Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 188

relation between temperature T and resistance R in the thermistor is obtained
similarly to the case of the intrinsic semiconductor:

1
273 15

1
273 15

Ê
Ë

ˆ
¯

RR = exp B T - W (4.61)

0

+ . T + .

0

where T 0 (K) is the reference temperature, T 0 a reference temperature which
has often the value of 0°C (273.15 K) or 20°C (293.15 K), R 0 the value of resist-
ance when temperature is T 0 , and B a thermistor constant given by the
expression

ln ( RR )

0

B = ∞ C

(4.62)

1 ( T + 273 15 . ) - 1 ( T + 273 15 . )

0

where ln is the natural log. From these expressions, temperature T can be
expressed as

- 1

=- (

)

1 ln RR

B

Ê
Ë

ˆ
¯

T

0

- 273 15 . ∞ C (4.63)

T

0

and the sensitivity of the thermistor is given by

B
T 2

a =- ¥ 100 % deg (4.64)

Thermistor sensors come in various shapes (Fig. 4.37). The bead-type thermis-
tor is covered with glass, making it possible to measure a high temperature
with reliability and stability. Since the tipped thermistor can be mass produced
at low cost and with high precision, it has been used as the electronic ther-
mometer. The disc-type thermistor has been utilized for consumer products
such as air conditioners and refrigerators, because it cannot make a small-size
thermometer. Since the thermistor is a resistive element, it cannot convert
temperature into an electric signal directly. Therefore, it converts temperature
into the electric signal with the aid of the external power source to feed the
sensor. The circuit structure is based on the following principles:

1. Temperature Measurement Method based on Relation of Tempera-
ture-Resistance Characteristics after Having Measured Thermistor Resis-
tance . This method yields temperature T from the measurement of R / R 0
by making the graphic relation between T and R / R 0 on the basis of the
expression (1).
2. Method of Converting Temperature Change into Voltage or Current Using
Wheatstone Bridge . This incorporates the thermistor into the Wheatstone
bridge to detect the temperature change as a change of voltage or
current using the detection method based on the zero method or the
fixing bridge. This bridge circuit method is mainly used for actual
thermometers.