Question 4.70: Resistance Temperature Transducer A resistive transducer use......
Resistance Temperature Transducer
A resistive transducer uses a sensing element whose resistance varies with temperature. For a particular transducer, the resistance varies as R_{TR} = 0.375T + 100 Ω, where T is temperature in °C. This transducer is to be included in a circuit to measure temperatures in the range from −200° C to 800° C. The circuit must convert the transducer resistance variation over this temperature range into an output voltage in the range from 0 to 5 V. Two proposed circuit designs are shown in Figure P4-70. Which of these circuits would you recommend for production and why? Hint: First verify that the circuits perform the required function. Use OrCAD to verify your results.
Compute the range of resistance values for the given temperature range.
Script File
clear all
format short eng
T_range = [-200,800];
RTR_range = 0.375*T_range+100
RTR_range =
25.0000e+000 400.0000e+000
Analyze the performance of each circuit for the given resistor range
% Circuit 1
disp('Circuit 1')
% Use voltage division to find vTR
vTR = 5*RTR_range./(RTR_range+100)
% Circuit 1 is configured as a subtractor
vo1 = vTR*(900+600)/900 - 2.5*600/900
Circuit 1
vTR =
1.0000e+000 4.0000e+000
vo1 =
0.0000e-003 5.0000e+000
% Circuit 2
disp('Circuit 2')
% Find a Thevenin equivalent for the voltage at the
% negative input terminal
vT = 5*100./(100+RTR_range)
RT = 600 + 1./(1./RTR_range + 1/100)
% Circuit 2 is configured as a subtractor
vo2 = 2.5*(RT+1e3)./RT - vT*1e3./RT
Circuit 2
vT =
4.0000e+000 1.0000e+000
RT =
620.0000e+000 680.0000e+000
vo2 =
80.6452e-003 4.7059e+000
Circuit 1 meets the specifications, but Circuit 2 does not. Confirm these results using OrCAD.
The output voltages from the two circuits are plotted on the same axes for direct comparison.
Although the output voltage from Circuit 2 is similar to the output voltage from Circuit 1, Circuit 2 does not meet the specifications and Circuit 1 does, as predicted with the MATLAB calculations.
Answer:
Choose Circuit 1 for production because it meets the specifications and Circuit 2 does not.
