## NTC Thermistors Temperature Measurement With Wheatstone Bridge

R 1 / R3 = R2 / Rt Þ condition of bridge being in balance @ 25°C

Where:

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

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

Vb = [ R3 / (R1 + R3) ] - [ Rt / (R2 + Rt) ] Vs eqn 3

Where:

1. Vs = Source Voltage = 7.55 volt DC Max

2. Vb = 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, Vb = 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.

Chart 1 - Temperature vs. Volts | |||||||

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 |