Siemens SIMATIC S7-400 Reference Manual page 217

Principle of operation of external compensation with compensating box
If you employ external compensation, the temperature of the reference junction of the
thermocouples is taken into account via a compensating box, for example.
The compensating box contains a bridge circuit calibrated for a definite reference junction
temperature. The reference junction is formed by the connecting ends of the thermocouple's
compensating lines.
The resistance of the temperature-sensitive bridge changes as a function of the difference
between the actual reference temperature and calibrating temperature. This difference induces
a positive or negative compensating voltage, which is added to the thermal voltage.
Note
The diagrams below do not show the connecting lines required for connecting the electrical
potentials of the analog input module and the sensors. This means that you must still read and
follow the instructions in the generally applicable information on connecting sensors.
Connection of thermocouples without compensation or using the reference temperature value
Wire the thermocouples either directly to the inputs of the module, or indirectly via
compensating lines. Each channel can use a thermocouple type supported by the analog
module independently of the other channels.
1
M
+
M :
-
(1)
Figure 5-16
Connecting the compensating box
The compensating box is looped in through the leads of each thermocouple. The compensating
box must be supplied on an isolated basis. The power supply module must provide adequate
noise filtering, for example, by means of grounded cable shielding.
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08
: Measuring line (positive)
Measuring line (negative)
Compensating leads (same material as thermocouple)
Connection of thermocouples without compensation or using the reference temperature
value to an isolated AI
M+
M-
M+
M-
Analog modules
5.13 Connecting thermocouples
217