Simatic S7 - 200 tips doc

SIMATIC S7-200 Tips

S7-200 Tip PT100 Tip No. 36

Group Topic

6 Measuring and Monitoring Temperature with a PT100 Resistance

Temperature Detector

CPUs required for this tip

CPU 210  CPU 212  CPU 214  CPU 215  CPU 216  OTHER 

Overview

This program tip discusses the following: measuring a temperature and monitoring the specified

limits by using the analog expansion module EM235; at one input channel of the analog module,

a temperature sensor PT100 is connected.

In order to convert the temperature-conditioned resistance change of the PT100 into a voltage,

the analog output is used as constant current source. The output supplies the PT100 sensor

with a 12.5mA constant current. With this circuit, a linear input voltage of 5mV/°C is generated.

The EM235 converts this voltage into a digital value, which the program reads out cyclically.

From the value read, the program calculates the temperature[°C], using the following formula

T[°C] = Digital Value - 0 ° C-Offset

1°C-Value

Digital Value = value stored in AIWx (x=0,2,4).

0°C-Offset = digital value, measured at 0°C. In the case in point, this offset is 4000.

1°C-Value = value difference at a temperature rise of 1°C. In this tip, 1°C-Value = 16.

The program calculates the value to the first decimal point and writes the result in the

embedded variable of Message 1: ’’Temperature = xxx.x°C’’, which is read out with a TD200. In

the initialization segment of the program, the user can enter a high and low temperature limit.

The program monitors the measured value and displays a warning via the TD200 if the

measured temperature leaves the selected range. Message 2: ‘’Temperature > xxx.x°C’’

appears in the second display line of the TD200 if the measured temperature exceeds the high

temperature limit; Message 3: ‘’Temperature < xxx.x°C" is displayed if the measured

temperature drops below the low temperature limit.

Copyright 1997 by SIEMENS page 1 / 19 rzz1406913668.doc

Status: 06/97 Version 1.3

SIMATIC S7-200 customers have free use of the application tips. These tips are only a general approach to using the S7-200 with various applications.

Your specific application may be different. It is your responsibility to use the SIMATIC S7-200 properly in your applications.