Wiring Resistance Thermometers And Resistors - Siemens SIMATIC S7-300 Manual

Priniples of analog value processing

4.5 Wiring resistance thermometers and resistors

4.5 Wiring resistance thermometers and resistors
4.5
Introduction
This chapter contains a description of wiring resistance thermometers and resistors and
points to which particular attention must be paid.
Signal transducers for measuring resistance which can be wired
● With 4-wire connection
● With 3-wire connection
● With 2-wire connection
Wiring resistance thermometers and resistors
During a resistance measurement, the module supplies a constant current through terminals
I and I
C+
voltage drop. It is imperative to wire the constant current cables directly to the resistance
thermometer/resistor.
Measurements with programmed 4-or 3-wire connection parameters compensate for line
resistance and therefore deliver considerably better accuracy than that gained from the
mesurement with a 2-wire connection.
Measurements with a programmed 2-wire connection also record the line resistance in
addition to their own resistance.
4-wire connection of a resistance thermometer
The voltage generated at the resistance thermometer is measured via the M
terminals. Watch out for the polarity when you wire the cable (wire (I
M- at the resistance thermometer.)
Always wire the I
Figure 4-9
4-10
. The constant current is fed to the resistor to be measured and then measured as
C-
+, M+, I - and M- cables directly to the resistance thermometer.
C C
4-wire connection of resistance thermometers to an electrically isolated analog input
and M
+
and M , and I
C+ +
S7-300 Automation System Module data
Manual, 08/2006, A5E00105505-04
-
and
C -