
can be brought about either externally by a change in ambient temperature or internally by self-heating resulting from a current
flowing through the device. This predictable change in resistance as temperature changes is the basis for all applications of
thermistors.
The semi conducting material of NTC thermistors usually consists of a sintered ceramic fabricated in a variety of shapes
and sizes from a mixture of oxides chosen from Mn, Co, Ni, Cu, Fe and etc….By varying the composition and the size of the
semi-conducting elements, resistance values between 100 and 106 ohms at room temperature can be achieved with
temperature coefficients lying between -2% and -6.5% per ℃.
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NTC Thermistor：
Glossary
„ Thermistor
A thermistor is a thermally sensitive semiconductor resistor whose primary function is to exhibit an important change in
electrical resistance with a change in body temperature.
„ Negative temperature coefficient thermistor (NTC)
An NTC thermistor is one in which the resistance decreases with increasing temperature.
„ Zero-power resistance (R T )
The zero-power resistance is the value of a resistance when measured at a specified temperature, under conditions such
that the change in resistance due to the internal generation of heat is negligible with respect to the total error of
measurement.
„ Rated zero-power resistance (R25 )
The rated zero-power resistance is the nominal value at the standard temperature of 25℃ unless otherwise specified.
„ B-value
An index of the thermal sensitivity expressed by the formula :
Or
Where
B: constant in Kelvins (K)
R1: resistance in ohms (Ω) at temperature T1
R2: resistance in ohms (Ω) at temperature T2
T1=298.15k (+25℃), T2=358.15k (+85℃)
The value given above for T1 and T2 are the preferred values. When the detail specification prescribes that the B-value
shall be measured at other temperatures, the specified value (in Kelvins) shall be used for T1 and T2 in the calculation in
place of the preferred values.
„ Operating temperature range
The operating temperature range is the range of ambient temperature for which the thermistor has been designed to
operate continuously at zero-power. The limits are the upper and lower operating temperature.
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NTC Thermistor：
Glossary
„ Maximum power rating (Pmax)
The maximum power rating is the maximum dissipation which can be applied to the thermistor for an extended period of
time, at an ambient temperature of 25℃ or at such temperature as may be specified in the detail specification. Thinking's
maximum power derating curve is given as below. At a certain ambient temperature, the power-rating shall be derated
Max power rating (%)
linearly to zero.
100
0
0
TL
25
TU
Ambient temperature (℃)
„ Dissipation constant (δ)
The dissipation constant is the quotient (in W/K), at a specified ambient temperature in a specified medium of a change in
power dissipation in a thermistor to the resultant body temperature change.
„ Thermal time constant(τ)
The time (in s) required for the temperature of a thermistor to change by 63.2% of the difference between its initial and final
temperatures when subjected to a step function change in temperature under zero-power conditions.
T0
Temperature
τ
63.2%
Ta
Time
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NTC Thermistor：
Glossary
● Table 1: Heating time constant and temperature change ratio.
Code
Rate of change (%) for T0-Ta
τ
63.2
2τ
86.5
3τ
95.0
4τ
98.2
5τ
99.4
6τ
99.8
7τ
99.9
„ Resistance/temperature characteristic
The relationship between the zero-power resistance of a thermistor and its body temperature. The resistance law follow
approximately the formula:
Where R and R1 are the value of the zero-power resistance of the thermistor at temperature T and T1 respectively,
expressed in Kelvins, and B is the thermal sensitivity index.
„ Voltage/current characteristic
The relationship in still air at 25℃ or at such a temperature as may be prescribed in the detail specification between the dc
or ac rms voltage across the thermistor and the applied steady-state current.
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NTC Thermistor：
Application Note
„ Temperature measurement
The NTC thermistor offers a practical, low-cost solution to most temperature measurement applications. One of the circuits
suitable for temperature measurement is a Wheatstone bridge with an NTC thermistor used as one bridge leg.
„ Temperature control
The NTC thermistors are the most sensitive of all temperature sensing elements they are popular in temperature control
systems. By using a thermistor in a voltage comparator circuit basic on-off temperature control as well as over-temperature
protection can be incorporated.
„ Temperature compensation
Many semiconductors and ICs exhibit a temperature coefficient and require temperature compensation to produce stable
performance over a wide temperature range. Owing to their high positive temperature coefficient, NTC thermistors are
particularly suitable for this kind of compensation.
R
T
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NTC Thermistor：
Application Note
„ Inrush current suppressing
Circuits including a capacitor, filament for a bulb, inverter for fluorescent lamp, a heater and etc., induce an inrush current
more than 100 times the normal current when the circuit switch is turned on. The NTC thermistors specially developed for
this application limit the current at turn-on by their relatively high cold resistance. During continuous operation, the NTC
thermistors heats up and its resistance remains negligibly low.
I
T
„ Liquid level sensing
Thermistor liquid level sensors depend for their action on the difference between the dissipation constant of the liquid and
the air or vapour above the liquid. When the thermistor is immersed in a liquid the dissipation constant increases, the
temperature decreases and the voltage lying across thermistor rise. Owing to this effect NTC thermistor are able to detect
the presence or absence of the liquid at one precise level.
V
I
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NTC Thermistor：
Application Note
„ Example
2. Temperature Sensor IC
1.
NTC thermistor
(Cost : 50% down)
(Cost : ￥20±0.25)
Driving voltage controlled
by temperature
Voltage vs. temperature
characteristics
NTC thermistor
Driving voltage
Temperature sensor IC
Temperature (℃)
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