NTC Thermistors - Temperature Measurement With Wheatstone Bridge
 

One of the easiest ways to measure temperature is to use a Wheatstone bridge (see drawing below). In a normal operation of the bridge, R₃ is a variable resistor or a potentiometer. We will zero the bridge at 25°C using the potentiometer installed at R₃ position. Next we will record the voltage versus temperature reading of the bridge. You will find that this is very linear, especially in the range of 0°C to 70°C. In this manner the output voltage would be a good indicator of temperature.

 R₁ / R₃ = R₂ / R_t   Þ condition of bridge being in balance @ 25°C

 Where:

 R₁ = 250W, R₂ =1000 W, R₃ = 2500W, R_t = 10,000W.

 The bridge voltage in circuit diagram (1) is now easily calculated using the formula below:

 V_b = [ R₃ / (R1 + R3) ] - [ R_t  / (R2 + R_t) ] V_s     eqn 3

 Where:

 1.                  V_s = Source Voltage = 7.55 volt DC Max

2.                  V_b = Bridge Voltage

 Now: using the equation 3 (eqn 3) and the resistance versus temperature curve for NTC thermistor we can accurately predict the voltage at any temperature or vice versa.

 This is very accurate and repeatable since the bridge aids linearize the NTC effect.

As an example:

 (EX) Predict the voltage drop at temperature of 40°C

 (A) At 40°C R_t = 5282Ω, from Ametherm curves.

Using eqn 3, V_b = í[2500/(2500+250)] - [ 5282/(5282 + 1000)]ý (7.55v) = 0.5154

Refer to chart #1: T= 39°C Þ 0.519V and T= 38°C = 0.482V

Interpolating to the tenth of °C we will have (0.519) - (0.482) / 10 =0.0037

Therefore if we subtract 0.519-0.0037 = 0.5153 =38.9°C

Although our target was for 40°C, this was done to illustrate the effect of selecting a part, which has a tolerance of ± 2.2°C.

DEG C

DCV

DEG C

DCV

DEG C

DCV

DEG C

DCV

-19.5

0.591

24

0.069

50

-0.974

76

-2.46

-19

0.589

25

0.002

51

-1.022

77

-2.54

-18.5

0.586

26

-0.019

52

-1.072

78

-2.6

-18

0.584

27

-0.06

53

-1.121

79

-2.66

-17

0.574

28

-0.1

54

-1.165

80

-2.71

-16

0.57

29

-0.137

55

-1.217

81

-2.78

-15

0.567

30

-0.175

56

-1.262

82

-2.83

-14

0.561

31

-0.213

57

-1.314

83

-2.88

-13

0.554

32

-0.248

58

-1.361

84

-2.92

-12

0.547

33

-0.289

59

-1.408

85

-2.97

-11

0.543

34

-0.326

60

-1.457

86

-3

-10

0.532

35

-0.368

61

-1.505

87

-3.06

-9

0.526

36

-0.403

62

-1.505

88

-3.08

-8

0.521

37

-0.443

63

-1.606

89

-3.11

-7

0.514

38

-0.482

64

-1.655

90

-3.16

-6

0.508

39

-0.519

65

-1.702

91

-3.2

-5

0.503

40

-0.558

66

-1.738

92

-3.25

-4

0.497

41

-0.598

67

-1.8

93

-3.29

-3

0.49

42

-0.638

68

-1.864

94

-3.33

-2

0.484

43

-0.681

69

-1.905

95

-3.39

-1

0.473

44

-0.718

70

-1.965

96

-3.44

0

0.456

45

-0.762

71

-2.1

97

-3.48

1

0.444

46

-0.8

72

-2.18

98

-3.55

2

0.456

47

-0.837

73

-2.26

99

-3.59

3

0.469

48

-0.887

74

-2.34

100

-3.63

23

0.117

49

-0.935

75

-2.39