NTC Thermistors

T (˚C)

0

–20

–40

20

40

60

80

100 120 140

1000

100

10

1

0.1

0.01

0.001

5000

6000

4000

3000

2000

B=1000

T (˚C)

2.4

1

2.9

3.4

3.9

4.4

125

85

50

25

0

–20

–40

10000000

1000000

100000

10000

10

1000

100

/50

=4

250

/50

=4

250

343

5

343

5

/50

=4

050

50

=4

70

0

50

=4

70

0

280

0

280

0

/50

=4

50

0

/50

=45

00

T

1

/50

=4

050

The NTC Thermistors

This is a Negative Temperature Coefficient Resistor whose resistance changes as ambient temperature changes. Therm-

istor comprises 2 or 4 kinds of metal oxides of iron, nickel, cobalt, manganese and copper, being shaped and sintered

at high temperature (1200 to 1500 °C)

● For temperature measurement or temperature detection :

thermometer, temperature controller

● For temperature compensation : transistor circuit,

measuring instruments

● Temperature Coefficient of Resistance is negative and

extremely large

● Various kinds of types especially smaller ones are

available.

● Resistance values are available from 22 Ω to 470 kΩ

Fig. 1

Fig. 2

Thermistor is a resistor sensitive to temperature utilizing

the large temperature-coefficient of metal oxide semi-

conductor. And its temperature dependency of resistance

value is indicated by the following equation:

B

.................................... (1)

B: Thermistor Constant (K)

So called Temperature Coefficient (

a) is generally

indicated as follows:

a=

.................................................................... (2)

But

a is not adequate for use as a constant, because a

change by temperature is considerably large, so B Value

is used as a coefficient of thermistor.

1

T

1

(

)

[

]

B

T

2

The relation between resistance and temperature of a

thermistor is linear as shown in Fig. 2, in which resistance

is shown in vertical direction in a logarithmic scale and

reciprocal of absolute temperature in horizontal direction.

Bias degrees in these straight lines are determined according

to the B Value expressed by the following equation.

B =

.................................................. (3)

When calculated from this equation, B Value is a variable

in a strict sense, and the resistance is expressed by the

following equation:

R = AT

In (4), C is a small positive or negative constant and quite

negligible except use in precision temperature-measuring

device, thereby the B Value is, in practical usage, to be

considered as a constant. In Fig. 1,

shown with T °C, in the horizontal direction.

1

1

Sep. 2010

00

2