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Temperature Dependent NTC Resistors
NTC (Negative Temperature Coefficient) resistors are semiconductors made of poly-crystalline, mixed-oxide ceramics and used mainly for temperature measurement. Since they are responsive to heat, they, and the positive-temperature coefficient resistors we shall later, are commonly dubbed with the name, thermistors. THe way they work is as follows: in semiconductors, the number of free charge carriers increases with the temperature, causing the electrical resistance to decrease. Hence the term negative temperature coefficient. At room temperature, this value is around -3 to -5% per degree. The temperature range, typically -60ºC to +200ºC, is given by the following equation:
T
:
Temperature in K
T_{0}
:
Reference temperature
B
:
Specific material constant
The reference temperature and specific material constant B are provided on the component's data sheet. Temperatures must be expressed in Kelvin (temperature referred to absolute zero). Conversion to the Kelvin scale is performed with the equation T = (J + 273°C).
Much more sensitive than metallic-resistor thermometers, NTC resistors are suitable for all types of temperature measurement and control. However, the fact that their resistance characteristic is exponential instead of linear frequently proves disadvantageous. In such cases, it may be necessary to perform a process of linearisation.
The basic values exemplified in the table below represent a NTC resistor with a reference temperature T_{0} = 25°C and a corresponding resistance R_{25} = 5 kW.
Table 1: Basic values of a NTC resistor (R_{25 }= 5kW)
Measured temperature in °C |
0 |
20 |
25 |
40 |
60 |
80 |
100 |
120 |
Basic resistance value in ohms |
16325 |
6245 |
5000 |
2663 |
1244 |
627.5 |
339 |
194.7 |
The diagram below shows the related characteristic (red) and that of a similar resistor with a reference value of 10 kW (blue).