ESP1104 Introduction to Electronic Systems Khursheed
Tutorial 6
The questions in this tutorial relate to the use of a NTC thermistor with nominal resistance value, R0 of 10 kΩ at 25ºC. This thermistor is often used as a temperature sensor, and its resistance, R, for temperature, T with respect to the reference temperature T0 (25ºC) can be modeled by the following exponential equation
0 0
1 exp 1
T R T
R
where all temperature values are in degrees Kelvin (absolute zero is at 273.15 K). Assume that β = 4400 ºK and that the maximum allowable dissipation for the thermistor is 0.9W.
1) Let R1 = R2 = R3 = R4 = 10 kΩ in the OP-Amp circuit shown below. The thermistor is denoted by RT. Plot the output voltage as the temperatures varies from 25 to 45ºC, do not take more than 10 points. Find the equation for a straight-line approximation. What range of temperatures can be measured by this circuit?
1
+15 V
RT R1
+ R2 -
R3
V0 -15 V
R4
2) The figure below shows an alternative OP-Amp thermistor sensor circuit to the one described in question 1. Derive an expression for the output voltage in terms of the thermistor resistance. What kind of OP-Amp circuit is depicted below? Explain how it differs from the one used in question 1? Let R1 = 15.5 kΩ, and R2 = R3 = 1 kΩ. Plot the output voltage as the temperatures varies from 25 to 45ºC, take a maximum of 10 points. What is the minimum temperature that can be measured by this circuit? What are the advantages/disadvantages of this OP-AMP sensor circuit compared to the one described in question 1.
2
+15 V
RT R1
+ R2 -
R3
V0 -15 V
