Temperature Measurement System

In this lab a temperature measurement system will construct. A thermistor is a variable resistor

which resistance changes with temperature.

Figure 1: Calculations for a Thermistor

The resistance versus temperature curve

of a thermistor is not linear curve, but it is

linear between 25 - 37 centigrade degrees.

In the circuit a NTC (Negative Temperature

Coefficient) is used. This means, when

temperature goes up, the thermistor resistance

goes down. Input voltage of the circuit is 5 v and output voltage varies by a minimum of 0.5 v over a

temperature range of 25 C to 37 C. So, output voltage must increase as temperature increase.

According to Fig 1, when the temperature goes up, R(th) goes down, and its drop voltage goes down.

Therefore, V(out) increase because V(in) = V(th) + V(out). According to the curve of thermistor,

R(25) = 10600 ohms and R(37) = 7400 ohms. V(out) = [R / (R + R(th) )] * V(in).

This formula is used Vout (37 C) - Vout (25 C) = 0.4 volts  and R = 4 k , R = 18 k is calculated.

Vout (37 C) = [ 4 / (4 + 7.4)]*5 = 1.8 volts , Vout (25 C) = [ 4 / (4 + 10.6)]*5 = 1.4 volts

1.8 - 1.4 = 0.4 volts.

Vout (37 C) = [ 18 / (18 + 7.4)]*5 = 3.5 volts , Vout (25 C) = [ 18 / (18 + 10.6)]*5 = 3.1 volts

3.5 - 3.1 = 0.4 volts.

Figure 2: Thermistor resistance of the body Temperature

According to the thermistor curve,

Rth (33 C) = 8.4 k  & Rth (25 C) = 10.6 k

Rth (33 C measured) = 7.3 k

Vout (33 C measured) = 1.88 v

Rth (25 C measured) = 10.95 k

Vout (25 C measured) = 1.32 v

percent voltage error (25 C) = [(1.4 - 1.32) / 1.32]*100 = 6.1%

percent voltage error (33 C) = [(1.88 - 1.8) / 1.8]*100 = 4.44%

Figure 3: A Schematic Circuit

Figure 4: Operation of Thermistor Circuit Movie

The output sensitivity of the device must be at least 0.1 V/C.

According to temperature - resistance curve in Laboratory Manual:

1) A linear equation is calculated.

Rth = -(266.67)*T + 17266.8 

Line Equation

According to above equation, when T ≥ 52.2496 °C → Δ ≥ 0

Therefore, quadratic equation for R has real root. But 52 °C is not in temperature interval of line equation.
So we can change dVout / dT = 0.1 Volts / °C.

So dVout / dT = 0.0314 is suitable , and R = 11450 ohms or R = 9812 ohms